Gather first letter from each string in a list of strings

Question

I'm learning Haskell on my own, and I'm wondering if this is idiomatic Haskell.

In particular: is it a common pattern to "wrap" the result in a list in order to make a function total? (that's what I ended up doing to ensure the function was total and no compiler warnings about not matching a pattern).

safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead (x:_) = Just x
firstLetters :: [String] -> [Char]
firstLetters ss = concat $
 map (\s -> case safeHead s of
 Nothing -> []
 Just c -> [c]) ss

Answer 1

Well a few thoughts, of all String = [Char] so we could really give the signature

firstLetters :: [String] -> String

Whether you think this is better is a matter of taste. Second, you should take a look at

Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]

It's exactly what you'd expect, it applies a function to Maybes and prunes out the Nothings. This simplifies the slightly ugly concat bit.

 firstLetters = mapMaybe safeHead

Much clearer :) In the interest of teaching you to fish, here's how I found the name of mapMaybe

• j-a: When you say "is a matter of taste" is that a 50/50 divide in the Haskell community, or more like a 90/10 towards using String?
• daniel gratzer: @j-a 50/50. In general you should prefer String when you're using well strings. In all other cases it's entirely up to you!

Answer 2

In addition to @jozefg's answer, Data.Maybe also contains listToMaybe.

listToMaybe :: [a] -> Maybe a

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Which reduces the code to

firstLetters = mapMaybe listToMaybe