Function Types

We know that functions in Haskell are expressions and each expression must have a type, so what types can a function have? Functions take in some arguments and create a result of some type which can be the same as some (or all) of its arguments or a different one. We have already touched upon the subject of function types in our example function triple:

triple :: Int -> Int
triple x = 3 * x

In this case, the type of our function triple is Int -> Int – it takes anIntas its only argument and returns anIntas the result. As functions are expressions, we can use them as any other type of data, for example, we can create a list of functions:

ghci> funList = [(+), (*)]
-- (+) and (*) are functions for addition and multiplication
ghci> :t funList
Num a => [a -> a -> a]

We can see that funList is a list of a certain type – specifically, a function type that takes two arguments of type Num and returns a Num type as its result.