text: ============================================================
  Kaleidoscope: Extending the Language: User-defined Operators
  ============================================================
  
  .. contents::
     :local:
  
  Chapter 6 Introduction
  ======================
  
  Welcome to Chapter 6 of the "`Implementing a language with
  LLVM <index.html>`_" tutorial. At this point in our tutorial, we now
  have a fully functional language that is fairly minimal, but also
  useful. There is still one big problem with it, however. Our language
  doesn't have many useful operators (like division, logical negation, or
  even any comparisons besides less-than).
  
  This chapter of the tutorial takes a wild digression into adding
  user-defined operators to the simple and beautiful Kaleidoscope
  language. This digression now gives us a simple and ugly language in
  some ways, but also a powerful one at the same time. One of the great
  things about creating your own language is that you get to decide what
  is good or bad. In this tutorial we'll assume that it is okay to use
  this as a way to show some interesting parsing techniques.
  
  At the end of this tutorial, we'll run through an example Kaleidoscope
  application that `renders the Mandelbrot set <#kicking-the-tires>`_. This gives an
  example of what you can build with Kaleidoscope and its feature set.
  
  User-defined Operators: the Idea
  ================================
  
  The "operator overloading" that we will add to Kaleidoscope is more
  general than languages like C++. In C++, you are only allowed to
  redefine existing operators: you can't programmatically change the
  grammar, introduce new operators, change precedence levels, etc. In this
  chapter, we will add this capability to Kaleidoscope, which will let the
  user round out the set of operators that are supported.
  
  The point of going into user-defined operators in a tutorial like this
  is to show the power and flexibility of using a hand-written parser.
  Thus far, the parser we have been implementing uses recursive descent
  for most parts of the grammar and operator precedence parsing for the
  expressions. See `Chapter 2 <OCamlLangImpl2.html>`_ for details. Without
  using operator precedence parsing, it would be very difficult to allow
  the programmer to introduce new operators into the grammar: the grammar
  is dynamically extensible as the JIT runs.
  
  The two specific features we'll add are programmable unary operators
  (right now, Kaleidoscope has no unary operators at all) as well as
  binary operators. An example of this is:
  
  ::
  
      # Logical unary not.
      def unary!(v)
        if v then
          0
        else
          1;
  
      # Define > with the same precedence as <.
      def binary> 10 (LHS RHS)
        RHS < LHS;
  
      # Binary "logical or", (note that it does not "short circuit")
      def binary| 5 (LHS RHS)
        if LHS then
          1
        else if RHS then
          1
        else
          0;
  
      # Define = with slightly lower precedence than relationals.
      def binary= 9 (LHS RHS)
        !(LHS < RHS | LHS > RHS);
  
  Many languages aspire to being able to implement their standard runtime
  library in the language itself. In Kaleidoscope, we can implement
  significant parts of the language in the library!
  
  We will break down implementation of these features into two parts:
  implementing support for user-defined binary operators and adding unary
  operators.
  
  User-defined Binary Operators
  =============================
  
  Adding support for user-defined binary operators is pretty simple with
  our current framework. We'll first add support for the unary/binary
  keywords:
  
  .. code-block:: ocaml
  
      type token =
        ...
        (* operators *)
        | Binary | Unary
  
      ...
  
      and lex_ident buffer = parser
        ...
            | "for" -> [< 'Token.For; stream >]
            | "in" -> [< 'Token.In; stream >]
            | "binary" -> [< 'Token.Binary; stream >]
            | "unary" -> [< 'Token.Unary; stream >]
  
  This just adds lexer support for the unary and binary keywords, like we
  did in `previous chapters <OCamlLangImpl5.html#lexer-extensions-for-if-then-else>`_. One nice
  thing about our current AST, is that we represent binary operators with
  full generalisation by using their ASCII code as the opcode. For our
  extended operators, we'll use this same representation, so we don't need
  any new AST or parser support.
  
  On the other hand, we have to be able to represent the definitions of
  these new operators, in the "def binary\| 5" part of the function
  definition. In our grammar so far, the "name" for the function
  definition is parsed as the "prototype" production and into the
  ``Ast.Prototype`` AST node. To represent our new user-defined operators
  as prototypes, we have to extend the ``Ast.Prototype`` AST node like
  this:
  
  .. code-block:: ocaml
  
      (* proto - This type represents the "prototype" for a function, which captures
       * its name, and its argument names (thus implicitly the number of arguments the
       * function takes). *)
      type proto =
        | Prototype of string * string array
        | BinOpPrototype of string * string array * int
  
  Basically, in addition to knowing a name for the prototype, we now keep
  track of whether it was an operator, and if it was, what precedence
  level the operator is at. The precedence is only used for binary
  operators (as you'll see below, it just doesn't apply for unary
  operators). Now that we have a way to represent the prototype for a
  user-defined operator, we need to parse it:
  
  .. code-block:: ocaml
  
      (* prototype
       *   ::= id '(' id* ')'
       *   ::= binary LETTER number? (id, id)
       *   ::= unary LETTER number? (id) *)
      let parse_prototype =
        let rec parse_args accumulator = parser
          | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
          | [< >] -> accumulator
        in
        let parse_operator = parser
          | [< 'Token.Unary >] -> "unary", 1
          | [< 'Token.Binary >] -> "binary", 2
        in
        let parse_binary_precedence = parser
          | [< 'Token.Number n >] -> int_of_float n
          | [< >] -> 30
        in
        parser
        | [< 'Token.Ident id;
             'Token.Kwd '(' ?? "expected '(' in prototype";
             args=parse_args [];
             'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
            (* success. *)
            Ast.Prototype (id, Array.of_list (List.rev args))
        | [< (prefix, kind)=parse_operator;
             'Token.Kwd op ?? "expected an operator";
             (* Read the precedence if present. *)
             binary_precedence=parse_binary_precedence;
             'Token.Kwd '(' ?? "expected '(' in prototype";
              args=parse_args [];
             'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
            let name = prefix ^ (String.make 1 op) in
            let args = Array.of_list (List.rev args) in
  
            (* Verify right number of arguments for operator. *)
            if Array.length args != kind
            then raise (Stream.Error "invalid number of operands for operator")
            else
              if kind == 1 then
                Ast.Prototype (name, args)
              else
                Ast.BinOpPrototype (name, args, binary_precedence)
        | [< >] ->
            raise (Stream.Error "expected function name in prototype")
  
  This is all fairly straightforward parsing code, and we have already
  seen a lot of similar code in the past. One interesting part about the
  code above is the couple lines that set up ``name`` for binary
  operators. This builds names like "binary@" for a newly defined "@"
  operator. This then takes advantage of the fact that symbol names in the
  LLVM symbol table are allowed to have any character in them, including
  embedded nul characters.
  
