Section SumExpr.
(* This is the infrastructure to support the mxdirect predicate. We use a     *)
(* bespoke canonical structure to decompose a matrix expression into binary   *)
(* and n-ary products, using some of the "quote" technology. This lets us     *)
(* characterize direct sums as set sums whose rank is equal to the sum of the *)
(* ranks of the individual terms. The mxsum_expr/proper_mxsum_expr structures *)
(* below supply both the decomposition and the calculation of the rank sum.   *)
(* The mxsum_spec dependent predicate family expresses the consistency of     *)
(* these two decompositions.                                                  *)
(*   The main technical difficulty we need to overcome is the fact that       *)
(* the "catch-all" case of canonical structures has a priority lower than     *)
(* constant expansion. However, it is undesireable that local abbreviations   *)
(* be opaque for the direct-sum predicate, e.g., not be able to handle        *)
(* let S := (\sum_(i | P i) LargeExpression i)%MS in mxdirect S -> ...).      *)
(*   As in "quote", we use the interleaving of constant expansion and         *)
(* canonical projection matching to achieve our goal: we use a "wrapper" type *)
(* (indeed, the wrapped T type defined in ssrfun.v) with a self-inserting     *)
(* non-primitive constructor to gain finer control over the type and          *)
(* structure inference process. The innermost, primitive, constructor flags   *)
(* trivial sums; it is initially hidden by an eta-expansion, which has been   *)
(* made into a (default) canonical structure -- this lets type inference      *)
(* automatically insert this outer tag.                                       *)
(*   In detail, we define three types                                         *)
(*  mxsum_spec S r <-> There exists a finite list of matrices A1, ..., Ak     *)
(*                     such that S is the set sum of the Ai, and r is the sum *)
(*                     of the ranks of the Ai, i.e., S = (A1 + ... + Ak)%MS   *)
(*                     and r = \rank A1 + ... + \rank Ak. Note that           *)
(*                     mxsum_spec is a recursive dependent predicate family   *)
(*                     whose elimination rewrites simultaneaously S, r and    *)
(*                     the height of S.                                       *)
(*   proper_mxsum_expr n == The interface for proper sum expressions; this is *)
(*                     a double-entry interface, keyed on both the matrix sum *)
(*                     value and the rank sum. The matrix value is restricted *)
(*                     to square matrices, as the "+"%MS operator always      *)
(*                     returns a square matrix. This interface has two        *)
(*                     canonical insances, for binary and n-ary sums.         *)
(*   mxsum_expr m n == The interface for general sum expressions, comprising  *)
(*                     both proper sums and trivial sums consisting of a      *)
(*                     single matrix. The key values are WRAPPED as this lets *)
(*                     us give priority to the "proper sum" interpretation    *)
(*                     (see below). To allow for trivial sums, the matrix key *)
(*                     can have any dimension. The mxsum_expr interface has   *)
(*                     two canonical instances, for trivial and proper sums,  *)
(*                     keyed to the Wrap and wrap constructors, respectively. *)
(* The projections for the two interfaces above are                           *)
(*   proper_mxsum_val, mxsum_val : these are respectively coercions to 'M_n   *)
(*                     and wrapped 'M_(m, n); thus, the matrix sum for an     *)
(*                     S : mxsum_expr m n can be written unwrap S.            *)
(*   proper_mxsum_rank, mxsum_rank : projections to the nat and wrapped nat,  *)
(*                     respectively; the rank sum for S : mxsum_expr m n is   *)
(*                     thus written unwrap (mxsum_rank S).                    *)
(* The mxdirect A predicate actually gets A in a phantom argument, which is   *)
(* used to infer an (implicit) S : mxsum_expr such that unwrap S = A; the     *)
(* actual definition is \rank (unwrap S) == unwrap (mxsum_rank S).            *)
(*   Note that the inference of S is inherently ambiguous: ANY matrix can be  *)
