| I've started using template haskell (replacing some preprocessor stuff) | | However I had real trouble when trying to convert | instance (Show d) => Show (C d) | where show _ = "C " ++ (show (undefined :: d)) This isn't a bug. Haskell 98 doesn't have scoped type variables, so your program means: instance (Show d) => Show (C d) where show _ = "C " ++ (show (undefined :: forall d. d)) If you want scoped type variables use -fglasgow-exts as well. (You should be able to say -fscoped-type-variables, but there's a separate small bug (which I'll fix) that means -fscoped-type-variables doesn't make the tyvars of an instance decl scope properly.) hope this helps Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On | Behalf Of Marc Weber | Sent: 16 June 2007 12:04 | To: glasgow-haskell-users@haskell.org | Subject: Template Haskell [d| .. trouble | | Hi. | | I've started using template haskell (replacing some preprocessor stuff) | | However I had real trouble when trying to convert | instance (Show d) => Show (C d) | where show _ = "C " ++ (show (undefined :: d)) | | into th. | Why? It didn't compile (http://hpaste.org/289) | | Heffalump on #haskell suggested that the d is already in scope so I | don't need the first list item of ForallT .. | And that does work fine. | | So is this a bug in the [d| .. parser / to abstract syntax tree | transformer ? | | Session showing this behviour: | | marc@localhost ~ $ cat ABC.hs | {-# OPTIONS_GHC -fglasgow-exts #-} | module ABC where | data C d | marc@localhost ~ $ ghci -fth | ___ ___ _ | / _ \ /\ /\/ __(_) | / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. | / /_\\/ __ / /___| | http://www.haskell.org/ghc/ | \____/\/ /_/\____/|_| Type :? for help. | | Loading package base ... linking ... done. | Prelude> :l ABC | [1 of 1] Compiling ABC ( ABC.hs, interpreted ) | Ok, modules loaded: ABC. | *ABC> :m +Language.Haskell.TH | *ABC Language.Haskell.TH> runQ [d| instance (Show d) => Show (C d) where show _ = "C " ++ (show | (undefined :: d)) |] >>= print | Loading package template-haskell ... linking ... done. | [InstanceD [AppT (ConT GHC.Show.Show) (VarT d_0)] (AppT (ConT GHC.Show.Show) (AppT (ConT ABC.C) | (VarT d_0))) [FunD show [Clause [WildP] (NormalB (InfixE (Just (LitE (StringL "C "))) (VarE | GHC.Base.++) (Just (AppE (VarE show) (SigE (VarE GHC.Err.undefined) (ForallT [d_1] [] (VarT d_1))))))) | []]]] | *ABC Language.Haskell.TH> | | | Now | (ForallT [d_1] [] (VarT d_1) | should be | (ForallT [] [] (VarT d_1) | shouldn't it? | | | Marc Weber | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users