问题
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 ( intToString :: [Int] -> [Char] ) using a fold, that mimics this map:
map intToDigit [5,2,8,3,4] == "52834"
And then says, "For the conversion, use intToDigit :: Int -> Char from Data.Char."
I'm not entirely sure I get the point... but yet it doesn't seem like it should be that hard -- you're just reading in the list (folding it in, I get how folds work in general) from either the left or right and doing the conversion... but I'm not sure how to set it up.
回答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]
来源：https://stackoverflow.com/questions/53294277/re-write-map-inttodigit-with-a-fold