99 questions/Solutions/22

Create a list containing all integers within a given range.

range x y = [x..y]

or

range = enumFromTo

or

range x y = take (y-x+1) \$ iterate (+1) x

or

range start stop
| start > stop  = reverse (range stop start)
| start == stop = [stop]
| start < stop  = start:range (start+1) stop

The following does the same but without using a reverse function

range :: Int -> Int -> [Int]
range n m
| n == m = [n]
| n < m = n:(range (n+1) m)
| n > m = n:(range (n-1) m)

or, a generic and shorter version of the above

range :: (Ord a, Enum a) => a -> a -> [a]
range a b | (a == b) = [a]
range a b = a:range ((if a < b then succ else pred) a) b

or with scanl

range l r = scanl (+) l (replicate (l - r) 1)

with support for both directions

range l r = scanl op l \$ replicate diff 1
where
op = if l < r then (+) else (-)
diff = abs \$ l - r

Since there's already syntactic sugar for ranges, there's usually no reason to define a function like 'range' in Haskell. In fact, the syntactic sugar is implemented using the enumFromTo function, which is exactly what 'range' should be.