[Template-haskell] RE: [ ghc-Bugs-820778 ] Malformed Predicate w/ Template Haskell

[Simon Peyton-Jones]

TH maestros

This bug turns out to be simply that Template Haskell is only set up for
Haskell 98, whereas Jon Cast wants to manipulate multi-parameter type
clases.

Should we add multi-param type classes to THSyntax?  Does anyone feel
like doing it?  It's a fairly routine matter, but it does mean changing
the data type.  (Another reason to use bracket syntax!)

Simon


| -----Original Message-----
| From: glasgow-haskell-bugs-bounces at haskell.org
[mailto:glasgow-haskell-bugs-
| bounces at haskell.org] On Behalf Of SourceForge.net
| Sent: 09 October 2003 18:51
| To: noreply at sourceforge.net
| Subject: [ ghc-Bugs-820778 ] Malformed Predicate w/ Template Haskell
| 
| Bugs item #820778, was opened at 2003-10-09 12:51
| Message generated for change (Tracker Item Submitted) made by Item
Submitter
| You can respond by visiting:
|
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=820778&grou
p_id=8032
| 
| Category: Compiler (Type checker)
| Group: 6.0.1
| Status: Open
| Resolution: None
| Priority: 5
| Submitted By: Jon Cast (jcast)
| Assigned to: Nobody/Anonymous (nobody)
| Summary: Malformed Predicate w/ Template Haskell
| 
| Initial Comment:
| The following two modules:
| 
|  --- Cut ---
| 
| module ScratchTemplates where
| 
| import Language.Haskell.THSyntax
| 
| newtype Interaction a = Interaction a deriving Show
| 
| ret = Interaction
| 
| instance Monad Interaction where
|   return = Interaction
|   Interaction x >>= f = f x
| 
| instance Functor Interaction where
|   fmap f x = x >>= return . f
| 
| interactionT t = tcon (TconName "Interaction") `tapp` t
| 
| class Flatten a b | a -> b where
|   flatten :: a -> b
| 
| flattenT :: TypQ -> TypQ -> TypQ
| flattenT t t' = tvar "Flatten" `tapp` t `tapp` t
| 
| baseType t = sequence [
|   inst (return []) (flattenT t t)
|        [val (pvar "flatten") (normal (var "id")) []],
|   inst (return []) (flattenT (interactionT t)
| (interactionT t))
|        [val (pvar "flatten") (normal (var "id")) []]]
| 
| instance Flatten a b =>
|          Flatten (Interaction (Interaction a))
| (Interaction b) where
|   flatten a = a >>= id >>= return . flatten
| 
| module Main where
| 
| import Monad
| import ScratchTemplates
| import Language.Haskell.THSyntax
| 
| $(baseType (tcon (Tuple 0)))
| 
| instance Flatten String String where
|   flatten a = a
| 
| instance Flatten (Interaction String) (Interaction
| String) where
|   flatten a = a
| 
| instance Flatten b c => Combine String b (String, c) where
|   combine a b = liftM2 (,) a (fmap flatten b)
| 
| instance Flatten Int Int where
|   flatten a = a
| 
| class Combine a b c | a b -> c where
|   combine :: Interaction a -> Interaction b ->
| Interaction c
| 
| instance Combine () b b where
|   combine a b = b
| 
| instance Flatten b c => Combine Int b (Int, c) where
|   combine a b = do
|     x <- a
|     y <- fmap flatten b
|     return (x, y)
| 
| instance (Flatten b c, Combine a c d) =>
|           Flatten (Interaction a, Interaction b)
| (Interaction d) where
|   flatten (a, b) = combine a (fmap flatten b)
| 
| main = main
| 
|  --- Cut here ---
| 
| loaded into ghci -fglasgow-exts produce the following
| error message:
| 
| Compiling ScratchTemplates ( ScratchTemplates.hs,
| interpreted )
| Compiling Main             ( scratch.hs, interpreted )
| ghc-6.0.1: panic! (the `impossible' happened, GHC
| version 6.0.1):
| 	Malformed predicate
| 
| Please report it as a compiler bug to
| glasgow-haskell-bugs at haskell.org,
| or http://sourceforge.net/projects/ghc/.
| 
| 
| ----------------------------------------------------------------------
| 
| You can respond by visiting:
|
https://sourceforge.net/tracker/?func=detail&atid=108032&aid=820778&grou
p_id=8032
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