diff options
| -rw-r--r-- | compiler/prelude/PrelNames.lhs | 3 | ||||
| -rw-r--r-- | compiler/typecheck/TcGenDeriv.lhs | 22 |
2 files changed, 22 insertions, 3 deletions
diff --git a/compiler/prelude/PrelNames.lhs b/compiler/prelude/PrelNames.lhs index 09835fb34e..fe1e8b1638 100644 --- a/compiler/prelude/PrelNames.lhs +++ b/compiler/prelude/PrelNames.lhs @@ -617,11 +617,12 @@ punc_RDR = dataQual_RDR lEX (fsLit "Punc") ident_RDR = dataQual_RDR lEX (fsLit "Ident") symbol_RDR = dataQual_RDR lEX (fsLit "Symbol") -step_RDR, alt_RDR, reset_RDR, prec_RDR :: RdrName +step_RDR, alt_RDR, reset_RDR, prec_RDR, pfail_RDR :: RdrName step_RDR = varQual_RDR rEAD_PREC (fsLit "step") alt_RDR = varQual_RDR rEAD_PREC (fsLit "+++") reset_RDR = varQual_RDR rEAD_PREC (fsLit "reset") prec_RDR = varQual_RDR rEAD_PREC (fsLit "prec") +pfail_RDR = varQual_RDR rEAD_PREC (fsLit "pfail") showList_RDR, showList___RDR, showsPrec_RDR, showString_RDR, showSpace_RDR, showParen_RDR :: RdrName diff --git a/compiler/typecheck/TcGenDeriv.lhs b/compiler/typecheck/TcGenDeriv.lhs index b8c7c8cc36..564756269e 100644 --- a/compiler/typecheck/TcGenDeriv.lhs +++ b/compiler/typecheck/TcGenDeriv.lhs @@ -881,7 +881,25 @@ The latter desugares to inline code for matching the Ident and the string, and this can be very voluminous. The former is much more compact. Cf Trac #7258, although that also concerned non-linearity in the occurrence analyser, a separate issue. - + +Note [Read for empty data types] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +What should we get for this? (Trac #7931) + data Emp deriving( Read ) -- No data constructors + +Here we want + read "[]" :: [Emp] to succeed, returning [] +So we do NOT want + instance Read Emp where + readPrec = error "urk" +Rather we want + instance Read Emp where + readPred = pfail -- Same as choose [] + +Because 'pfail' allows the parser to backtrack, but 'error' doesn't. +These instances are also useful for Read (Either Int Emp), where +we want to be able to parse (Left 3) just fine. + \begin{code} gen_Read_binds :: FixityEnv -> SrcSpan -> TyCon -> (LHsBinds RdrName, BagDerivStuff) @@ -902,7 +920,7 @@ gen_Read_binds get_fixity loc tycon read_prec = mkHsVarBind loc readPrec_RDR (nlHsApp (nlHsVar parens_RDR) read_cons) - read_cons | null data_cons = error_Expr "Derived Read on empty data type" -- Trac #7931 + read_cons | null data_cons = nlHsVar pfail_RDR -- See Note [Read for empty data types] | otherwise = foldr1 mk_alt (read_nullary_cons ++ read_non_nullary_cons) read_non_nullary_cons = map read_non_nullary_con non_nullary_cons |