Hello All,

I was working on a homework problem where the task is to write the iterate function in terms of scanl. Came up with this:

myIterate f x = scanl (const.f) x (repeat x)

I went looking around for other solutions to check my work and found the following solution on the Haskell Wiki (https://wiki.haskell.org/Iterate):

iterate f x = scanl f x (repeat x)

myIterate seems to work checked against the Prelude iterate but I don't know if it's a good solution or not, because the iterate solution on the Haskell Wiki throws a type error. Types of Prelude iterate and scanl are different, so I can see why, or am I missing something? Here is some REPL output:

Prelude iterate:

λ> take 10 (iterate (+1) 1)
[1,2,3,4,5,6,7,8,9,10]

myIterate:

λ> take 10 (myIterate (+1) 1)
[1,2,3,4,5,6,7,8,9,10]

Haskell Wiki solution:

λ> take 10 (iterate' (+1) 1)

<interactive>:63:20: error:
    • Occurs check: cannot construct the infinite type: a ~ a -> a
      Expected type: a -> a -> a
        Actual type: (a -> a) -> a -> a
    • In the first argument of ‘iterate'’, namely ‘(+ 1)’
      In the second argument of ‘take’, namely ‘(iterate' (+ 1) 1)’
      In the expression: take 10 (iterate' (+ 1) 1)
    • Relevant bindings include it :: [a] (bound at <interactive>:63:1)

It will work if you pass it a function that takes two parameters though:

λ> take 10 (iterate' (+) 1)
[1,2,3,4,5,6,7,8,9,10]

Any thoughts would be much appreciated!

Andrea