Haskell recursively applying function to every element but first?

Question

So I've been doing this program which receives a function f, a number a and a list b and it should return a list [a, f(a,b), f(f(a,b),b, ..] iterating through the list b and using recursion. Do you guys know how I can optimize my code?

calculate :: (a -> b -> a) -> a -> [b] -> [a]
help :: (a -> b -> a) -> a -> [b] -> [a]

help f a (x:xs) = (f a x) : (calculate f (f a x) xs)
help f a [] = []

calculate f a b = a : (help f a b)

"Do you guys know how to optimise my code?" What do you mean by "optimise"? Have you timed its performance? What are you asking? — AJF
– AJF, Commented Jan 16, 2019 at 20:17

Will Ness · Accepted Answer · 2019-01-17 05:57:08Z

2

calculate f a b = tail . concatMap (replicate 2) . scanl f a $ b.

The replicate bit is probably in error. If so, then simply calculate = scanl.

This translates the code, as the "[a, f(a,b), f(f(a,b),b, ..]" from the text contradicts it (and it contradicts the text itself, which talks of "iterating through the list b").

edited Jan 17, 2019 at 5:57

answered Jan 16, 2019 at 20:39

Will Ness

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2 Comments

codingbudgie Over a year ago

This works perfectly fine if i use it as calculate f a b = tail . concatMap (replicate 1) . scanl f a $ b

Will Ness Over a year ago

OK, but concatMap (replicate 1) is the same as id and id . f is the same as simply f itself. And with replicate 1 you don't need the tail either.

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