-%
-% (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 HsSyn ( OutPat(..), HsLit, HsExpr )
-import DsHsSyn ( outPatType )
+import {-# SOURCE #-} Match ( match )
+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 ( GenId{-instances-}, SYN_IE(Id) )
-import Util ( panic, assertPanic )
+import Id ( Id )
+import Type ( Type )
+import ListSetOps ( equivClassesByUniq )
+import SrcLoc ( unLoc, Located(..) )
+import Unique ( Uniquable(..) )
+import Outputable
\end{code}
We are confronted with the first column of patterns in a set of
@match_cons_used@ does all the real work.
\begin{code}
matchConFamily :: [Id]
+ -> Type
-> [EquationInfo]
- -> [EquationInfo] -- Shadows
-> DsM MatchResult
-
-matchConFamily (var:vars) eqns_info shadows
- = match_cons_used vars eqns_info shadows `thenDs` \ alts ->
- mkCoAlgCaseMatchResult var alts
+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 { eqn_pats = ConPatOut (L _ data_con) _ _ _ _ _ : _}) = getUnique data_con
+ in
+ -- Now make a case alternative out of each group
+ 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_cons_used _ [{- no more eqns -}] _ = returnDs []
+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
-match_cons_used vars eqns_info@(EqnInfo (ConPat data_con _ arg_pats : ps1) _ : eqns) shadows
- = let
- (eqns_for_this_con, eqns_not_for_this_con) = splitByCon eqns_info
- (shadows_for_this_con, shadows_not_for_this_con) = splitByCon shadows
- 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 shadows_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 ->
-
- -- Now do the business to make the alt for _this_ ConPat ...
- match (new_vars++vars)
- (map shift_con_pat eqns_for_this_con)
- (map shift_con_pat shadows_for_this_con) `thenDs` \ match_result ->
-
- returnDs (
- (data_con, new_vars, match_result)
- : rest_of_alts
- )
+ ; match_result <- match (arg_vars ++ vars) ty (shiftEqns eqns)
+
+ ; binds <- mapM ds_binds [ bind | ConPatOut _ _ _ bind _ _ <- pats,
+ not (isEmptyLHsBinds bind) ]
+
+ ; let match_result' = bindInMatchResult (line_up other_pats) $
+ mkCoLetsMatchResult binds match_result
+
+ ; return (data_con, tvs1 ++ dicts1 ++ 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)
- WildPat _ -> (info:rest_yes, info:rest_no)
- -- WildPats will be in the shadows only,
- -- and they go into both groups
- other_pat -> (rest_yes, info:rest_no)
- where
- (rest_yes, rest_no) = splitByCon rest
-
- shift_con_pat :: EquationInfo -> EquationInfo
- shift_con_pat (EqnInfo (ConPat _ _ pats': pats) match_result)
- = EqnInfo (pats' ++ pats) match_result
- shift_con_pat (EqnInfo (WildPat _: pats) match_result) -- Will only happen in shadow
- = EqnInfo ([WildPat (outPatType arg_pat) | arg_pat <- arg_pats] ++ pats) match_result
- shift_con_pat other = panic "matchConFamily:match_cons_used:shift_con_pat"
+ pats@(pat1 : other_pats) = map firstPat eqns
+ ConPatOut (L _ data_con) tvs1 dicts1 _ (PrefixCon arg_pats1) pat_ty = pat1
+
+ ds_binds bind = do { prs <- dsHsNestedBinds bind; return (Rec prs) }
+
+ line_up pats
+ | null tvs1 && null dicts1 = [] -- Common case
+ | otherwise = [ pr | ConPatOut _ ts ds _ _ _ <- pats,
+ pr <- (ts `zip` tvs1) ++ (ds `zip` dicts1)]
+
+ -- 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 data_con) inst_tys)
+ (dataConOrigArgTys data_con)
+ inst_tys | isVanillaDataCon data_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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