  The next interesting thing to add, is codegen support for these binary
  operators. Given our current structure, this is a simple addition of a
  default case for our existing binary operator node:
  
  .. code-block:: ocaml
  
      let codegen_expr = function
        ...
        | Ast.Binary (op, lhs, rhs) ->
            let lhs_val = codegen_expr lhs in
            let rhs_val = codegen_expr rhs in
            begin
              match op with
              | '+' -> build_add lhs_val rhs_val "addtmp" builder
              | '-' -> build_sub lhs_val rhs_val "subtmp" builder
              | '*' -> build_mul lhs_val rhs_val "multmp" builder
              | '<' ->
                  (* Convert bool 0/1 to double 0.0 or 1.0 *)
                  let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
                  build_uitofp i double_type "booltmp" builder
              | _ ->
                  (* If it wasn't a builtin binary operator, it must be a user defined
                   * one. Emit a call to it. *)
                  let callee = "binary" ^ (String.make 1 op) in
                  let callee =
                    match lookup_function callee the_module with
                    | Some callee -> callee
                    | None -> raise (Error "binary operator not found!")
                  in
                  build_call callee [|lhs_val; rhs_val|] "binop" builder
            end
  
  As you can see above, the new code is actually really simple. It just
  does a lookup for the appropriate operator in the symbol table and
  generates a function call to it. Since user-defined operators are just
  built as normal functions (because the "prototype" boils down to a
  function with the right name) everything falls into place.
  
  The final piece of code we are missing, is a bit of top level magic:
  
  .. code-block:: ocaml
  
      let codegen_func the_fpm = function
        | Ast.Function (proto, body) ->
            Hashtbl.clear named_values;
            let the_function = codegen_proto proto in
  
            (* If this is an operator, install it. *)
            begin match proto with
            | Ast.BinOpPrototype (name, args, prec) ->
                let op = name.[String.length name - 1] in
                Hashtbl.add Parser.binop_precedence op prec;
            | _ -> ()
            end;
  
            (* Create a new basic block to start insertion into. *)
            let bb = append_block context "entry" the_function in
            position_at_end bb builder;
            ...
  
  Basically, before codegening a function, if it is a user-defined
  operator, we register it in the precedence table. This allows the binary
  operator parsing logic we already have in place to handle it. Since we
  are working on a fully-general operator precedence parser, this is all
  we need to do to "extend the grammar".
  
  Now we have useful user-defined binary operators. This builds a lot on
  the previous framework we built for other operators. Adding unary
  operators is a bit more challenging, because we don't have any framework
  for it yet - lets see what it takes.
  
  User-defined Unary Operators
  ============================
  
  Since we don't currently support unary operators in the Kaleidoscope
  language, we'll need to add everything to support them. Above, we added
  simple support for the 'unary' keyword to the lexer. In addition to
  that, we need an AST node:
  
  .. code-block:: ocaml
  
      type expr =
        ...
        (* variant for a unary operator. *)
        | Unary of char * expr
        ...
  
  This AST node is very simple and obvious by now. It directly mirrors the
  binary operator AST node, except that it only has one child. With this,
  we need to add the parsing logic. Parsing a unary operator is pretty
  simple: we'll add a new function to do it:
  
  .. code-block:: ocaml
  
      (* unary
       *   ::= primary
       *   ::= '!' unary *)
      and parse_unary = parser
        (* If this is a unary operator, read it. *)
        | [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] ->
            Ast.Unary (op, operand)
  
        (* If the current token is not an operator, it must be a primary expr. *)
        | [< stream >] -> parse_primary stream
  
  The grammar we add is pretty straightforward here. If we see a unary
  operator when parsing a primary operator, we eat the operator as a
  prefix and parse the remaining piece as another unary operator. This
  allows us to handle multiple unary operators (e.g. "!!x"). Note that
  unary operators can't have ambiguous parses like binary operators can,
  so there is no need for precedence information.
  
  The problem with this function, is that we need to call ParseUnary from
  somewhere. To do this, we change previous callers of ParsePrimary to
  call ``parse_unary`` instead:
  
  .. code-block:: ocaml
  
      (* binoprhs
       *   ::= ('+' primary)* *)
      and parse_bin_rhs expr_prec lhs stream =
              ...
              (* Parse the unary expression after the binary operator. *)
              let rhs = parse_unary stream in
              ...
  
      ...
  
      (* expression
       *   ::= primary binoprhs *)
      and parse_expr = parser
        | [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream
  
  With these two simple changes, we are now able to parse unary operators
  and build the AST for them. Next up, we need to add parser support for
  prototypes, to parse the unary operator prototype. We extend the binary
  operator code above with:
  
  .. code-block:: ocaml
  
      (* prototype
       *   ::= id '(' id* ')'
       *   ::= binary LETTER number? (id, id)
       *   ::= unary LETTER number? (id) *)
      let parse_prototype =
        let rec parse_args accumulator = parser
          | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
          | [< >] -> accumulator
        in
        let parse_operator = parser
          | [< 'Token.Unary >] -> "unary", 1
          | [< 'Token.Binary >] -> "binary", 2
        in
        let parse_binary_precedence = parser
          | [< 'Token.Number n >] -> int_of_float n
          | [< >] -> 30
        in
        parser
        | [< 'Token.Ident id;
             'Token.Kwd '(' ?? "expected '(' in prototype";
             args=parse_args [];
             'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
            (* success. *)
            Ast.Prototype (id, Array.of_list (List.rev args))
        | [< (prefix, kind)=parse_operator;
             'Token.Kwd op ?? "expected an operator";
             (* Read the precedence if present. *)
             binary_precedence=parse_binary_precedence;
             'Token.Kwd '(' ?? "expected '(' in prototype";
              args=parse_args [];
             'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
            let name = prefix ^ (String.make 1 op) in
            let args = Array.of_list (List.rev args) in
  
