Am Sonntag 31 Mai 2009 15:37:27 schrieb Ivan Uemlianin:
Dear Andrew and Lee
Thanks for your comments.
Andrew Wagner wrote:
As for why it actually happens in this case, it's no doubt related to the particular algorithm ghci uses to do the type inference.
In particular the part where GHC assigns names to type variables.
Interesting. I tried it in hugs and it doesn't happen:
Main> :type safeSecond safeSecond :: [a] -> Maybe a Main> :type tidySecond tidySecond :: [a] -> Maybe a Main>
So, it's a property of the ghci interpreter rather than of the language itself.
Yes.
Just out of curiousity, I'd be interested to know what's going on in ghci to produce this effect. Are there different types of type variable?
There are type variables of different *kind*, e.g. in class Functor f where fmap :: (a -> b) -> f a -> f b a and b are type variables of kind * (the kind of ordinary types) and f is a type variable of kind (* -> *) (the kind of type constructors which take an ordinary type and produce another ordinary type, examples are [] and Maybe). But that has nothing to do with the phenomenon, in the inferred type signatures of safeSecond and tidySecond, the 'a' resp. 't' are both type variables of kind *. The only difference is that in one case the name supply delivered 'a' and in the other it delivered 't'.