HUnit TestCase with a Type Error
I've written a function similar to LISP's flatten
:
data NestedList a = Elem a | List [NestedList a]
flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (List xs) = concatMap flatten xs
These two test-cases work fine:
test1 = TestCase (assertEqual "test singleton" [5] (flatten (Elem 5)))
test2 = TestCase (assertEqual "test mixed"
[1,2,3,4]
(flatten (List [Elem 1,
List [Elem 2, Elem 3],
Elem 4])))
However this one reports a type error:
test3 = TestCase (assertEqual "test empty" [] (flatten (List [])))
Testing from the REPL works fine:
*Main> [] == flatten (List [])
True
Why am I getting an error, and how should I write a test-case for an empty list?
EDIT: Here is the exact error message:
Ambiguous type variable `a0' in the constraints:
(Show a0) arising from a use of `assertEqual'
at D:\haskell\sou开发者_StackOverflow中文版rce.hs:61:19-29
(Eq a0) arising from a use of `assertEqual'
at D:\haskell\source.hs:61:19-29
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `TestCase', namely
`(assertEqual "test empty" [] (flatten (List [])))'
I'm guessing the problem is the compiler can't figure out the type of the empty list. It could be [Int]
, [Char]
, anything really. Try giving it some type, which type exactly doesn't matter.
test3 = TestCase (assertEqual "test empty" ([] :: [Int]) (flatten (List [])))
Note in the other cases the compiler can deduce the type of list.
As the error message already tells you, you have to add a type signature to the list in the units test. Possibly like this:
test3 = TestCase (assertEqual "test empty" ([]::[()]) (flatten (List [])))
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