            (* Verify right number of arguments for operator. *)
            if Array.length args != kind
            then raise (Stream.Error "invalid number of operands for operator")
            else
              if kind == 1 then
                Ast.Prototype (name, args)
              else
                Ast.BinOpPrototype (name, args, binary_precedence)
        | [< >] ->
            raise (Stream.Error "expected function name in prototype")
  
  As with binary operators, we name unary operators with a name that
  includes the operator character. This assists us at code generation
  time. Speaking of, the final piece we need to add is codegen support for
  unary operators. It looks like this:
  
  .. code-block:: ocaml
  
      let rec codegen_expr = function
        ...
        | Ast.Unary (op, operand) ->
            let operand = codegen_expr operand in
            let callee = "unary" ^ (String.make 1 op) in
            let callee =
              match lookup_function callee the_module with
              | Some callee -> callee
              | None -> raise (Error "unknown unary operator")
            in
            build_call callee [|operand|] "unop" builder
  
  This code is similar to, but simpler than, the code for binary
  operators. It is simpler primarily because it doesn't need to handle any
  predefined operators.
  
  Kicking the Tires
  =================
  
  It is somewhat hard to believe, but with a few simple extensions we've
  covered in the last chapters, we have grown a real-ish language. With
  this, we can do a lot of interesting things, including I/O, math, and a
  bunch of other things. For example, we can now add a nice sequencing
  operator (printd is defined to print out the specified value and a
  newline):
  
  ::
  
      ready> extern printd(x);
      Read extern: declare double @printd(double)
      ready> def binary : 1 (x y) 0;  # Low-precedence operator that ignores operands.
      ..
      ready> printd(123) : printd(456) : printd(789);
      123.000000
      456.000000
      789.000000
      Evaluated to 0.000000
  
  We can also define a bunch of other "primitive" operations, such as:
  
  ::
  
      # Logical unary not.
      def unary!(v)
        if v then
          0
        else
          1;
  
      # Unary negate.
      def unary-(v)
        0-v;
  
      # Define > with the same precedence as <.
      def binary> 10 (LHS RHS)
        RHS < LHS;
  
      # Binary logical or, which does not short circuit.
      def binary| 5 (LHS RHS)
        if LHS then
          1
        else if RHS then
          1
        else
          0;
  
      # Binary logical and, which does not short circuit.
      def binary& 6 (LHS RHS)
        if !LHS then
          0
        else
          !!RHS;
  
      # Define = with slightly lower precedence than relationals.
      def binary = 9 (LHS RHS)
        !(LHS < RHS | LHS > RHS);
  
  Given the previous if/then/else support, we can also define interesting
  functions for I/O. For example, the following prints out a character
  whose "density" reflects the value passed in: the lower the value, the
  denser the character:
  
  ::
  
      ready>
  
      extern putchard(char)
      def printdensity(d)
        if d > 8 then
          putchard(32)  # ' '
        else if d > 4 then
          putchard(46)  # '.'
        else if d > 2 then
          putchard(43)  # '+'
        else
          putchard(42); # '*'
      ...
      ready> printdensity(1): printdensity(2): printdensity(3) :
                printdensity(4): printdensity(5): printdensity(9): putchard(10);
      *++..
      Evaluated to 0.000000
  
  Based on these simple primitive operations, we can start to define more
  interesting things. For example, here's a little function that solves
  for the number of iterations it takes a function in the complex plane to
  converge:
  
  ::
  
      # determine whether the specific location diverges.
      # Solve for z = z^2 + c in the complex plane.
      def mandelconverger(real imag iters creal cimag)
        if iters > 255 | (real*real + imag*imag > 4) then
          iters
        else
          mandelconverger(real*real - imag*imag + creal,
                          2*real*imag + cimag,
                          iters+1, creal, cimag);
  
      # return the number of iterations required for the iteration to escape
      def mandelconverge(real imag)
        mandelconverger(real, imag, 0, real, imag);
  
  This "z = z\ :sup:`2`\  + c" function is a beautiful little creature
  that is the basis for computation of the `Mandelbrot
  Set <http://en.wikipedia.org/wiki/Mandelbrot_set>`_. Our
  ``mandelconverge`` function returns the number of iterations that it
  takes for a complex orbit to escape, saturating to 255. This is not a
  very useful function by itself, but if you plot its value over a
  two-dimensional plane, you can see the Mandelbrot set. Given that we are
  limited to using putchard here, our amazing graphical output is limited,
  but we can whip together something using the density plotter above:
  
  ::
  
      # compute and plot the mandelbrot set with the specified 2 dimensional range
      # info.
      def mandelhelp(xmin xmax xstep   ymin ymax ystep)
        for y = ymin, y < ymax, ystep in (
          (for x = xmin, x < xmax, xstep in
             printdensity(mandelconverge(x,y)))
          : putchard(10)
        )
  
      # mandel - This is a convenient helper function for plotting the mandelbrot set
      # from the specified position with the specified Magnification.
      def mandel(realstart imagstart realmag imagmag)
        mandelhelp(realstart, realstart+realmag*78, realmag,
                   imagstart, imagstart+imagmag*40, imagmag);
  
  Given this, we can try plotting out the mandelbrot set! Lets try it out:
  
