Template Haskell [d| .. trouble

[Simon Peyton-Jones]

| 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 at haskell.org [mailto:glasgow-haskell-users-bounces at haskell.org] On
| Behalf Of Marc Weber
| Sent: 16 June 2007 12:04
| To: glasgow-haskell-users at 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 at localhost ~ $ cat ABC.hs
|   {-# OPTIONS_GHC -fglasgow-exts #-}
|   module ABC where
|   data C d
|   marc at 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
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