I would like to create my own list in Haskell where I could put in the number 6 and get the result [1,2,3,4,5,6]
I thought I could write something like
ones :: Int -> [a]
ones 0 = []
ones n = [n, n-1 ... n==0]
Could someone help me?
1 Answer
There are many ways to do this. Below a non-exhaustive list.
You can use Haskells list ranges:
ones :: (Enum n, Num n) -> n -> [n]
ones n = [1 .. n]
You can use the enumFromTo :: Enum a => a -> a -> [a] function:
ones :: (Enum n, Num n) -> n -> [n]
ones = enumFromTo 1
Or we can use explicit recursion:
ones :: (Ord n, Num n) => n -> [n]
ones n = go 1
where go i | i <= n = i : go (i+1)
| otherwise = []
Or by using iterate :: (a -> a) -> a -> [a] and take :: Int -> [a] -> [a]:
ones :: Num n => Int -> [n]
ones n = take n (iterate (1+) 1)
Note that not all approaches have the same signature. Some implementations do not require the numerical type to be an instance of the Enum type class, which can make the function more flexible in the sense that numerical types that can not be enumerated, can still get processed.
ones n = [1..n]ones?