  ::
  
      ready> mandel(-2.3, -1.3, 0.05, 0.07);
      *******************************+++++++++++*************************************
      *************************+++++++++++++++++++++++*******************************
      **********************+++++++++++++++++++++++++++++****************************
      *******************+++++++++++++++++++++.. ...++++++++*************************
      *****************++++++++++++++++++++++.... ...+++++++++***********************
      ***************+++++++++++++++++++++++.....   ...+++++++++*********************
      **************+++++++++++++++++++++++....     ....+++++++++********************
      *************++++++++++++++++++++++......      .....++++++++*******************
      ************+++++++++++++++++++++.......       .......+++++++******************
      ***********+++++++++++++++++++....                ... .+++++++*****************
      **********+++++++++++++++++.......                     .+++++++****************
      *********++++++++++++++...........                    ...+++++++***************
      ********++++++++++++............                      ...++++++++**************
      ********++++++++++... ..........                        .++++++++**************
      *******+++++++++.....                                   .+++++++++*************
      *******++++++++......                                  ..+++++++++*************
      *******++++++.......                                   ..+++++++++*************
      *******+++++......                                     ..+++++++++*************
      *******.... ....                                      ...+++++++++*************
      *******.... .                                         ...+++++++++*************
      *******+++++......                                    ...+++++++++*************
      *******++++++.......                                   ..+++++++++*************
      *******++++++++......                                   .+++++++++*************
      *******+++++++++.....                                  ..+++++++++*************
      ********++++++++++... ..........                        .++++++++**************
      ********++++++++++++............                      ...++++++++**************
      *********++++++++++++++..........                     ...+++++++***************
      **********++++++++++++++++........                     .+++++++****************
      **********++++++++++++++++++++....                ... ..+++++++****************
      ***********++++++++++++++++++++++.......       .......++++++++*****************
      ************+++++++++++++++++++++++......      ......++++++++******************
      **************+++++++++++++++++++++++....      ....++++++++********************
      ***************+++++++++++++++++++++++.....   ...+++++++++*********************
      *****************++++++++++++++++++++++....  ...++++++++***********************
      *******************+++++++++++++++++++++......++++++++*************************
      *********************++++++++++++++++++++++.++++++++***************************
      *************************+++++++++++++++++++++++*******************************
      ******************************+++++++++++++************************************
      *******************************************************************************
      *******************************************************************************
      *******************************************************************************
      Evaluated to 0.000000
      ready> mandel(-2, -1, 0.02, 0.04);
      **************************+++++++++++++++++++++++++++++++++++++++++++++++++++++
      ***********************++++++++++++++++++++++++++++++++++++++++++++++++++++++++
      *********************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++.
      *******************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++...
      *****************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++.....
      ***************++++++++++++++++++++++++++++++++++++++++++++++++++++++++........
      **************++++++++++++++++++++++++++++++++++++++++++++++++++++++...........
      ************+++++++++++++++++++++++++++++++++++++++++++++++++++++..............
      ***********++++++++++++++++++++++++++++++++++++++++++++++++++........        .
      **********++++++++++++++++++++++++++++++++++++++++++++++.............
      ********+++++++++++++++++++++++++++++++++++++++++++..................
      *******+++++++++++++++++++++++++++++++++++++++.......................
      ******+++++++++++++++++++++++++++++++++++...........................
      *****++++++++++++++++++++++++++++++++............................
      *****++++++++++++++++++++++++++++...............................
      ****++++++++++++++++++++++++++......   .........................
      ***++++++++++++++++++++++++.........     ......    ...........
      ***++++++++++++++++++++++............
      **+++++++++++++++++++++..............
      **+++++++++++++++++++................
      *++++++++++++++++++.................
      *++++++++++++++++............ ...
      *++++++++++++++..............
      *+++....++++................
      *..........  ...........
      *
      *..........  ...........
      *+++....++++................
      *++++++++++++++..............
      *++++++++++++++++............ ...
      *++++++++++++++++++.................
      **+++++++++++++++++++................
      **+++++++++++++++++++++..............
      ***++++++++++++++++++++++............
      ***++++++++++++++++++++++++.........     ......    ...........
      ****++++++++++++++++++++++++++......   .........................
      *****++++++++++++++++++++++++++++...............................
      *****++++++++++++++++++++++++++++++++............................
      ******+++++++++++++++++++++++++++++++++++...........................
      *******+++++++++++++++++++++++++++++++++++++++.......................
      ********+++++++++++++++++++++++++++++++++++++++++++..................
      Evaluated to 0.000000
      ready> mandel(-0.9, -1.4, 0.02, 0.03);
      *******************************************************************************
      *******************************************************************************
      *******************************************************************************
      **********+++++++++++++++++++++************************************************
      *+++++++++++++++++++++++++++++++++++++++***************************************
      +++++++++++++++++++++++++++++++++++++++++++++**********************************
      ++++++++++++++++++++++++++++++++++++++++++++++++++*****************************
      ++++++++++++++++++++++++++++++++++++++++++++++++++++++*************************
      +++++++++++++++++++++++++++++++++++++++++++++++++++++++++**********************
      +++++++++++++++++++++++++++++++++.........++++++++++++++++++*******************
      +++++++++++++++++++++++++++++++....   ......+++++++++++++++++++****************
      +++++++++++++++++++++++++++++.......  ........+++++++++++++++++++**************
      ++++++++++++++++++++++++++++........   ........++++++++++++++++++++************
      +++++++++++++++++++++++++++.........     ..  ...+++++++++++++++++++++**********
      ++++++++++++++++++++++++++...........        ....++++++++++++++++++++++********
      ++++++++++++++++++++++++.............       .......++++++++++++++++++++++******
      +++++++++++++++++++++++.............        ........+++++++++++++++++++++++****
      ++++++++++++++++++++++...........           ..........++++++++++++++++++++++***
      ++++++++++++++++++++...........                .........++++++++++++++++++++++*
      ++++++++++++++++++............                  ...........++++++++++++++++++++
      ++++++++++++++++...............                 .............++++++++++++++++++
      ++++++++++++++.................                 ...............++++++++++++++++
      ++++++++++++..................                  .................++++++++++++++
      +++++++++..................                      .................+++++++++++++
      ++++++........        .                               .........  ..++++++++++++
      ++............                                         ......    ....++++++++++
      ..............                                                    ...++++++++++
      ..............                                                    ....+++++++++
      ..............                                                    .....++++++++
      .............                                                    ......++++++++
      ...........                                                     .......++++++++
      .........                                                       ........+++++++
      .........                                                       ........+++++++
      .........                                                           ....+++++++
      ........                                                             ...+++++++
      .......                                                              ...+++++++
                                                                          ....+++++++
                                                                         .....+++++++
                                                                          ....+++++++
                                                                          ....+++++++
                                                                          ....+++++++
      Evaluated to 0.000000
      ready> ^D
  
  At this point, you may be starting to realize that Kaleidoscope is a
  real and powerful language. It may not be self-similar :), but it can be
  used to plot things that are!
  
