It depends on what you mean when you say a function is in normal form.
Unfortunately the instance doesn't capture either of the definitions
that immediately come to my mind:
- All values it returns are in normal form? This requires an NFData b
constraint.
- The body of the function is in normal form? This requires HNF, which
we don't have in Haskell. I guess the current instance is going for
this, but approximates HNF with WHNF.
I'm +1 to removing this, simply because there are at least three
(including yours) reasonable definitions. If someone wants an NFData
instance for a value which includes functions, they should know exactly
how they want to handle that.
On Sun, 1 May 2016 16:38:07 +0200 (CEST)
Henning Thielemann
According to Haddock comments, between deepseq-1.2 and deepseq-1.3 an instance for NFData on functions was introduced without previous discussion. I find this instance pretty problematic since it has no superclasses. The correct instance would be certainly something like
instance (Enumerate a, NFData b) => NFData (a -> b)
where Enumerate would be a new class that allows to enumerate all values of a type. This would be hardly useful because it is pretty inefficient. I'd prefer that the instance is removed, again, or even better, be replaced by a non-implementable instance. Alternatively we should replace it by a correct implementation with corresponding superclasses. If we do the second, then we could still omit the Enumerate instance for types where enumeration of all values of the type is too expensive.
I assume that the instance was added to simplify automatic derivation of NFData instances. However, I think it would be better if people insert custom deepseq implementation for the expected functions then. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
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