Re-write 'map intToDigit' with a fold…

Question

So this one seems like it should be super-simple... but I\'m not sure where to stick the \'fold\' in (obviously you could fold either way)...

It says \"write a function

Answer 1

It is not difficult, think about the definition foldr (or foldl) of List:

 foldr::(a -> b -> b) -> b -> [a] -> b

Here (a->b->b) is the step function which will be applied on each element of list [a], b is your target.

Now, you have a list of Int ([Int]), and need to convert to [Char] (or String).

Relpace [a] by [5,2,8,3,4], b by []::[Char] (your target) and (a->b->b) by step function :

foldr step ([]::[Char]) [5,2,8,3,4]

We have known that step function has type (a->b->b), specifically, (Int->[Char]->[Char]), the job need to do just convert Int to [Char], as mentioned in question: intToDigit can be helped to convert Int to Char and (:) operator can append element at the head of List so:

step x s = intToDigit x : s

where s is [Char] (or String), put them all:

foldr (\x s->intToDigit x : s) [] [5,2,8,3,4]