  With this, we conclude the "adding user-defined operators" chapter of
  the tutorial. We have successfully augmented our language, adding the
  ability to extend the language in the library, and we have shown how
  this can be used to build a simple but interesting end-user application
  in Kaleidoscope. At this point, Kaleidoscope can build a variety of
  applications that are functional and can call functions with
  side-effects, but it can't actually define and mutate a variable itself.
  
  Strikingly, variable mutation is an important feature of some languages,
  and it is not at all obvious how to `add support for mutable
  variables <OCamlLangImpl7.html>`_ without having to add an "SSA
  construction" phase to your front-end. In the next chapter, we will
  describe how you can add variable mutation without building SSA in your
  front-end.
  
  Full Code Listing
  =================
  
  Here is the complete code listing for our running example, enhanced with
  the if/then/else and for expressions.. To build this example, use:
  
  .. code-block:: bash
  
      # Compile
      ocamlbuild toy.byte
      # Run
      ./toy.byte
  
  Here is the code:
  
  \_tags:
      ::
  
          <{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
          <*.{byte,native}>: g++, use_llvm, use_llvm_analysis
          <*.{byte,native}>: use_llvm_executionengine, use_llvm_target
          <*.{byte,native}>: use_llvm_scalar_opts, use_bindings
  
  myocamlbuild.ml:
      .. code-block:: ocaml
  
          open Ocamlbuild_plugin;;
  
          ocaml_lib ~extern:true "llvm";;
          ocaml_lib ~extern:true "llvm_analysis";;
          ocaml_lib ~extern:true "llvm_executionengine";;
          ocaml_lib ~extern:true "llvm_target";;
          ocaml_lib ~extern:true "llvm_scalar_opts";;
  
          flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
          dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
  
  token.ml:
      .. code-block:: ocaml
  
          (*===----------------------------------------------------------------------===
           * Lexer Tokens
           *===----------------------------------------------------------------------===*)
  
          (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
           * these others for known things. *)
          type token =
            (* commands *)
            | Def | Extern
  
            (* primary *)
            | Ident of string | Number of float
  
            (* unknown *)
            | Kwd of char
  
            (* control *)
            | If | Then | Else
            | For | In
  
            (* operators *)
            | Binary | Unary
  
  lexer.ml:
      .. code-block:: ocaml
  
          (*===----------------------------------------------------------------------===
           * Lexer
           *===----------------------------------------------------------------------===*)
  
          let rec lex = parser
            (* Skip any whitespace. *)
            | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
  
            (* identifier: [a-zA-Z][a-zA-Z0-9] *)
            | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
                let buffer = Buffer.create 1 in
                Buffer.add_char buffer c;
                lex_ident buffer stream
  
            (* number: [0-9.]+ *)
            | [< ' ('0' .. '9' as c); stream >] ->
                let buffer = Buffer.create 1 in
                Buffer.add_char buffer c;
                lex_number buffer stream
  
            (* Comment until end of line. *)
            | [< ' ('#'); stream >] ->
                lex_comment stream
  
            (* Otherwise, just return the character as its ascii value. *)
            | [< 'c; stream >] ->
                [< 'Token.Kwd c; lex stream >]
  
            (* end of stream. *)
            | [< >] -> [< >]
  
          and lex_number buffer = parser
            | [< ' ('0' .. '9' | '.' as c); stream >] ->
                Buffer.add_char buffer c;
                lex_number buffer stream
            | [< stream=lex >] ->
                [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
  
          and lex_ident buffer = parser
            | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
                Buffer.add_char buffer c;
                lex_ident buffer stream
            | [< stream=lex >] ->
                match Buffer.contents buffer with
                | "def" -> [< 'Token.Def; stream >]
                | "extern" -> [< 'Token.Extern; stream >]
                | "if" -> [< 'Token.If; stream >]
                | "then" -> [< 'Token.Then; stream >]
                | "else" -> [< 'Token.Else; stream >]
                | "for" -> [< 'Token.For; stream >]
                | "in" -> [< 'Token.In; stream >]
                | "binary" -> [< 'Token.Binary; stream >]
                | "unary" -> [< 'Token.Unary; stream >]
                | id -> [< 'Token.Ident id; stream >]
  
          and lex_comment = parser
            | [< ' ('\n'); stream=lex >] -> stream
            | [< 'c; e=lex_comment >] -> e
            | [< >] -> [< >]
  
  ast.ml:
      .. code-block:: ocaml
  
          (*===----------------------------------------------------------------------===
           * Abstract Syntax Tree (aka Parse Tree)
           *===----------------------------------------------------------------------===*)
  
          (* expr - Base type for all expression nodes. *)
          type expr =
            (* variant for numeric literals like "1.0". *)
            | Number of float
  
            (* variant for referencing a variable, like "a". *)
            | Variable of string
  
            (* variant for a unary operator. *)
            | Unary of char * expr
  
            (* variant for a binary operator. *)
            | Binary of char * expr * expr
  
            (* variant for function calls. *)
            | Call of string * expr array
  
            (* variant for if/then/else. *)
            | If of expr * expr * expr
  
            (* variant for for/in. *)
            | For of string * expr * expr * expr option * expr
  
          (* proto - This type represents the "prototype" for a function, which captures
           * its name, and its argument names (thus implicitly the number of arguments the
           * function takes). *)
          type proto =
            | Prototype of string * string array
            | BinOpPrototype of string * string array * int
  
          (* func - This type represents a function definition itself. *)
          type func = Function of proto * expr
  
  parser.ml:
      .. code-block:: ocaml
  
          (*===---------------------------------------------------------------------===
           * Parser
           *===---------------------------------------------------------------------===*)
  
          (* binop_precedence - This holds the precedence for each binary operator that is
           * defined *)
          let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
  
          (* precedence - Get the precedence of the pending binary operator token. *)
          let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
  
