-%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
+
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[MatchCon]{Pattern-matching constructors}
\begin{code}
-#include "HsVersions.h"
-
module MatchCon ( matchConFamily ) where
-IMP_Ubiq()
-#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ <= 201
-IMPORT_DELOOPER(DsLoop) ( match ) -- break match-ish loop
-#else
-import {-# SOURCE #-} Match
-#endif
+#include "HsVersions.h"
+
+import {-# SOURCE #-} Match ( match )
-import HsSyn ( OutPat(..), HsLit, HsExpr )
-import DsHsSyn ( outPatType )
+import HsSyn ( Pat(..), HsConDetails(..) )
import DsMonad
import DsUtils
-import Id ( GenId{-instances-}, SYN_IE(Id) )
-import Util ( panic, assertPanic )
+import Id ( Id )
+import Subst ( mkSubst, mkInScopeSet, bindSubst, substExpr )
+import CoreFVs ( exprFreeVars )
+import VarEnv ( emptySubstEnv )
+import ListSetOps ( equivClassesByUniq )
+import Unique ( Uniquable(..) )
\end{code}
We are confronted with the first column of patterns in a set of
-> DsM MatchResult
matchConFamily (var:vars) eqns_info
- = match_cons_used vars eqns_info `thenDs` \ alts ->
- mkCoAlgCaseMatchResult var alts
+ = let
+ -- Sort into equivalence classes by the unique on the constructor
+ -- All the EqnInfos should start with a ConPat
+ eqn_groups = equivClassesByUniq get_uniq eqns_info
+ get_uniq (EqnInfo _ _ (ConPatOut data_con _ _ _ _ : _) _) = getUnique data_con
+ in
+ -- Now make a case alternative out of each group
+ mapDs (match_con vars) eqn_groups `thenDs` \ alts ->
+
+ returnDs (mkCoAlgCaseMatchResult var alts)
\end{code}
And here is the local function that does all the work. It is
more-or-less the @matchCon@/@matchClause@ functions on page~94 in
Wadler's chapter in SLPJ.
-\begin{code}
-match_cons_used _ [{- no more eqns -}] = returnDs []
-match_cons_used vars eqns_info@(EqnInfo n ctx (ConPat data_con _ arg_pats : ps1) _ : eqns)
- = let
- (eqns_for_this_con, eqns_not_for_this_con) = splitByCon eqns_info
- in
- -- Go ahead and do the recursive call to make the alts
- -- for the other ConPats in this con family...
- match_cons_used vars eqns_not_for_this_con `thenDs` \ rest_of_alts ->
-
- -- Make new vars for the con arguments; avoid new locals where possible
- selectMatchVars arg_pats `thenDs` \ new_vars ->
+\begin{code}
+match_con vars (eqn1@(EqnInfo _ _ (ConPatOut data_con (PrefixCon arg_pats) _ ex_tvs ex_dicts : _) _)
+ : other_eqns)
+ = -- Make new vars for the con arguments; avoid new locals where possible
+ mapDs selectMatchVar arg_pats `thenDs` \ arg_vars ->
-- Now do the business to make the alt for _this_ ConPat ...
- match (new_vars++vars)
- (map shift_con_pat eqns_for_this_con) `thenDs` \ match_result ->
-
- returnDs (
- (data_con, new_vars, match_result)
- : rest_of_alts
- )
+ match (arg_vars ++ vars)
+ (map shift_con_pat (eqn1:other_eqns)) `thenDs` \ match_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
+ | null other_eqns = match_result
+ | otherwise = adjustMatchResult do_subst match_result
+ in
+
+ returnDs (data_con, ex_tvs ++ ex_dicts ++ arg_vars, match_result')
where
- splitByCon :: [EquationInfo] -> ([EquationInfo], [EquationInfo])
- splitByCon [] = ([],[])
- splitByCon (info@(EqnInfo _ _ (pat : _) _) : rest)
- = case pat of
- ConPat n _ _ | n == data_con -> (info:rest_yes, rest_no)
- other_pat -> (rest_yes, info:rest_no)
- where
- (rest_yes, rest_no) = splitByCon rest
-
shift_con_pat :: EquationInfo -> EquationInfo
- shift_con_pat (EqnInfo n ctx (ConPat _ _ pats': pats) match_result)
- = EqnInfo n ctx (pats' ++ pats) match_result
- shift_con_pat (EqnInfo n ctx (WildPat _: pats) match_result) -- Will only happen in shadow
- = EqnInfo n ctx ([WildPat (outPatType arg_pat) | arg_pat <- arg_pats] ++ pats) match_result
- shift_con_pat other = panic "matchConFamily:match_cons_used:shift_con_pat"
+ shift_con_pat (EqnInfo n ctx (ConPatOut _ (PrefixCon 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)
+ | ConPatOut _ _ _ 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
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.
+