Question

I am writing a test for a binary search function I wrote.

module Tests where

import Data.List (sort)
import Test.QuickCheck
import BinarySearch (binarySearch)

prop_equals_elem x xs = (binarySearch x $ sort xs) == (x `elem` xs)

args = Args {replay = Nothing, maxSuccess = 200, maxDiscard=200, maxSize=200, chatty = False}

main = do
    quickCheck (prop_equals_elem :: (Ord a) => a -> [a] -> Bool)

It works well using quickCheck in ghci but when I try to run main it gives the error

Tests.hs:12:5:
    Ambiguous type variable `a0' in the constraints:
      (Arbitrary a0) arising from a use of `quickCheckWith'
          at Tests.hs:12:5-18
      (Show a0) arising from a use of `quickCheckWith'
          at Tests.hs:12:5-18
      (Ord a0) arising from an expression type signature
          at Tests.hs:12:26-72

Why does this not work in main but does in ghci?

Answer 1

This is likely caused by the extended defaulting rules in GHCi.

When testing a function like this, you need to use a concrete element type. GHCi will default the element type to () because of the extended rules, but this will not happen when compiling the code normally, so GHC is telling you that it cannot figure out which element type to use.

You can for example use Int instead for the test. () is pretty useless for testing this function, as all elements would be the same.

quickCheck (prop_equals_elem :: Int -> [Int] -> Bool)

If it works for Int, it should work for any type due to parametricity.