          (* primary
           *   ::= identifier
           *   ::= numberexpr
           *   ::= parenexpr
           *   ::= ifexpr
           *   ::= forexpr *)
          let rec parse_primary = parser
            (* numberexpr ::= number *)
            | [< 'Token.Number n >] -> Ast.Number n
  
            (* parenexpr ::= '(' expression ')' *)
            | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
  
            (* identifierexpr
             *   ::= identifier
             *   ::= identifier '(' argumentexpr ')' *)
            | [< 'Token.Ident id; stream >] ->
                let rec parse_args accumulator = parser
                  | [< e=parse_expr; stream >] ->
                      begin parser
                        | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
                        | [< >] -> e :: accumulator
                      end stream
                  | [< >] -> accumulator
                in
                let rec parse_ident id = parser
                  (* Call. *)
                  | [< 'Token.Kwd '(';
                       args=parse_args [];
                       'Token.Kwd ')' ?? "expected ')'">] ->
                      Ast.Call (id, Array.of_list (List.rev args))
  
                  (* Simple variable ref. *)
                  | [< >] -> Ast.Variable id
                in
                parse_ident id stream
  
            (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
            | [< 'Token.If; c=parse_expr;
                 'Token.Then ?? "expected 'then'"; t=parse_expr;
                 'Token.Else ?? "expected 'else'"; e=parse_expr >] ->
                Ast.If (c, t, e)
  
            (* forexpr
                  ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
            | [< 'Token.For;
                 'Token.Ident id ?? "expected identifier after for";
                 'Token.Kwd '=' ?? "expected '=' after for";
                 stream >] ->
                begin parser
                  | [<
                       start=parse_expr;
                       'Token.Kwd ',' ?? "expected ',' after for";
                       end_=parse_expr;
                       stream >] ->
                      let step =
                        begin parser
                        | [< 'Token.Kwd ','; step=parse_expr >] -> Some step
                        | [< >] -> None
                        end stream
                      in
                      begin parser
                      | [< 'Token.In; body=parse_expr >] ->
                          Ast.For (id, start, end_, step, body)
                      | [< >] ->
                          raise (Stream.Error "expected 'in' after for")
                      end stream
                  | [< >] ->
                      raise (Stream.Error "expected '=' after for")
                end stream
  
            | [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
  
          (* unary
           *   ::= primary
           *   ::= '!' unary *)
          and parse_unary = parser
            (* If this is a unary operator, read it. *)
            | [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] ->
                Ast.Unary (op, operand)
  
            (* If the current token is not an operator, it must be a primary expr. *)
            | [< stream >] -> parse_primary stream
  
          (* binoprhs
           *   ::= ('+' primary)* *)
          and parse_bin_rhs expr_prec lhs stream =
            match Stream.peek stream with
            (* If this is a binop, find its precedence. *)
            | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
                let token_prec = precedence c in
  
                (* If this is a binop that binds at least as tightly as the current binop,
                 * consume it, otherwise we are done. *)
                if token_prec < expr_prec then lhs else begin
                  (* Eat the binop. *)
                  Stream.junk stream;
  
                  (* Parse the unary expression after the binary operator. *)
                  let rhs = parse_unary stream in
  
                  (* Okay, we know this is a binop. *)
                  let rhs =
                    match Stream.peek stream with
                    | Some (Token.Kwd c2) ->
                        (* If BinOp binds less tightly with rhs than the operator after
                         * rhs, let the pending operator take rhs as its lhs. *)
                        let next_prec = precedence c2 in
                        if token_prec < next_prec
                        then parse_bin_rhs (token_prec + 1) rhs stream
                        else rhs
                    | _ -> rhs
                  in
  
                  (* Merge lhs/rhs. *)
                  let lhs = Ast.Binary (c, lhs, rhs) in
                  parse_bin_rhs expr_prec lhs stream
                end
            | _ -> lhs
  
          (* expression
           *   ::= primary binoprhs *)
          and parse_expr = parser
            | [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream
  
          (* prototype
           *   ::= id '(' id* ')'
           *   ::= binary LETTER number? (id, id)
           *   ::= unary LETTER number? (id) *)
          let parse_prototype =
            let rec parse_args accumulator = parser
              | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
              | [< >] -> accumulator
            in
            let parse_operator = parser
              | [< 'Token.Unary >] -> "unary", 1
              | [< 'Token.Binary >] -> "binary", 2
            in
            let parse_binary_precedence = parser
              | [< 'Token.Number n >] -> int_of_float n
              | [< >] -> 30
            in
            parser
            | [< 'Token.Ident id;
                 'Token.Kwd '(' ?? "expected '(' in prototype";
                 args=parse_args [];
                 'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
                (* success. *)
                Ast.Prototype (id, Array.of_list (List.rev args))
            | [< (prefix, kind)=parse_operator;
                 'Token.Kwd op ?? "expected an operator";
                 (* Read the precedence if present. *)
                 binary_precedence=parse_binary_precedence;
                 'Token.Kwd '(' ?? "expected '(' in prototype";
                  args=parse_args [];
                 'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
                let name = prefix ^ (String.make 1 op) in
                let args = Array.of_list (List.rev args) in
  
                (* Verify right number of arguments for operator. *)
                if Array.length args != kind
                then raise (Stream.Error "invalid number of operands for operator")
                else
                  if kind == 1 then
                    Ast.Prototype (name, args)
                  else
                    Ast.BinOpPrototype (name, args, binary_precedence)
            | [< >] ->
                raise (Stream.Error "expected function name in prototype")
  
          (* definition ::= 'def' prototype expression *)
          let parse_definition = parser
            | [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
                Ast.Function (p, e)
  
          (* toplevelexpr ::= expression *)
          let parse_toplevel = parser
            | [< e=parse_expr >] ->
                (* Make an anonymous proto. *)
                Ast.Function (Ast.Prototype ("", [||]), e)
  
