Haskell#concat Implementation
You could eta-contract the definition in two places: \acc x -&gt; acc ++ x is the eta-expanded version of (++) and concat' ys = (...) ys is the eta-expanded version of concat' = (...). So that would take you to:

concat' :: [[a]] -&gt; [a]
concat' = foldl (++) []


Now, in that case it turns out that it's better to use foldr rather than foldl and there is a good write-up about concat on the haskell wiki.