Question

I am testing a function called extractions that operates over any list.

extractions :: [a] -> [(a,[a])] extractions [] = [] extractions l = extract l []     where extract [] _ = []           extract (x:xs) prev = (x, prev++xs) : extract xs (x : prev) 

I want to test it, for example, with

import Test.QuickCheck.Batch     prop_len l = length l == length (extractions l) main = runTests 'extractions' defOpt [run prop_len] 

But this won’t compile; I have to supply a type either for run or prop_len, because QuickCheck can’t generate [a], it has to generate something concrete. So I chose Int:

main = runTests 'extractions' defOpt [r prop_len]     where r = run :: ([Int] -> Bool) -> TestOptions -> IO TestResult 

Is there any way to get QuickCheck to choose a for me instead of having it specified in the type of run?

Answer 1

The quickcheck manual says "no":

Properties must have monomorphic types. `Polymorphic’ properties, such as the one above, must be restricted to a particular type to be used for testing. It is convenient to do so by stating the types of one or more arguments in a

where types = (x1 :: t1, x2 :: t2, …)

clause…