I am playing with concatenating lists so I start with this: testconcat :: [[a]] -> [a] testconcat [[]] = [] testconcat [(x:xs)] = x : xs Now, testconcat [[]] and testconcat [[1,2]] works So now I want to try with a 2nd inner list so I do this: testconcat :: [[a]] -> [a] testconcat [[]] = [] testconcat [(x:xs)] = x : xs testconcat [(x:xs), (y:ys)] = x : xs : y : ys and I get load error: Couldn't match expected type `a' with actual type `[a]' `a' is a rigid type variable bound by the type signature for testconcat :: [[a]] -> [a] at prog_haskell.hs:218:15 In the first argument of `(:)', namely `xs' In the second argument of `(:)', namely `xs : y : ys' In the expression: x : xs : y : ys But this loads ok: testconcat :: [[a]] -> [a] testconcat [[]] = [] testconcat [(x:xs)] = x : xs testconcat [(x:xs), (y:ys)] = x : xs Even this line cannot be loaded: testconcat [(x:xs), (y:ys)] = x : xs : [] Couldn't match expected type `a' with actual type `[a]' `a' is a rigid type variable bound by the type signature for testconcat :: [[a]] -> [a] at prog_haskell.hs:218:15 In the first argument of `(:)', namely `xs' In the second argument of `(:)', namely `xs : []' In the expression: x : xs : [] Failed, modules loaded: none. if x : xs works why not x : xs : <something else> ??? Can someone please explain?