I'am learning haskell and I don't realy understand function definitions, for example:
module TestModule where
iter :: Int -> (a -> a) -> (a -> a)
iter n f
| n>0 = f . iter (n-1) f
| otherwise = id
iter_2 n f = foldr (.) id (replicate n f)
What I suppose to give this function? I have loaded this function via console and don't really understand how to use it.
My question how to use this function via console. Like "iter 5 ???"
read the type of the function as
iter :: Int -> (a -> a) -> a -> a
^^^ ^^^^^^^^ ^ ^
arg arg arg result
meaning if you provide an Int, a (a->a) function, and an a, it will give you back another a. Here a stands for any type. For example
iter 4 (+1) 0
will be 4, that is performs (+1) on 0 four times.
You need to give it a function, more specifically, an endofunction (mapping any type to itself). For example,
*Main> iter 3 sqrt 256
2.0
Of course, this can also be a custom function, like
*Main> let f x = x+2
*Main> iter 3 f 7
13
or even an anonymous one defined right in the invocation:
*Main> iter 3 (\str -> "("++str++")") "..."
"(((...)))"
