On 7/10/07, Brian L. Troutwine
I'm rather new to Haskell and need, in typical newbie style, a bit of help understanding the type system.
The Nth even Fibonacci number, EF(n) can be defined by the recursive relation EF(0) = 2, EF(n) = [EF(n-1) * (phi**3)], where phi is the golden ratio and [] is the nearest integer function. An infinite lazy list of this sequence would be nice to have for my Project Euler, er, project. Defining phi thusly,
phi :: (Floating t) => t phi = (1+sqrt(5))/2
With phi in place, if I understood types properly (and if I understand iterate correctly as I think), the lazy list should be a relatively quick matter.
even_fibs :: (Num t) => [t] even_fibs = iterate (\x -> round(x * (phi**3))) 2
Dynamically typed even_fibs :: (Floating t, Integral t, RealFrac t) => [t], assuming I pass -fno-monomorphism-restriction to ghci. That's not at all the type I assumed even_fibs would take, as can be seen from above. So, I went on a bit of sojourn. Having seen the sights of the Haskell Report section 6.4, the marvels of the references cited in the wiki's article on the monomorphism restriction and the Gentle Introduction's chapter 10 I must say I'm rather more terribly confused than when I started out, possibly.
That was your first mistake :-) (As a beginner, anyway.) Look at the type of round: Prelude> :t round round :: forall a b. (RealFrac a, Integral b) => a -> b So the argument x in the lambda-expression being passed to iterate must have a type that's an instance of RealFrac. It must also have a type that's an instance of Integral, since the result of multiplying it with phi gets passed into the next iteration. Finally, it has to have a type that's an instance of Floating, since phi is declared as a Floating. You can probably see the problem. Cheers, Tim -- Tim Chevalier* catamorphism.org *Often in error, never in doubt "There's no money in poetry, but there's no poetry in money, either." --Robert Graves