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Haskell has no for or while — there's no mutable state to advance. Iteration is recursion.

sumTo :: Int -> Int
sumTo 0 = 0
sumTo n = n + sumTo (n - 1)

This is two equations of sumTo — the compiler tries patterns top-to-bottom. The base case (sumTo 0 = 0) terminates the recursion.

List recursion uses the (x:xs) pattern (x is the head, xs is the rest):

length' :: [a] -> Int
length' []     = 0
length' (_:xs) = 1 + length' xs

In practice, you rarely write recursion by hand — the standard library has higher-order functions (map, filter, foldr, foldl) that abstract the common patterns:

sumTo n = sum [1..n]
sumTo n = foldr (+) 0 [1..n]