How to produce list of all integers within two integers (minimum and maximum) in Haskell?

Question

I'm new to Haskell and trying to see how I can write function that will take two integers and produce a list of all integers within that range.

Here's what I have so far but no luck.

rangeList :: Integer -> Integer -> [Integer]
rangeList n m
 | n == m = []
 | otherwise = n : n + (rangeList n (m-1))

Answer 1

What you aim to accomplish already exists. If both bounds are inclusive, this is:

rangeList :: Enum a => a -> a -> [a]
rangeList n m = [n .. m]

or if the upperbound is exclusive, you can make use of pred :: Enum a => a -> a:

rangeListExcl :: Enum a => a -> a -> [a]
rangeListExcl n m = [n .. pred m]

The main problem with your own implementation is that n + rangeList n (m-1) makes not much sense, since rangeList n (m-1) will be a list of elements, and n is here an Integer, so you can not add these together. Even if you could doo that, it would still not work correctly, since the rangeList n (m-1) would make a sequence between n and m-1, by adding n to that result, you get a range between 2*n and n+m-1. The recursion shoud increment the upperbound with one, for example with:

rangeListExl :: (Ord a, Num a) => a -> a -> [a]
rangeListExcl n m
    | n >= m = []
    | otherwise = n : rangeListExcl (n+1) m

Answer 2

Your rangeList function already exists in the Prelude, under the name enumFromTo.