Strange error with QuickCheck property

[marcosh]

If I try to copy the example property from the documentation

-- |
-- prop> \xs -> sort xs == (sort . sort) (xs :: [Int])

I get the following error message when I run stack test

Progress 1/2: haskell-training/home/marcosh/projects/tweag/haskell-training/src/Forms.hs:52: failure in expression `\xs -> sort xs == (sort . sort) (xs :: [Int])'

<interactive>:44:2: error:
    • No instance for (Show ([Int] -> [Int]))
        arising from a use of ‘Test.QuickCheck.Test.quickCheck’
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: Test.QuickCheck.Test.quickCheck doctest_prop
      In an equation for ‘it’:
          it = (Test.QuickCheck.Test.quickCheck doctest_prop)

[martijnbastiaan]

It looks like this error is caused by sort being out of scope. Note entirely sure why this error message pops up, but adding the following setup statement works:

{- $setup
>>> import Data.List (sort)
-}

[marcosh]

oh, I see! Thanks!

Probably sort, if not imported, is interpreted as a value to be generated.

It's probably enough to clarify this in the documentation

[martijnbastiaan]

Makes sense. I'll change the example to something in scope by default too, like abs.