Commit 98b23d27 authored by simonpj's avatar simonpj
Browse files

[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.
parent 6aa2bf20
......@@ -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.
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