Joining a foldable collection of Monoids by some separator element.

JoinMonoid.hs

import Data.Maybe
import Data.Monoid

-- | Join a Foldable collection of Monoids by a separator element.
join :: (Monoid a, Foldable t) => a -> t a -> a
join sep = fromMaybe mempty . fst . foldr combine (Nothing, True)
  where combine each (into, isFirst) = if isFirst
                                       then (Just each, False)
                                       else (Just each <> Just sep <> into, False)

Results.hs

join ", " []              -- => ""
join ", " ["a"]           -- => "a"
join ", " ["a", "b"]      -- => "a, b"
join ", " ["a", "b", "c"] -- => "a, b, c"