-%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1995
+
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[MatchCon]{Pattern-matching constructors}
\begin{code}
+module MatchCon ( matchConFamily ) where
+
#include "HsVersions.h"
-module MatchCon (
- matchConFamily
-) where
-
-import AbsSyn -- the stuff being desugared
-import PlainCore -- the output of desugaring;
- -- importing this module also gets all the
- -- CoreSyn utility functions
-import DsMonad -- the monadery used in the desugarer
-
-import AbsUniType ( mkTyVarTy, splitType, TyVar, TyVarTemplate,
- getTyConDataCons,
- instantiateTauTy, TyCon, Class, UniType,
- TauType(..), InstTyEnv(..)
- IF_ATTACK_PRAGMAS(COMMA instantiateTy)
- )
+import {-# SOURCE #-} Match ( match )
+
+import HsSyn ( Pat(..), HsConDetails(..) )
+
+import DsMonad
import DsUtils
-import Id ( eqId, getInstantiatedDataConSig,
- getIdUniType, isDataCon, DataCon(..)
- )
-import Maybes ( Maybe(..) )
-import Match ( match )
-import Util
-\end{code}
-\subsection[matchConFamily]{Making alternatives for a constructor family}
+import Id ( Id )
+import Subst ( mkSubst, mkInScopeSet, bindSubst, substExpr )
+import CoreFVs ( exprFreeVars )
+import VarEnv ( emptySubstEnv )
+import ListSetOps ( equivClassesByUniq )
+import SrcLoc ( unLoc )
+import Unique ( Uniquable(..) )
+\end{code}
We are confronted with the first column of patterns in a set of
equations, all beginning with constructors from one ``family'' (e.g.,
@[]@ and @:@ make up the @List@ ``family''). We want to generate the
-alternatives for a @CoCase@ expression. There are several choices:
+alternatives for a @Case@ expression. There are several choices:
\begin{enumerate}
\item
Generate an alternative for every constructor in the family, whether
chapter does.
\begin{description}
\item[Advantages:]
-(a)~Simple. (b)~It may also be that large sparsely-used constructor families are mainly
-handled by the code for literals.
+(a)~Simple. (b)~It may also be that large sparsely-used constructor
+families are mainly handled by the code for literals.
\item[Disadvantages:]
-(a)~Not practical for large sparsely-used constructor families, e.g., the
-ASCII character set. (b)~Have to look up (in the TDE environment) a
-list of what constructors make up the whole family. So far, this is
-the only part of desugaring that needs information from the environments.
+(a)~Not practical for large sparsely-used constructor families, e.g.,
+the ASCII character set. (b)~Have to look up a list of what
+constructors make up the whole family.
\end{description}
\item
\end{description}
\end{enumerate}
-We are implementing the ``do-it-right'' option for now.
-The arguments to @matchConFamily@ are the same as to @match@; the extra
-@Int@ returned is the number of constructors in the family.
+We are implementing the ``do-it-right'' option for now. The arguments
+to @matchConFamily@ are the same as to @match@; the extra @Int@
+returned is the number of constructors in the family.
The function @matchConFamily@ is concerned with this
-have-we-used-all-the-constructors question; the local function
+have-we-used-all-the-constructors? question; the local function
@match_cons_used@ does all the real work.
\begin{code}
matchConFamily :: [Id]
-> [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) 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 _ _ (ConPatOut data_con _ _ _ _ : _) _) = getUnique data_con
+ in
+ -- Now make a case alternative out of each group
+ mappM (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.
+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_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
+ mappM selectMatchVarL arg_pats `thenDs` \ arg_vars ->
-match_cons_used vars eqns_info@(EqnInfo (ConPat data_con _ arg_pats : ps1) _ : eqns) shadows
- = ASSERT(isDataCon data_con)
+ -- Now do the business to make the alt for _this_ ConPat ...
+ 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
- (eqns_for_this_con, eqns_not_for_this_con) = splitByCon eqns_info
- (shadows_for_this_con, shadows_not_for_this_con) = splitByCon shadows
+ match_result' | null ex_tvs = match_result
+ | null other_eqns = match_result
+ | otherwise = adjustMatchResult do_subst match_result
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
- )
+
+ 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 `eqId` 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 (typeOfPat 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 (map unLoc 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.
+
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