Fix Trac #1759: do not let ticks get in the way of spotting trivially-true guards

[ghc-hetmet.git] / compiler / deSugar / DsGRHSs.lhs

diff --git a/compiler/deSugar/DsGRHSs.lhs b/compiler/deSugar/DsGRHSs.lhs
index 12e0f0b..265df11 100644 (file)
--- a/compiler/deSugar/DsGRHSs.lhs
+++ b/compiler/deSugar/DsGRHSs.lhs
@@ -6,6 +6,13 @@
 Matching guarded right-hand-sides (GRHSs)
 
 \begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
 module DsGRHSs ( dsGuarded, dsGRHSs ) where
 
 #include "HsVersions.h"
@@ -20,13 +27,12 @@ import Type
 
 import DsMonad
 import DsUtils
-import DsBreakpoint
-import Unique
 import PrelInfo
 import TysWiredIn
 import PrelNames
 import Name
 import SrcLoc
+
 \end{code}
 
 @dsGuarded@ is used for both @case@ expressions and pattern bindings.
@@ -56,14 +62,15 @@ dsGRHSs :: HsMatchContext Name -> [Pat Id]  -- These are to build a MatchContext
        -> 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 ->
+dsGRHSs hs_ctx pats grhssa@(GRHSs grhss binds) rhs_ty = do
+    match_results <- mappM (dsGRHS hs_ctx pats rhs_ty) grhss
     let 
        match_result1 = foldr1 combineMatchResults match_results
-       match_result2 = adjustMatchResultDs (dsLocalBinds binds) match_result1
+       match_result2 = adjustMatchResultDs 
+                                 (\e -> dsLocalBinds binds e) 
+                                 match_result1
                -- NB: nested dsLet inside matchResult
-    in
+    --
     returnDs match_result2
 
 dsGRHS hs_ctx pats rhs_ty (L loc (GRHS guards rhs))
@@ -98,10 +105,8 @@ matchGuards [] ctx rhs rhs_ty
        -- 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 (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 (ExprStmt e _ _ : stmts) ctx rhs rhs_ty
+  | isTrueLHsExpr e
   = matchGuards stmts ctx rhs rhs_ty
 
 matchGuards (ExprStmt expr _ _ : stmts) ctx rhs rhs_ty
@@ -121,6 +126,19 @@ 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
+
+isTrueLHsExpr :: LHsExpr Id -> Bool
+-- Returns True if we're sure that the expression is True
+-- I.e.   * 'True' datacon
+--       * 'otherwise' Id
+--       * Trivial wappings of these
+isTrueLHsExpr (L _ (HsVar v)) =  v `hasKey` otherwiseIdKey
+                             || v `hasKey` getUnique trueDataConId     
+       -- trueDataConId doesn't have the same unique as trueDataCon
+isTrueLHsExpr (L _ (HsTick    _ _ e)) = isTrueLHsExpr e
+isTrueLHsExpr (L _ (HsBinTick _ _ e)) = isTrueLHsExpr e
+isTrueLHsExpr (L _ (HsPar e))         = isTrueLHsExpr e
+isTrueLHsExpr other = False
 \end{code}
 
 Should {\em fail} if @e@ returns @D@