CODEWITHSUNDEEP
functionhaskellfunction-composition
Joachim Breitner
Joachim Breitner

Reputation: 25763

Most idiomatic implementation of `[a -> a] -> (a -> a)`

If I have a list of functions, each of the type a -> a for some type, what is the most shortest, elegant and idiomatic way to combine them; preferably without adding extra dependencies?

Some variants include

foo (x:xs) = x . (foo xs)
foo [] = id

and

foo = foldr (.) id

and

foo = appEndo . mconcat . map Endo

but for some reason I’m expecting to find something nicer.

Upvotes: 4

Views: 496

Answers (2)

Ørjan Johansen
Ørjan Johansen

Reputation: 18189

Another one, which may not be shorter than foldr (.) id but which I think is cute:

foo = flip (foldr id)

Upvotes: 6

daniel gratzer
daniel gratzer

Reputation: 53871

I'd say you're not going to beat

comp = foldr (.) id

Why? Well we have a list of things and we're trying to reduce it in a right associative way.

If you look at the implementations of and, sum, maximum and similar, you'll see that this is how they're implemented in the standard library, I don't think you get more idiomatic than that :)

Tangent: I hesitate to add the foldr1 variant mentioned in comments because I'd say that it's unexpected behavior for this to be partial, unlike say maximum where it clearly must be.

Upvotes: 19

Related Questions