          (*  external ::= 'extern' prototype *)
          let parse_extern = parser
            | [< 'Token.Extern; e=parse_prototype >] -> e
  
  codegen.ml:
      .. code-block:: ocaml
  
          (*===----------------------------------------------------------------------===
           * Code Generation
           *===----------------------------------------------------------------------===*)
  
          open Llvm
  
          exception Error of string
  
          let context = global_context ()
          let the_module = create_module context "my cool jit"
          let builder = builder context
          let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
          let double_type = double_type context
  
          let rec codegen_expr = function
            | Ast.Number n -> const_float double_type n
            | Ast.Variable name ->
                (try Hashtbl.find named_values name with
                  | Not_found -> raise (Error "unknown variable name"))
            | Ast.Unary (op, operand) ->
                let operand = codegen_expr operand in
                let callee = "unary" ^ (String.make 1 op) in
                let callee =
                  match lookup_function callee the_module with
                  | Some callee -> callee
                  | None -> raise (Error "unknown unary operator")
                in
                build_call callee [|operand|] "unop" builder
            | Ast.Binary (op, lhs, rhs) ->
                let lhs_val = codegen_expr lhs in
                let rhs_val = codegen_expr rhs in
                begin
                  match op with
                  | '+' -> build_add lhs_val rhs_val "addtmp" builder
                  | '-' -> build_sub lhs_val rhs_val "subtmp" builder
                  | '*' -> build_mul lhs_val rhs_val "multmp" builder
                  | '<' ->
                      (* Convert bool 0/1 to double 0.0 or 1.0 *)
                      let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
                      build_uitofp i double_type "booltmp" builder
                  | _ ->
                      (* If it wasn't a builtin binary operator, it must be a user defined
                       * one. Emit a call to it. *)
                      let callee = "binary" ^ (String.make 1 op) in
                      let callee =
                        match lookup_function callee the_module with
                        | Some callee -> callee
                        | None -> raise (Error "binary operator not found!")
                      in
                      build_call callee [|lhs_val; rhs_val|] "binop" builder
                end
            | Ast.Call (callee, args) ->
                (* Look up the name in the module table. *)
                let callee =
                  match lookup_function callee the_module with
                  | Some callee -> callee
                  | None -> raise (Error "unknown function referenced")
                in
                let params = params callee in
  
                (* If argument mismatch error. *)
                if Array.length params == Array.length args then () else
                  raise (Error "incorrect # arguments passed");
                let args = Array.map codegen_expr args in
                build_call callee args "calltmp" builder
            | Ast.If (cond, then_, else_) ->
                let cond = codegen_expr cond in
  
                (* Convert condition to a bool by comparing equal to 0.0 *)
                let zero = const_float double_type 0.0 in
                let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
  
                (* Grab the first block so that we might later add the conditional branch
                 * to it at the end of the function. *)
                let start_bb = insertion_block builder in
                let the_function = block_parent start_bb in
  
                let then_bb = append_block context "then" the_function in
  
                (* Emit 'then' value. *)
                position_at_end then_bb builder;
                let then_val = codegen_expr then_ in
  
                (* Codegen of 'then' can change the current block, update then_bb for the
                 * phi. We create a new name because one is used for the phi node, and the
                 * other is used for the conditional branch. *)
                let new_then_bb = insertion_block builder in
  
                (* Emit 'else' value. *)
                let else_bb = append_block context "else" the_function in
                position_at_end else_bb builder;
                let else_val = codegen_expr else_ in
  
                (* Codegen of 'else' can change the current block, update else_bb for the
                 * phi. *)
                let new_else_bb = insertion_block builder in
  
                (* Emit merge block. *)
                let merge_bb = append_block context "ifcont" the_function in
                position_at_end merge_bb builder;
                let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
                let phi = build_phi incoming "iftmp" builder in
  
                (* Return to the start block to add the conditional branch. *)
                position_at_end start_bb builder;
                ignore (build_cond_br cond_val then_bb else_bb builder);
  
                (* Set a unconditional branch at the end of the 'then' block and the
                 * 'else' block to the 'merge' block. *)
                position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
                position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
  
                (* Finally, set the builder to the end of the merge block. *)
                position_at_end merge_bb builder;
  
                phi
            | Ast.For (var_name, start, end_, step, body) ->
                (* Emit the start code first, without 'variable' in scope. *)
                let start_val = codegen_expr start in
  
                (* Make the new basic block for the loop header, inserting after current
                 * block. *)
                let preheader_bb = insertion_block builder in
                let the_function = block_parent preheader_bb in
                let loop_bb = append_block context "loop" the_function in
  
                (* Insert an explicit fall through from the current block to the
                 * loop_bb. *)
                ignore (build_br loop_bb builder);
  
                (* Start insertion in loop_bb. *)
                position_at_end loop_bb builder;
  
                (* Start the PHI node with an entry for start. *)
                let variable = build_phi [(start_val, preheader_bb)] var_name builder in
  
                (* Within the loop, the variable is defined equal to the PHI node. If it
                 * shadows an existing variable, we have to restore it, so save it
                 * now. *)
                let old_val =
                  try Some (Hashtbl.find named_values var_name) with Not_found -> None
                in
                Hashtbl.add named_values var_name variable;
  
                (* Emit the body of the loop.  This, like any other expr, can change the
                 * current BB.  Note that we ignore the value computed by the body, but
                 * don't allow an error *)
                ignore (codegen_expr body);
  
                (* Emit the step value. *)
                let step_val =
                  match step with
                  | Some step -> codegen_expr step
                  (* If not specified, use 1.0. *)
                  | None -> const_float double_type 1.0
                in
  
                let next_var = build_add variable step_val "nextvar" builder in
  
                (* Compute the end condition. *)
                let end_cond = codegen_expr end_ in
  
                (* Convert condition to a bool by comparing equal to 0.0. *)
                let zero = const_float double_type 0.0 in
                let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
  
                (* Create the "after loop" block and insert it. *)
                let loop_end_bb = insertion_block builder in
                let after_bb = append_block context "afterloop" the_function in
  
                (* Insert the conditional branch into the end of loop_end_bb. *)
                ignore (build_cond_br end_cond loop_bb after_bb builder);
  
                (* Any new code will be inserted in after_bb. *)
                position_at_end after_bb builder;
  
                (* Add a new entry to the PHI node for the backedge. *)
                add_incoming (next_var, loop_end_bb) variable;
  
