I was wondering why replace is not in base. I thought, it's because it must be easy to implement with standard combinators. Although I do not like prefix syntax of Applicative I wrote it this way:

replace a b = map (bool <$> id <*> (const b) <*> (== a))

And was slightly surprised after seeing replace in Data.List.Utils:

replace old new l = join new . split old $ l

Are there non-opinion based reasons to prefer one than the other in public projects?


1 Answer 1


Your code is not a replacement for replace (pun intended). Its type signature is wrong. Your code has type Eq b => b -> b -> [b] -> [b], but replace should have type Eq a => [a] -> [a] -> [a] -> [a].

<$> and <*> are infix, not prefix.

I'd prefer this to your implementation:

replace' :: Eq b => b -> b -> [b] -> [b]
replace' a b = map (\x -> if (a == x) then b else x)

It does the exact same thing in the exact same way, but is simpler and more readable (even a novice Haskell programmer who has never heard of bool or <$> or <*> can read it). Even more important: it has a type signature.

If you'd decided to cut-and-paste the type signature of the function you wanted to replicate before starting, you would have found out that your version doesn't work as a replacement at compile time -- and if you'd looked at and thought about the type signature, you probably would have wondered why the extra []s were there, and figured it out even earlier.


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