On Sun, May 1, 2016 at 10:58 AM, Michael Walker
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.
Even when talking about term rewriting, reducing an application of two normal forms does not necessarily yield a normal form directly.
- 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.
Requiring the body to be in normal form would actually be normal form. Head normal form is actually a weaker condition, where the body must not be a redex. But the distinction is rather academic, because neither of these can easily be achieved in GHC. Anyhow, I would suggest not getting hung up on what 'normal form' means, because it is actually just a bad name for what is going on once functions are involved. Really, that's why the class is named `NFData` in my mind, because talking about what it does as being the 'normal form' only really makes sense when you're talking about pure, sum-of-products algebraic data, and functions are not that. The more important question is, what is desirable behavior, and why? Why would enumerating all possible results of a function and deep seqing them be the desired behavior of deep seqing a function? It doesn't necessarily, for instance, have the sort of, 'pull everything into memory to free another scarce resource,' effect mentioned, because functions don't work that way. I would guess that the main argument for this behavior is to say that it is the only 'allowed' behavior, but, being useless, it should just be removed. But it would be better to argue that the instance should be removed directly. -- Dan