Question

Is there a nice way to find the first occurrence of a constructor in a list, without the explicit recursion in the sample below?

data Elem = A Int | B Char deriving Show

getA :: [Elem] -> Maybe Elem
getA [] = Nothing
getA (e:es) = 
    case e of 
        A a -> Just (A a)
        _   -> getA es

Answer 1

Simply

import Data.Maybe (listToMaybe)
getA xs = listToMaybe [e | e@(A _) <- xs]

Addendum: even better, future-proofed using an empty record pattern (kudos hammar):

getA xs = listToMaybe [e | e@(A{}) <- xs]

Note however, that this only works out so neatly for matching constructors. For general properties, find is nicer:

get prop xs = listToMaybe [e | e <- xs, prop e]
get prop xs = listToMaybe (filter prop xs)
get prop xs = find prop xs