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(* COMPUTATIONS *)
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(* This is a computation language intended to be interpreted into the * state monad. It has `get` and `put` operations, a way to combine * computations via `bind`, and a way to conclude with a literal value * via `return`. A neat thing about monads is that all of our * intuitions about `+` and expression languages from above can be * carried over. `bind` is actually associative (like `+`), `return` * is a unit for `bind` (just like `0` was for `+`). Most * importantly, we can compile this tree-structured thing into an * equivalent list-structured thing, and the ability to do so * crucially relies on associativity and unit laws. *)
Inductive computation_tree : Set :=
| Bind_CT : computation_tree -> (nat -> computation_tree) -> computation_tree
| Return_CT : nat -> computation_tree
| Get_CT : computation_tree
| Put_CT : nat -> computation_tree.
(* This is an interpretation of computation trees in terms of * functions that manipulate a single cell of state *)
(*                                                                output state              *)
(*                                                                       ↓↓↓                *)
Fixpoint interp_computation_tree (e : computation_tree) (state : nat) : (nat * nat) :=
(*                                                                             ↑↑↑          *)
(*                                                                             return value *)
match e with
| Bind_CT e' f =>
match interp_computation_tree e' state with
| (state', n) => interp_computation_tree (f n) state'
end
| Return_CT n => (state, n)
| Get_CT => (state, state)
| Put_CT n => (n, n)
end.
(* This is our "first try" at list-ifying the computation tree. This * is just like a list that has two different types of "cons cells": * one for `get` and one for `put`. We can fold these together into a * single "cons cell" using a helper definition, so let's do that. *)
Inductive computation_list_first_try : Set :=
| Get_CLFT : (nat -> computation_list_first_try) -> computation_list_first_try
| Put_CLFT : nat -> (nat -> computation_list_first_try) -> computation_list_first_try
| Done_CLFT : computation_list_first_try.
(* This is the inductive type for "commands" in our language *)
Inductive computation_op : Set :=
| Get_OP : computation_op
| Put_OP : nat -> computation_op.
(* This is the inductive type for computations in our language. It's * just a list, but where the cons cells allow for the tail of the * list to depend on a "return value". The tail of the list also holds * a final value. *)
Inductive computation_list : Set :=
| Op_CL : computation_op -> (nat -> computation_list) -> computation_list
| Done_CL : nat -> computation_list.
(* This is an interpretation of these computation lists in terms of * the same state monad semantics *)
(*                                                                       state              *)
(*                                                                       ↓↓↓                *)
Fixpoint interp_computation_list (e : computation_list) (state : nat) : (nat * nat) :=
(*                                                                             ↑↑↑          *)
(*                                                                             return value *)
match e with
| Op_CL o f =>
match o with
| Get_OP => interp_computation_list (f state) state
| Put_OP n => interp_computation_list (f n) n
end
| Done_CL n => (state, n)
end.
(* And finally, we have the compiler from computation trees to * computation lists. *)
(* Magically, we need an append operation for our computation lists * before defining our compiler for computation trees, just like we * needed an append operation on expression lists before defining our * compiler for expression trees. *)
Fixpoint append_computation_list (e : computation_list) (f : nat -> computation_list) : computation_list :=
match e with
| Op_CL o f1 => Op_CL o (fun x => append_computation_list (f1 x) f)
| Done_CL n => f n
end.
(* And now we have the compiler from computation trees to computation * lists. We should be able to prove that the denotations of * computation trees and computation lists (via compilation) are * equal, and you'll notice that doing so relies on associativity and * unit laws for the denotation space (the state monad). *)
Fixpoint compile_computation_tree (e : computation_tree) : computation_list :=
match e with
| Bind_CT e f => append_computation_list (compile_computation_tree e) (fun n => compile_computation_tree (f n))
| Return_CT n => Done_CL n
| Get_CT => Op_CL Get_OP (fun n => Done_CL n)
| Put_CT n => Op_CL (Put_OP n) (fun n' => Done_CL n')
end.

Theorem comp_tree_correct: forall e n, interp_computation_list(compile_computation_tree e) n = interp_computation_tree e n.