[Haskell-cafe] Why does "instance Ord Pat" causes <<loop>>

[Luke Palmer]

On Mon, Dec 8, 2008 at 2:04 AM, Martin Hofmann <
martin.hofmann at uni-bamberg.de> wrote:

> I am storing the TH data types 'Exp' and 'Pat' in Maps and Sets. As a
> first attempt to quickly get rid of typechecker's complaints I defined
> some naive instances of Ord for Exp and Pat.
>
> Now it took me about a week to realise, that 'instance Ord Pat' causes
> ghc to loop. Apparently, there is a reason, why Pat and Exp are not
> instances of Ord. What is so bad about it?
>
> If Pat and Exp should not be instances of Ord, maybe a note in the
> source code or haddock would be helpful. On the other hand, what would
> argue against a lexicographic ordering (apart from the inefficient
> implementation of the following one)?
>
> Following some literate code to reproduce the <<loop>> (or stack
> overflow in GHCi), by commenting and uncommenting the appropriate lines:


Try this:

data Foo = Foo deriving Eq

instance Ord Foo

Then try Foo < Foo.

instance Ord Foo is not the same as "deriving Ord"; it declares an instance
using all default definitions, which are self-referential.

It would be nice if typeclass authors could somehow declare the minimal
complete definition, so we could get a warning in this case.

Luke


>
>
>
>
> > {-# OPTIONS_GHC -fglasgow-exts -fth #-}
> > module Test where
> > import Language.Haskell.TH
> > import Data.Set
>
>
> -------------------
>  naive Ord
>
> > instance Ord Exp
>
> > instance Ord Pat
>
> -------------------
>  lexicographic Ord
>
>  instance Ord Exp where
>     compare l r = compare (show l) (show r)
>
>  instance Ord Pat where
>     compare l r = compare (show l) (show r)
>
> -------------------
>
>
> > mkVP s = VarP $ mkName s
> > mkVE s = VarE $ mkName s
> > rule1 = (,) [mkVP "x_14"] (mkVE "y_14")
> > rule2 = (,) [InfixP (mkVP "x1_15") '(:) (mkVP "x_16")] (InfixE (Just
> (mkVE "y1_15")) (ConE '(:)) (Just (mkVE "ys_16")))
>
> > stack_overflow = fromList [rule1,rule2]
>
>
> Thanks,
>
> Martin
>
>
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