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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, dsLocalBinds )
import {-# SOURCE #-} Match ( matchSinglePat )
import HsSyn ( Stmt(..), HsExpr(..), GRHSs(..), GRHS(..),
LHsExpr, HsMatchContext(..), Pat(..) )
import CoreSyn ( CoreExpr )
import Var ( Id )
import Type ( Type )
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 -> Type -> DsM CoreExpr
dsGuarded grhss rhs_ty
= dsGRHSs PatBindRhs [] grhss rhs_ty `thenDs` \ match_result ->
mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_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
-> Type -- Type of RHS
-> DsM MatchResult
dsGRHSs hs_ctx pats (GRHSs grhss binds) rhs_ty
= mappM (dsGRHS hs_ctx pats rhs_ty) grhss `thenDs` \ match_results ->
let
match_result1 = foldr1 combineMatchResults match_results
match_result2 = adjustMatchResultDs (dsLocalBinds binds) match_result1
-- NB: nested dsLet inside matchResult
in
returnDs match_result2
dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
= matchGuards (map unLoc guards) hs_ctx rhs rhs_ty
\end{code}
%************************************************************************
%* *
%* matchGuard : make a MatchResult from a guarded RHS *
%* *
%************************************************************************
\begin{code}
matchGuards :: [Stmt Id] -- Guard
-> HsMatchContext Name -- Context
-> LHsExpr Id -- RHS
-> Type -- Type of RHS of guard
-> 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)
matchGuards [] ctx rhs rhs_ty
= do { core_rhs <- dsLExpr rhs
; return (cantFailMatchResult core_rhs) }
-- ExprStmts must be guards
-- Turn an "otherwise" guard is a no-op. This ensures that
-- you don't get a "non-exhaustive eqns" message when the guards
-- finish in "otherwise".
-- NB: The success of this clause depends on the typechecker not
-- wrapping the 'otherwise' in empty HsTyApp or HsCoerce constructors
-- If it does, you'll get bogus overlap warnings
matchGuards (ExprStmt (L _ (HsVar v)) _ _ : stmts) ctx rhs rhs_ty
| v `hasKey` otherwiseIdKey
|| v `hasKey` getUnique trueDataConId
-- trueDataConId doesn't have the same unique as trueDataCon
= matchGuards stmts ctx rhs rhs_ty
matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
= matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
dsLExpr expr `thenDs` \ pred_expr ->
returnDs (mkGuardedMatchResult pred_expr match_result)
matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty
= matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
returnDs (adjustMatchResultDs (dsLocalBinds binds) match_result)
-- NB the dsLet occurs inside the match_result
-- Reason: dsLet takes the body expression as its argument
-- so we can't desugar the bindings without the
-- body expression in hand
matchGuards (BindStmt pat bind_rhs _ _ : stmts) ctx rhs rhs_ty
= matchGuards stmts ctx rhs rhs_ty `thenDs` \ match_result ->
dsLExpr bind_rhs `thenDs` \ core_rhs ->
matchSinglePat core_rhs ctx pat rhs_ty 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}
|
