[commit: ghc] master: Comments about the let/app invariant (04ded40)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/04ded40857fdaeea1e48bb39da54f1a5e9f91570/ghc

>---------------------------------------------------------------

commit 04ded40857fdaeea1e48bb39da54f1a5e9f91570
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Wed Oct 1 14:39:00 2014 +0100

    Comments about the let/app invariant


>---------------------------------------------------------------

04ded40857fdaeea1e48bb39da54f1a5e9f91570
 compiler/coreSyn/MkCore.lhs | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/compiler/coreSyn/MkCore.lhs b/compiler/coreSyn/MkCore.lhs
index 6987f06..81f0533 100644
--- a/compiler/coreSyn/MkCore.lhs
+++ b/compiler/coreSyn/MkCore.lhs
@@ -129,8 +129,8 @@ mkCoreLets binds body = foldr mkCoreLet body binds
 -- | Construct an expression which represents the application of one expression
 -- to the other
 mkCoreApp :: CoreExpr -> CoreExpr -> CoreExpr
--- Check the invariant that the arg of an App is ok-for-speculation if unlifted
--- See CoreSyn Note [CoreSyn let/app invariant]
+-- Respects the let/app invariant by building a case expression where necessary
+--   See CoreSyn Note [CoreSyn let/app invariant]
 mkCoreApp fun (Type ty) = App fun (Type ty)
 mkCoreApp fun (Coercion co) = App fun (Coercion co)
 mkCoreApp fun arg       = ASSERT2( isFunTy fun_ty, ppr fun $$ ppr arg )
@@ -141,18 +141,21 @@ mkCoreApp fun arg       = ASSERT2( isFunTy fun_ty, ppr fun $$ ppr arg )
 
 -- | Construct an expression which represents the application of a number of
 -- expressions to another. The leftmost expression in the list is applied first
+-- Respects the let/app invariant by building a case expression where necessary
+--   See CoreSyn Note [CoreSyn let/app invariant]
 mkCoreApps :: CoreExpr -> [CoreExpr] -> CoreExpr
 -- Slightly more efficient version of (foldl mkCoreApp)
 mkCoreApps orig_fun orig_args
   = go orig_fun (exprType orig_fun) orig_args
   where
-    go fun _      []               = fun
-    go fun fun_ty (Type ty : args) = go (App fun (Type ty)) (applyTy fun_ty ty) args
+    go fun _      []                   = fun
+    go fun fun_ty (Type ty     : args) = go (App fun (Type ty)) (applyTy fun_ty ty) args
     go fun fun_ty (Coercion co : args) = go (App fun (Coercion co)) (applyCo fun_ty co) args
-    go fun fun_ty (arg     : args) = ASSERT2( isFunTy fun_ty, ppr fun_ty $$ ppr orig_fun $$ ppr orig_args )
-                                     go (mk_val_app fun arg arg_ty res_ty) res_ty args
-                                   where
-                                     (arg_ty, res_ty) = splitFunTy fun_ty
+    go fun fun_ty (arg         : args) = ASSERT2( isFunTy fun_ty, ppr fun_ty $$ ppr orig_fun
+                                                                  $$ ppr orig_args )
+                                         go (mk_val_app fun arg arg_ty res_ty) res_ty args
+                                       where
+                                         (arg_ty, res_ty) = splitFunTy fun_ty
 
 -- | Construct an expression which represents the application of a number of
 -- expressions to that of a data constructor expression. The leftmost expression
@@ -160,13 +163,16 @@ mkCoreApps orig_fun orig_args
 mkCoreConApps :: DataCon -> [CoreExpr] -> CoreExpr
 mkCoreConApps con args = mkCoreApps (Var (dataConWorkId con)) args
 
------------
 mk_val_app :: CoreExpr -> CoreExpr -> Type -> Type -> CoreExpr
-mk_val_app fun arg arg_ty _        -- See Note [CoreSyn let/app invariant]
+-- Build an application (e1 e2),
+-- or a strict binding  (case e2 of x -> e1 x)
+-- using the latter when necessary to respect the let/app invariant
+--   See Note [CoreSyn let/app invariant]
+mk_val_app fun arg arg_ty res_ty
   | not (needsCaseBinding arg_ty arg)
   = App fun arg                -- The vastly common case
 
-mk_val_app fun arg arg_ty res_ty
+  | otherwise
   = Case arg arg_id res_ty [(DEFAULT,[],App fun (Var arg_id))]
   where
     arg_id = mkWildValBinder arg_ty
@@ -179,6 +185,7 @@ mk_val_app fun arg arg_ty res_ty
         -- is if you take apart this case expression, and pass a
         -- fragmet of it as the fun part of a 'mk_val_app'.
 
+-----------
 mkWildEvBinder :: PredType -> EvVar
 mkWildEvBinder pred = mkWildValBinder pred