-%
+
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[MatchCon]{Pattern-matching constructors}
import {-# SOURCE #-} Match ( match )
-import HsSyn ( OutPat(..) )
-
+import HsSyn ( Pat(..), HsConDetails(..), isEmptyLHsBinds )
+import DsBinds ( dsHsNestedBinds )
+import DataCon ( isVanillaDataCon, dataConTyVars, dataConOrigArgTys )
+import TcType ( tcTyConAppArgs )
+import Type ( substTys, zipTopTvSubst, mkTyVarTys )
+import CoreSyn
import DsMonad
import DsUtils
import Id ( Id )
-import CoreSyn
-import Type ( mkTyVarTys )
-import Unique ( Uniquable(..), Unique )
-import UniqFM -- Until equivClassesUniq moves to Util
+import Type ( Type )
+import ListSetOps ( equivClassesByUniq )
+import SrcLoc ( unLoc, Located(..) )
+import Unique ( Uniquable(..) )
import Outputable
\end{code}
@match_cons_used@ does all the real work.
\begin{code}
matchConFamily :: [Id]
+ -> Type
-> [EquationInfo]
-> DsM MatchResult
-
-matchConFamily (var:vars) eqns_info
+matchConFamily (var:vars) ty eqns_info
= 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 _ _ (ConPat data_con _ _ _ _ : _) _) = getUnique data_con
+ get_uniq (EqnInfo { eqn_pats = ConPatOut (L _ 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)
+ mappM (match_con vars ty) eqn_groups `thenDs` \ alts ->
+ returnDs (mkCoAlgCaseMatchResult var ty 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.
+Wadler's chapter in SLPJ. The function @shift_con_pats@ does what the
+list comprehension in @matchClause@ (SLPJ, p.~94) does, except things
+are trickier in real 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@.
\begin{code}
-match_con vars all_eqns@(EqnInfo n ctx (ConPat data_con _ ex_tvs ex_dicts arg_pats : pats1) match_result1 : 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 (ex_dicts ++ arg_vars ++ vars)
- (map shift_con_pat all_eqns) `thenDs` \ match_result ->
-
- -- Substitute over the result
- let
- match_result' | null ex_tvs = match_result
- | otherwise = adjustMatchResult subst_it 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
-
-
--- Belongs in Util.lhs
-equivClassesByUniq :: (a -> Unique) -> [a] -> [[a]]
- -- NB: it's *very* important that if we have the input list [a,b,c],
- -- where a,b,c all have the same unique, then we get back the list
- -- [a,b,c]
- -- not
- -- [c,b,a]
- -- Hence the use of foldr, plus the reversed-args tack_on below
-equivClassesByUniq get_uniq xs
- = eltsUFM (foldr add emptyUFM xs)
+match_con vars ty eqns
+ = do { -- Make new vars for the con arguments; avoid new locals where possible
+ arg_vars <- selectMatchVars (map unLoc arg_pats1) arg_tys
+ ; eqns' <- mapM shift eqns
+ ; match_result <- match (arg_vars ++ vars) ty eqns'
+ ; return (con, tvs1 ++ dicts1 ++ arg_vars, match_result) }
where
- add a ufm = addToUFM_C tack_on ufm (get_uniq a) [a]
- tack_on old new = new++old
+ ConPatOut (L _ con) tvs1 dicts1 _ (PrefixCon arg_pats1) pat_ty = firstPat (head eqns)
+
+ shift eqn@(EqnInfo { eqn_wrap = wrap,
+ eqn_pats = ConPatOut _ tvs ds bind (PrefixCon arg_pats) _ : pats })
+ = do { prs <- dsHsNestedBinds bind
+ ; return (eqn { eqn_wrap = wrap . wrapBinds (tvs `zip` tvs1)
+ . wrapBinds (ds `zip` dicts1)
+ . mkDsLet (Rec prs),
+ eqn_pats = map unLoc arg_pats ++ pats }) }
+
+ -- Get the arg types, which we use to type the new vars
+ -- to match on, from the "outside"; the types of pats1 may
+ -- be more refined, and hence won't do
+ arg_tys = substTys (zipTopTvSubst (dataConTyVars con) inst_tys)
+ (dataConOrigArgTys con)
+ inst_tys | isVanillaDataCon con = tcTyConAppArgs pat_ty -- Newtypes opaque!
+ | otherwise = mkTyVarTys tvs1
\end{code}
-Note on @shift_con_pats@ just above: does what the list comprehension in
-@matchClause@ (SLPJ, p.~94) does, except things are trickier in real
-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@.
+Note [Existentials in shift_con_pat]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+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 b) (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! **
+Hence
+ False -> ....((/\b\(e:Ord b).expr2) a d)....
+
+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.
+
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