Haskell Function Signature

filter :: (a -> Bool) -> [a] -> [a]

This is saying:

• filter is the identifier of a function.
• :: is the type annotation.
• (a -> Bool) is the first argument. a means it can be any type. So this is saying: a function that takes an argument of type a and return a Boolean.
• -> is the type separator.
• [a] is the second argument. A list of elements of type a.
• -> another type separator, this time the return type of filter.
• [a] means a list of type a.