Because the type of cons (:) is a -> [a] -> [a].
Thus, when you do list : something else, you get a type mismatch. You are
basically giving it [a] -> [a] -> [a].
You got yourself a type violation.
On Mon, Dec 23, 2013 at 7:25 PM, 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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