How can I constrain a QuickCheck parameter to a list of non-empty Strings?

Question

I have a property that takes a list of Strings:

myProp :: [String] -> Bool

I need to constrain the inputs that QuickCheck generates so that only non-empty strings are in the list.

How can I do this?

Answer 1

You use forAll together with listOf (which generates lists) and listOf1 (which generates non-empty lists).

Examples

quickCheck $ forAll (listOf $ listOf1 arbitrary) $ myProp

-- more verbose alternative to make things clear
nonEmptyString :: Gen String
nonEmptyString = listOf1 arbitrary

quickCheck $ forAll (listOf nonEmptyString) $ myProp

Answer 2

import Test.QuickCheck.Modifiers (NonEmptyList (..))

myProp :: [NonEmptyList Char] -> Bool
myProp xs0 =
  let xs = map getNonEmpty xs0
  in ...

Answer 3

Or, from first principles (no library functions):

quickCheck $ \ h t -> let {s :: String ; s = h : t } in length s > 0

here s runs through all non-empty values.