I can't run your code to be sure, but I think your problem is the ==.
It implies that whatever a is, it has an Eq constraint, but none of
your type signatures imply that. Try this:
case pop EmptyStack) of
(Nothing, EmptyStack) -> blah
otherwise -> do something else or nothing
That way you are using pattern matching instead of equality testing.
On Sat, Jul 30, 2011 at 4:19 PM, Ovidiu Deac
I'm playing with Haskell so I wrote a stack module (see the code below). I have a problem with the pop function which returns a tuple (Nothing, EmptyStack) if called with an EmptyStack.
I kind of understand that the compiler cannot cannot figure out what type to use for a. But how could I tell the compiler that if the list is empty I don't care about that type?
Thanks, Ovidiu
///////////////// This is the hspec ... it "pop empty stack gives Nothing" ( (pop EmptyStack) ≡ (Nothing, EmptyStack)) ...
This is the code: module Stack where import Prelude
data Stack a = EmptyStack | StackEntry a (Stack a) deriving(Show, Eq) ... pop :: Stack a → (Maybe a, Stack a) pop EmptyStack = (Nothing, EmptyStack) pop (StackEntry a s) = ((Just a), s)
...and this is the error I get: test/TestStack.hs:20:28: Ambiguous type variable `a0' in the constraint: (Eq a0) arising from a use of `==' Probable fix: add a type signature that fixes these type variable(s) In the second argument of `it', namely `((pop EmptyStack) == (Nothing, EmptyStack))' In the expression: it "pop empty stack gives Nothing" ((pop EmptyStack) == (Nothing, EmptyStack)) In the second argument of `describe', namely `[it "empty stack is empty" (isEmpty EmptyStack), it "non-empty stack is not empty" (not (isEmpty (push 10 EmptyStack))), it "push then pop retrieves the same value" ((pop $ push 10 EmptyStack) == (Just 10, EmptyStack)), it "push push then pop retrieves the last value" ((pop $ push 2 (push 1 EmptyStack)) == (Just 2, (push 1 EmptyStack))), ....]' make: *** [test] Error 1
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