"Rijk J. C. van Haaften" <rjchaaft@cs.uu.nl> wrote:

Recently, I wrote a function similar to

x :: a x = x 42

which is type-correct (Hugs, Ghc, THIH). Still, from the expression it is clear that the type shoud have a function type. The definition

x :: a -> b x = x 42

is equally well accepted, though I can't see why this type would be correct. (I'd expect it to be too general.)

For what reasons are these types considered correct?

When you say x :: a you are asking that the compiler check that everything you say about x is consistent with x being acceptable where /any/ type is required. In the application x 42, it requires that x be a function, which is fine, because x has any type, and this includes functions. When you say x = x 42, this requires that the type returned from x 42 is the same as the type of x, again fine because if x::a, then x:: Integer -> a also, so x 42:: a. It works out in practise because x = x 42 gives x the value bottom, and bottom::a for all a. Jón -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk 31 Chalmers Road jf@cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)