On Mon, Dec 8, 2008 at 2:04 AM, Martin Hofmann < martin.hofmann@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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