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?