On 12/23/2013 08:55 AM, Angus Comber wrote:
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
The colon operator (:) takes a single element on the left, and a list on the right. In, testconcat [(x:xs), (y:ys)] = x : xs : y : ys you've got lists on both the left and the right. This would work: testconcat [(x:xs), (y:ys)] = x : ( y : (xs ++ ys) ) (note: not the same function as you wanted!) since it keeps all of the single-elements on the left-hand side of (:). You're going to need to use (++) in at least one place, though, since you need to combine the lists 'xs' and 'ys' somehow. The function you really want is, testconcat [(x:xs), (y:ys)] = (x:xs) ++ (y:ys) but of course this is just, testconcat [l1, l2] = l1 ++ l2