[Haskell-beginners] Question on : usage

[Angus Comber]

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?
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