Panic on Malformed instance A => B => C

[br1]

instance

A => B => C where

foo = undefined

[simonpj@…]

commit dd847cb899275adb5f3250e1447198cb61875bea

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Fri Mar 30 12:43:45 2012 +0100

    Don't crash if there's a malformed instance!
    
    Fixes Trac #5951

 compiler/rename/RnSource.lhs |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

[dag]

I ran into this by leaving out the fat arrow completely; does the fix solve that case as well?

[simonpj]

I don't know without a concrete example. If you suggest a program I guess I can test it with HEAD? Or maybe you can too?

Simon

[dag]

I couldn't reproduce the issue without equality constraints, so those may be necessary:

{-# LANGUAGE FlexibleInstances, TypeFamilies, MultiParamTypeClasses #-}

import qualified Data.Set as Set

class IsList a b c where
    fromList :: [b] -> a c

-- Note the missing => before IsList here
instance (Ord b, b ~ c) IsList Set.Set b c where
    fromList = Set.fromList

*** Exception: compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,
                                                        _,
                                                        SrcLoc.L _ cls,
                                                        _)

[dag]

Actually, I suspect that it's the comma and not the equality constraint, and that multiple contexts would work as well. Otherwise, it's interpreted to be an instance declaration for the Ord class itself, and the error is that fromList is not a visible member.

[simonpj]

Happily HEAD says

T5951.hs:11:10:
    Malformed instance: (Ord b, b ~ c) IsList Set.Set b c

on your example. So yes, I think this is fixed.

[guest]

Hello,

I get the same error on GHC 7.4.2 with the following (slightly different) example:

instance ExprCoercion? a -> b where

fromExpr (FunctionExpr? fn) = fn toExpr fn = FunctionExpr? $ \expr -> toExpr $ fn (fromExpr expr)

*** Exception: compiler/rename/RnSource.lhs:430:14-81: Irrefutable pattern failed for pattern Data.Maybe.Just (inst_tyvars,

_, SrcLoc?.L _ cls, _)

[simonpj]

It's fixed in HEAD, but on in 7.4.2.

Simon

[boris]

I ran into this issue with -> arrows in GHC 7.4.1. Could you please check this code in HEAD?

-- no language extensions
class Clazz a where
  func :: a -> String

instance Clazz a -> Clazz (Bool -> a) where
  func = undefined

[simonpj]

Yes, it's fine in HEAD

simonpj@cam-05-unx:~/tmp$ ghc-stage2 -c Foo.hs

Foo.hs:9:10: Malformed instance: Clazz a -> Clazz (Bool -> a)