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%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[DsGRHSs]{Matching guarded right-hand-sides (GRHSs)}
\begin{code}
module DsGRHSs ( dsGuarded, dsGRHSs ) where
#include "HsVersions.h"
import {-# SOURCE #-} DsExpr ( dsLExpr, dsLet )
import {-# SOURCE #-} Match ( matchSinglePat )
import HsSyn ( Stmt(..), HsExpr(..), GRHSs(..), GRHS(..),
HsMatchContext(..), Pat(..), LStmt )
import CoreSyn ( CoreExpr )
import Type ( Type )
import Var ( Id )
import DsMonad
import DsUtils
import Unique ( Uniquable(..) )
import PrelInfo ( nON_EXHAUSTIVE_GUARDS_ERROR_ID )
import TysWiredIn ( trueDataConId )
import PrelNames ( otherwiseIdKey, hasKey )
import Name ( Name )
import SrcLoc ( unLoc, Located(..) )
\end{code}
@dsGuarded@ is used for both @case@ expressions and pattern bindings.
It desugars:
\begin{verbatim}
| g1 -> e1
...
| gn -> en
where binds
\end{verbatim}
producing an expression with a runtime error in the corner if
necessary. The type argument gives the type of the @ei@.
\begin{code}
dsGuarded :: GRHSs Id -> DsM CoreExpr
dsGuarded grhss
= dsGRHSs PatBindRhs [] grhss `thenDs` \ (err_ty, match_result) ->
mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID err_ty "" `thenDs` \ error_expr ->
extractMatchResult match_result error_expr
\end{code}
In contrast, @dsGRHSs@ produces a @MatchResult@.
\begin{code}
dsGRHSs :: HsMatchContext Name -> [Pat Id] -- These are to build a MatchContext from
-> GRHSs Id -- Guarded RHSs
-> DsM (Type, MatchResult)
dsGRHSs kind pats (GRHSs grhss binds ty)
= mappM (dsGRHS kind pats) grhss `thenDs` \ match_results ->
let
match_result1 = foldr1 combineMatchResults match_results
match_result2 = adjustMatchResultDs (dsLet binds) match_result1
-- NB: nested dsLet inside matchResult
in
returnDs (ty, match_result2)
dsGRHS kind pats (L loc (GRHS guard))
= matchGuard (map unLoc guard) (DsMatchContext kind pats loc)
\end{code}
%************************************************************************
%* *
%* matchGuard : make a MatchResult from a guarded RHS *
%* *
%************************************************************************
\begin{code}
matchGuard :: [Stmt Id] -- Guard
-> DsMatchContext -- Context
-> DsM MatchResult
-- See comments with HsExpr.Stmt re what an ExprStmt means
-- Here we must be in a guard context (not do-expression, nor list-comp)
matchGuard [ResultStmt expr] ctx
= dsLExpr expr `thenDs` \ core_expr ->
returnDs (cantFailMatchResult core_expr)
-- ExprStmts must be guards
-- Turn an "otherwise" guard is a no-op
matchGuard (ExprStmt (L _ (HsVar v)) _ : stmts) ctx
| v `hasKey` otherwiseIdKey
|| v `hasKey` getUnique trueDataConId
-- trueDataConId doesn't have the same
-- unique as trueDataCon
= matchGuard stmts ctx
matchGuard (ExprStmt expr _ : stmts) ctx
= matchGuard stmts ctx `thenDs` \ match_result ->
dsLExpr expr `thenDs` \ pred_expr ->
returnDs (mkGuardedMatchResult pred_expr match_result)
matchGuard (LetStmt binds : stmts) ctx
= matchGuard stmts ctx `thenDs` \ match_result ->
returnDs (adjustMatchResultDs (dsLet binds) match_result)
-- NB the dsLet occurs inside the match_result
matchGuard (BindStmt pat rhs : stmts) ctx
= matchGuard stmts ctx `thenDs` \ match_result ->
dsLExpr rhs `thenDs` \ core_rhs ->
matchSinglePat core_rhs ctx pat match_result
\end{code}
Should {\em fail} if @e@ returns @D@
\begin{verbatim}
f x | p <- e', let C y# = e, f y# = r1
| otherwise = r2
\end{verbatim}
|