(* viewed as a trivial sum, including one whose description is manifestly a   *)
(* proper sum. We use the wrapped type and the interaction between delta      *)
(* reduction and canonical structure inference to resolve this ambiguity in   *)
(* favor of proper sums, as follows:                                          *)
(*    - The phantom type sets up a unification problem of the form            *)
(*         unwrap (mxsum_val ?S) = A                                          *)
(*      with unknown evar ?S : mxsum_expr m n.                                *)
(*    - As the constructor wrap is also a default Canonical instance for the  *)
(*      wrapped type, so A is immediately replaced with unwrap (wrap A) and   *)
(*      we get the residual unification problem                               *)
(*         mxsum_val ?S = wrap A                                              *)
(*    - Now Coq tries to apply the proper sum Canonical instance, which has   *)
(*      key projection wrap (proper_mxsum_val ?PS) where ?PS is a fresh evar  *)
(*      (of type proper_mxsum_expr n). This can only succeed if m = n, and if *)
(*      a solution can be found to the recursive unification problem          *)
(*         proper_mxsum_val ?PS = A                                           *)
(*      This causes Coq to look for one of the two canonical constants for    *)
(*      proper_mxsum_val (addsmx or bigop) at the head of A, delta-expanding  *)
(*      A as needed, and then inferring recursively mxsum_expr structures for *)
(*      the last argument(s) of that constant.                                *)
(*    - If the above step fails then the wrap constant is expanded, revealing *)
(*      the primitive Wrap constructor; the unification problem now becomes   *)
(*         mxsum_val ?S = Wrap A                                              *)
(*      which fits perfectly the trivial sum canonical structure, whose key   *)
(*      projection is Wrap ?B where ?B is a fresh evar. Thus the inference    *)
(*      succeeds, and returns the trivial sum.                                *)
(* Note that the rank projections also register canonical values, so that the *)
(* same process can be used to infer a sum structure from the rank sum. In    *)
(* that case, however, there is no ambiguity and the inference can fail,      *)
(* because the rank sum for a trivial sum is not an arbitrary integer -- it   *)
(* must be of the form \rank ?B. It is nevertheless necessary to use the      *)
(* wrapped nat type for the rank sums, because in the non-trivial case the    *)
(* head constant of the nat expression is determined by the proper_mxsum_expr *)
(* canonical structure, so the mxsum_expr structure must use a generic        *)
(* constant, namely wrap.                                                     *)
Inductive mxsum_spec n : forall m, 'M[F]_(m, n) -> nat -> Prop :=
| TrivialMxsum m A
: @mxsum_spec n m A (\rank A)
| ProperMxsum m1 m2 T1 T2 r1 r2 of
@mxsum_spec n m1 T1 r1 & @mxsum_spec n m2 T2 r2
: mxsum_spec (T1 + T2)%MS (r1 + r2)%N.
Arguments mxsum_spec {n%N m%N} T%MS r%N.
Structure mxsum_expr m n := Mxsum {
mxsum_val :> wrapped 'M_(m, n);
mxsum_rank : wrapped nat;
_ : mxsum_spec (unwrap mxsum_val) (unwrap mxsum_rank)
}.
Canonical trivial_mxsum m n A :=
@Mxsum m n (Wrap A) (Wrap (\rank A)) (TrivialMxsum A).
Structure proper_mxsum_expr n := ProperMxsumExpr {
proper_mxsum_val :> 'M_n;
proper_mxsum_rank : nat;
_ : mxsum_spec proper_mxsum_val proper_mxsum_rank
}.
Definition proper_mxsumP n (S : proper_mxsum_expr n) :=
let: ProperMxsumExpr _ _ termS := S return mxsum_spec S (proper_mxsum_rank S)
in termS.
Canonical sum_mxsum n (S : proper_mxsum_expr n) :=
@Mxsum n n (wrap (S : 'M_n)) (wrap (proper_mxsum_rank S)) (proper_mxsumP S).
Section Binary.
Variable (m1 m2 n : nat) (S1 : mxsum_expr m1 n) (S2 : mxsum_expr m2 n).
Fact binary_mxsum_proof :
mxsum_spec (unwrap S1 + unwrap S2)
(unwrap (mxsum_rank S1) + unwrap (mxsum_rank S2)).
Canonical nary_mxsum_expr := ProperMxsumExpr nary_mxsum_proof.
End Nary.
Definition mxdirect_def m n T of phantom 'M_(m, n) (unwrap (mxsum_val T)) :=
\rank (unwrap T) == unwrap (mxsum_rank T).
End SumExpr.
Notation mxdirect A := (mxdirect_def (Phantom 'M_(_,_) A%MS)).

Lemma mxdirectE m n (S : mxsum_expr m n) :
mxdirect (unwrap S) = (\rank (unwrap S) == unwrap (mxsum_rank S)).