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{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
Matching guarded right-hand-sides (GRHSs)
-}
{-# LANGUAGE CPP #-}
module DsGRHSs ( dsGuarded, dsGRHSs, dsGRHS, isTrueLHsExpr ) where
#include "HsVersions.h"
import GhcPrelude
import {-# SOURCE #-} DsExpr ( dsLExpr, dsLocalBinds )
import {-# SOURCE #-} Match ( matchSinglePat )
import HsSyn
import MkCore
import CoreSyn
import DsMonad
import DsUtils
import TysWiredIn
import PrelNames
import Type ( Type )
import Module
import Name
import Util
import SrcLoc
import Outputable
{-
@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@.
-}
dsGuarded :: GRHSs GhcTc (LHsExpr GhcTc) -> Type -> DsM CoreExpr
dsGuarded grhss rhs_ty = do
match_result <- dsGRHSs PatBindRhs grhss rhs_ty
error_expr <- mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID rhs_ty empty
extractMatchResult match_result error_expr
-- In contrast, @dsGRHSs@ produces a @MatchResult@.
dsGRHSs :: HsMatchContext Name
-> GRHSs GhcTc (LHsExpr GhcTc) -- Guarded RHSs
-> Type -- Type of RHS
-> DsM MatchResult
dsGRHSs hs_ctx (GRHSs grhss binds) rhs_ty
= ASSERT( notNull grhss )
do { match_results <- mapM (dsGRHS hs_ctx rhs_ty) grhss
; let match_result1 = foldr1 combineMatchResults match_results
match_result2 = adjustMatchResultDs (dsLocalBinds binds) match_result1
-- NB: nested dsLet inside matchResult
; return match_result2 }
dsGRHS :: HsMatchContext Name -> Type -> LGRHS GhcTc (LHsExpr GhcTc)
-> DsM MatchResult
dsGRHS hs_ctx rhs_ty (L _ (GRHS guards rhs))
= matchGuards (map unLoc guards) (PatGuard hs_ctx) rhs rhs_ty
{-
************************************************************************
* *
* matchGuard : make a MatchResult from a guarded RHS *
* *
************************************************************************
-}
matchGuards :: [GuardStmt GhcTc] -- Guard
-> HsStmtContext Name -- Context
-> LHsExpr GhcTc -- RHS
-> Type -- Type of RHS of guard
-> DsM MatchResult
-- See comments with HsExpr.Stmt re what a BodyStmt means
-- Here we must be in a guard context (not do-expression, nor list-comp)
matchGuards [] _ rhs _
= do { core_rhs <- dsLExpr rhs
; return (cantFailMatchResult core_rhs) }
-- BodyStmts 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 HsWrap constructors
-- If it does, you'll get bogus overlap warnings
matchGuards (BodyStmt e _ _ _ : stmts) ctx rhs rhs_ty
| Just addTicks <- isTrueLHsExpr e = do
match_result <- matchGuards stmts ctx rhs rhs_ty
return (adjustMatchResultDs addTicks match_result)
matchGuards (BodyStmt expr _ _ _ : stmts) ctx rhs rhs_ty = do
match_result <- matchGuards stmts ctx rhs rhs_ty
pred_expr <- dsLExpr expr
return (mkGuardedMatchResult pred_expr match_result)
matchGuards (LetStmt binds : stmts) ctx rhs rhs_ty = do
match_result <- matchGuards stmts ctx rhs rhs_ty
return (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 = do
match_result <- matchGuards stmts ctx rhs rhs_ty
core_rhs <- dsLExpr bind_rhs
matchSinglePat core_rhs (StmtCtxt ctx) pat rhs_ty match_result
matchGuards (LastStmt {} : _) _ _ _ = panic "matchGuards LastStmt"
matchGuards (ParStmt {} : _) _ _ _ = panic "matchGuards ParStmt"
matchGuards (TransStmt {} : _) _ _ _ = panic "matchGuards TransStmt"
matchGuards (RecStmt {} : _) _ _ _ = panic "matchGuards RecStmt"
matchGuards (ApplicativeStmt {} : _) _ _ _ =
panic "matchGuards ApplicativeLastStmt"
isTrueLHsExpr :: LHsExpr GhcTc -> Maybe (CoreExpr -> DsM CoreExpr)
-- Returns Just {..} if we're sure that the expression is True
-- I.e. * 'True' datacon
-- * 'otherwise' Id
-- * Trivial wappings of these
-- The arguments to Just are any HsTicks that we have found,
-- because we still want to tick then, even it they are always evaluated.
isTrueLHsExpr (L _ (HsVar _ (L _ v))) | v `hasKey` otherwiseIdKey
|| v `hasKey` getUnique trueDataConId
= Just return
-- trueDataConId doesn't have the same unique as trueDataCon
isTrueLHsExpr (L _ (HsConLikeOut _ con))
| con `hasKey` getUnique trueDataCon = Just return
isTrueLHsExpr (L _ (HsTick _ tickish e))
| Just ticks <- isTrueLHsExpr e
= Just (\x -> do wrapped <- ticks x
return (Tick tickish wrapped))
-- This encodes that the result is constant True for Hpc tick purposes;
-- which is specifically what isTrueLHsExpr is trying to find out.
isTrueLHsExpr (L _ (HsBinTick _ ixT _ e))
| Just ticks <- isTrueLHsExpr e
= Just (\x -> do e <- ticks x
this_mod <- getModule
return (Tick (HpcTick this_mod ixT) e))
isTrueLHsExpr (L _ (HsPar _ e)) = isTrueLHsExpr e
isTrueLHsExpr _ = Nothing
{-
Should {\em fail} if @e@ returns @D@
\begin{verbatim}
f x | p <- e', let C y# = e, f y# = r1
| otherwise = r2
\end{verbatim}
-}
|
