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?
Oh, to answer this, my guess is that such an instance is just kind of silly. It is a meaningless, arbitrary ordering, and is brittle to splitting/combining cases.
To put them in sets and maps, go ahead an define an arbitrary ordering however you can, but wrap it in a newtype like this:
newtype OrdExp = OrdExp Exp
instance Ord OrdExp where
compare (OrdExp a) (OrdExp b) = compare (show a) (show b)
An orphan instance is one which is defined in a module where neither the class nor the type being instantiated is defined. This newtype wrapping avoids orphan instances, and associates the arbitrary ordering to your own wrapper so if somebody else defined a different arbitrary ordering, they won't conflict.
Orphan instances (almost?) always indicate a nonmodular design decision.
Luke
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:
> {-# 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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