[project @ 2002-02-14 15:03:38 by simonpj]

------------------------------------
	Desugar existential matches correctly
	------------------------------------

Consider
	data T = forall a. Ord a => T a (a->Int)

	f (T x f) True  = ...expr1...
	f (T y g) False = ...expr2..

When we put in the tyvars etc we get

	f (T a (d::Ord a) (x::a) (f::a->Int)) True =  ...expr1...
	f (T b (e::Ord a) (y::a) (g::a->Int)) True =  ...expr2...

After desugaring etc we'll get a single case:

	f = \t::T b::Bool ->
	    case t of
	       T a (d::Ord a) (x::a) (f::a->Int)) ->
	    case b of
		True  -> ...expr1...
		False -> ...expr2...

*** We have to substitute [a/b, d/e] in expr2! **

Originally I tried to use
	(\b -> let e = d in expr2) a
to do this substitution.  While this is "correct" in a way, it fails
Lint, because e::Ord b but d::Ord a.

So now I simply do the substitution properly using substExpr.

diff --git a/ghc/compiler/deSugar/MatchCon.lhs b/ghc/compiler/deSugar/MatchCon.lhs
index 4094342..6fb0fff 100644 (file)
--- a/ghc/compiler/deSugar/MatchCon.lhs
+++ b/ghc/compiler/deSugar/MatchCon.lhs
@@ -16,8 +16,9 @@ import DsMonad
 import DsUtils
 
 import Id              ( Id )
-import CoreSyn
-import Type            ( mkTyVarTys )
+import Subst           ( mkSubst, mkInScopeSet, bindSubst, substExpr )
+import CoreFVs         ( exprFreeVars )
+import VarEnv          ( emptySubstEnv )
 import ListSetOps      ( equivClassesByUniq )
 import Unique          ( Uniquable(..) )
 \end{code}
@@ -95,31 +96,44 @@ more-or-less the @matchCon@/@matchClause@ functions on page~94 in
 Wadler's chapter in SLPJ.
 
 \begin{code}
-match_con vars all_eqns@(EqnInfo n ctx (ConPat data_con _ ex_tvs ex_dicts arg_pats : pats1) match_result1 : other_eqns)
+match_con vars (eqn1@(EqnInfo _ _ (ConPat data_con _ ex_tvs ex_dicts arg_pats : _) _)
+               : other_eqns)
   = -- Make new vars for the con arguments; avoid new locals where possible
-    mapDs selectMatchVar arg_pats                         `thenDs` \ arg_vars ->
+    mapDs selectMatchVar arg_pats      `thenDs` \ arg_vars ->
 
     -- Now do the business to make the alt for _this_ ConPat ...
-    match (ex_dicts ++ arg_vars ++ vars)
-         (map shift_con_pat all_eqns)  `thenDs` \ match_result ->
+    match (arg_vars ++ vars) 
+         (map shift_con_pat (eqn1:other_eqns)) `thenDs` \ match_result ->
 
-       -- Substitute over the result
+    --         [See "notes on do_subst" below this function]
+    -- Make the ex_tvs and ex_dicts line up with those
+    -- in the first pattern.  Remember, they are all guaranteed to be variables
     let
-       match_result' | null ex_tvs = match_result
-                     | otherwise   = adjustMatchResult subst_it match_result
-    in 
+       match_result' | null ex_tvs     = match_result
+                     | null other_eqns = match_result
+                     | otherwise       = adjustMatchResult do_subst match_result
+    in
+       
     returnDs (data_con, ex_tvs ++ ex_dicts ++ arg_vars, match_result')
   where
     shift_con_pat :: EquationInfo -> EquationInfo
-    shift_con_pat (EqnInfo n ctx (ConPat _ _ ex_tvs' ex_dicts' arg_pats: pats) match_result)
-      = EqnInfo n ctx (new_pats  ++ pats) match_result
-      where
-       new_pats  = map VarPat ex_dicts' ++ arg_pats 
-
-       -- We 'substitute' by going: (/\ tvs' -> e) tvs
-    subst_it e = foldr subst_one e other_eqns
-    subst_one (EqnInfo _ _ (ConPat _ _ ex_tvs' _ _ : _) _) e = mkTyApps (mkLams ex_tvs' e) ex_tys
-    ex_tys = mkTyVarTys ex_tvs
+    shift_con_pat (EqnInfo n ctx (ConPat _ _ _ _ arg_pats : pats) match_result)
+      = EqnInfo n ctx (arg_pats ++ pats) match_result
+
+    other_pats = [p | EqnInfo _ _ (p:_) _ <- other_eqns]
+
+    var_prs = concat [ (ex_tvs'   `zip` ex_tvs) ++ 
+                      (ex_dicts' `zip` ex_dicts) 
+                    | ConPat _ _ ex_tvs' ex_dicts' _ <- other_pats ]
+
+    do_subst e = substExpr subst e
+              where
+                subst    = foldl (\ s (v', v) -> bindSubst s v' v) in_scope var_prs
+                in_scope = mkSubst (mkInScopeSet (exprFreeVars e)) emptySubstEnv
+                       -- We put all the free variables of e into the in-scope 
+                       -- set of the substitution, not because it is necessary,
+                       -- but to suppress the warning in Subst.lookupInScope
+                       -- Tiresome, but doing the substitution at all is rare.
 \end{code}
 
 Note on @shift_con_pats@ just above: does what the list comprehension in
@@ -127,3 +141,37 @@ Note on @shift_con_pats@ just above: does what the list comprehension in
 life.  Works for @ConPats@, and we want it to fail catastrophically
 for anything else (which a list comprehension wouldn't).
 Cf.~@shift_lit_pats@ in @MatchLits@.
+
+
+Notes on do_subst stuff
+~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+       data T = forall a. Ord a => T a (a->Int)
+
+       f (T x f) True  = ...expr1...
+       f (T y g) False = ...expr2..
+
+When we put in the tyvars etc we get
+
+       f (T a (d::Ord a) (x::a) (f::a->Int)) True =  ...expr1...
+       f (T b (e::Ord a) (y::a) (g::a->Int)) True =  ...expr2...
+
+After desugaring etc we'll get a single case:
+
+       f = \t::T b::Bool -> 
+           case t of
+              T a (d::Ord a) (x::a) (f::a->Int)) ->
+           case b of
+               True  -> ...expr1...
+               False -> ...expr2...
+
+*** We have to substitute [a/b, d/e] in expr2! **
+That is what do_subst is doing.
+
+Originally I tried to use 
+       (\b -> let e = d in expr2) a 
+to do this substitution.  While this is "correct" in a way, it fails
+Lint, because e::Ord b but d::Ord a.  
+
+So now I simply do the substitution properly using substExpr.
+