                (* Restore the unshadowed variable. *)
                begin match old_val with
                | Some old_val -> Hashtbl.add named_values var_name old_val
                | None -> ()
                end;
  
                (* for expr always returns 0.0. *)
                const_null double_type
  
          let codegen_proto = function
            | Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) ->
                (* Make the function type: double(double,double) etc. *)
                let doubles = Array.make (Array.length args) double_type in
                let ft = function_type double_type doubles in
                let f =
                  match lookup_function name the_module with
                  | None -> declare_function name ft the_module
  
                  (* If 'f' conflicted, there was already something named 'name'. If it
                   * has a body, don't allow redefinition or reextern. *)
                  | Some f ->
                      (* If 'f' already has a body, reject this. *)
                      if block_begin f <> At_end f then
                        raise (Error "redefinition of function");
  
                      (* If 'f' took a different number of arguments, reject. *)
                      if element_type (type_of f) <> ft then
                        raise (Error "redefinition of function with different # args");
                      f
                in
  
                (* Set names for all arguments. *)
                Array.iteri (fun i a ->
                  let n = args.(i) in
                  set_value_name n a;
                  Hashtbl.add named_values n a;
                ) (params f);
                f
  
          let codegen_func the_fpm = function
            | Ast.Function (proto, body) ->
                Hashtbl.clear named_values;
                let the_function = codegen_proto proto in
  
                (* If this is an operator, install it. *)
                begin match proto with
                | Ast.BinOpPrototype (name, args, prec) ->
                    let op = name.[String.length name - 1] in
                    Hashtbl.add Parser.binop_precedence op prec;
                | _ -> ()
                end;
  
                (* Create a new basic block to start insertion into. *)
                let bb = append_block context "entry" the_function in
                position_at_end bb builder;
  
                try
                  let ret_val = codegen_expr body in
  
                  (* Finish off the function. *)
                  let _ = build_ret ret_val builder in
  
                  (* Validate the generated code, checking for consistency. *)
                  Llvm_analysis.assert_valid_function the_function;
  
                  (* Optimize the function. *)
                  let _ = PassManager.run_function the_function the_fpm in
  
                  the_function
                with e ->
                  delete_function the_function;
                  raise e
  
  toplevel.ml:
      .. code-block:: ocaml
  
          (*===----------------------------------------------------------------------===
           * Top-Level parsing and JIT Driver
           *===----------------------------------------------------------------------===*)
  
          open Llvm
          open Llvm_executionengine
  
          (* top ::= definition | external | expression | ';' *)
          let rec main_loop the_fpm the_execution_engine stream =
            match Stream.peek stream with
            | None -> ()
  
            (* ignore top-level semicolons. *)
            | Some (Token.Kwd ';') ->
                Stream.junk stream;
                main_loop the_fpm the_execution_engine stream
  
            | Some token ->
                begin
                  try match token with
                  | Token.Def ->
                      let e = Parser.parse_definition stream in
                      print_endline "parsed a function definition.";
                      dump_value (Codegen.codegen_func the_fpm e);
                  | Token.Extern ->
                      let e = Parser.parse_extern stream in
                      print_endline "parsed an extern.";
                      dump_value (Codegen.codegen_proto e);
                  | _ ->
                      (* Evaluate a top-level expression into an anonymous function. *)
                      let e = Parser.parse_toplevel stream in
                      print_endline "parsed a top-level expr";
                      let the_function = Codegen.codegen_func the_fpm e in
                      dump_value the_function;
  
                      (* JIT the function, returning a function pointer. *)
                      let result = ExecutionEngine.run_function the_function [||]
                        the_execution_engine in
  
                      print_string "Evaluated to ";
                      print_float (GenericValue.as_float Codegen.double_type result);
                      print_newline ();
                  with Stream.Error s | Codegen.Error s ->
                    (* Skip token for error recovery. *)
                    Stream.junk stream;
                    print_endline s;
                end;
                print_string "ready> "; flush stdout;
                main_loop the_fpm the_execution_engine stream
  
  toy.ml:
      .. code-block:: ocaml
  
          (*===----------------------------------------------------------------------===
           * Main driver code.
           *===----------------------------------------------------------------------===*)
  
          open Llvm
          open Llvm_executionengine
          open Llvm_target
          open Llvm_scalar_opts
  
          let main () =
            ignore (initialize_native_target ());
  
            (* Install standard binary operators.
             * 1 is the lowest precedence. *)
            Hashtbl.add Parser.binop_precedence '<' 10;
            Hashtbl.add Parser.binop_precedence '+' 20;
            Hashtbl.add Parser.binop_precedence '-' 20;
            Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
  
            (* Prime the first token. *)
            print_string "ready> "; flush stdout;
            let stream = Lexer.lex (Stream.of_channel stdin) in
  
            (* Create the JIT. *)
            let the_execution_engine = ExecutionEngine.create Codegen.the_module in
            let the_fpm = PassManager.create_function Codegen.the_module in
  
            (* Set up the optimizer pipeline.  Start with registering info about how the
             * target lays out data structures. *)
            DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
  
            (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
            add_instruction_combination the_fpm;
  
            (* reassociate expressions. *)
            add_reassociation the_fpm;
  
            (* Eliminate Common SubExpressions. *)
            add_gvn the_fpm;
  
            (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
            add_cfg_simplification the_fpm;
  
            ignore (PassManager.initialize the_fpm);
  
            (* Run the main "interpreter loop" now. *)
            Toplevel.main_loop the_fpm the_execution_engine stream;
  
            (* Print out all the generated code. *)
            dump_module Codegen.the_module
          ;;
  
          main ()
  
  bindings.c
      .. code-block:: c
  
          #include <stdio.h>
  
          /* putchard - putchar that takes a double and returns 0. */
          extern double putchard(double X) {
            putchar((char)X);
            return 0;
          }
  
          /* printd - printf that takes a double prints it as "%f\n", returning 0. */
          extern double printd(double X) {
            printf("%f\n", X);
            return 0;
          }
  
  `Next: Extending the language: mutable variables / SSA
  construction <OCamlLangImpl7.html>`_
  
  