haskell - Positive integer type

Question

In many articles about Haskell they say it allows to make some checks during compile time instead of run time. So, I want to implement the simplest check possible - allow a function to be called only on integers greater than zero. How can I do it?

Answer 1

module Positive (toPositive, Positive(unPositive)) where

newtype Positive = Positive { unPositive :: Int }

toPositive :: Int -> Maybe Positive
toPositive n = if (n < 0) then Nothing else Just (Positive n)

The above module doesn't export the constructor, so the only way to build a value of type Positive is to supply toPositive with a positive integer, which you can then unwrap using unPositive to access the actual value.

You can then write a function that only accepts positive integers using:

positiveInputsOnly :: Positive -> ...