On Mon, Dec 23, 2013 at 8:55 AM, Angus Comber
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> ???
x is an element. xs is a list of those elements. x : xs prepends a single element to a list. x : xs : y would attempt to prepend a single element *and* a list to something else --- but if you deconstructed a list to get x and xs, then x and xs are not the same type (since xs's type is that of a list of x). Perhaps the thing to understand is that in Haskell, a list is built up of elements all of the same type using (:): [x,y,z] is the same as x : y : z : [] (If you're familiar with Lisp, (:) is exactly cons and [] is nil.) But because Haskell is strictly typed, a list must contain elements all the same type. So you can't have y in that be a list of the same type as x, it must be a value the same type as x. Maybe you are looking for (++) instead? But note that that takes lists, not items, so it would have to be ([x] ++ xs ++ ...). -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net