Question

i went though several Haskell learning examples but i could not figure out how to write user defined higher order functions in Haskell

if we are taking a parameter as a function how the type of the function id defined?

Answer 1

Let’s use the function map as a simple example. map takes a function and a list and applies the function to all elements of the list. If you write the signature of map, it runs like this:

First, you need a function. Any function is OK, so the type of the first argument is a -> b. Then, you need a list of input values. Since the type of the list’s elements must fit to the function’s input, the type of the list is [a]. For the output: What is the result of a function a -> b when applied to a value of type a? Right, it is b. So the result type is [b]. Assembled together, the type of our function runs like this:

map :: (a -> b) -> [a] -> [b]

And is defined like this:

map f []     = []
map f (x:xs) = f x : map f xs

Does this help you to understand the concept of high order functions?