Test.LeanCheck.Function

This module is part of LeanCheck, a simple enumerative property-based testing library.

This module exports Listable and Show function typeclass instances. These can be useful for testing higher-order properties --- properties that take functions as arguments.

> import Test.LeanCheck
> import Test.LeanCheck.Function

> check $ \f p xs -> filter p (map f xs) == map f (filter p xs :: [Int])
*** Failed! Falsifiable (after 36 tests):
\_ -> 0
\x -> case x of
      0 -> True
      _ -> False
[1]

> check $ \f p xs -> filter p (map f xs) == map f (filter p xs :: [Bool])
*** Failed! Falsifiable (after 20 tests):
\_ -> False
\x -> case x of
      False -> False
      True -> True
[True]

> check $ \f z xs -> foldr f z xs == foldl f z (xs :: [Int])
*** Failed! Falsifiable (after 75 tests):
\x _ -> case x of
        0 -> 1
        _ -> 0
0
[0,0]

Warning: this is only intended to be used in testing modules. Avoid importing this on modules that are used as libraries.

The Listable and Show function instance are defined in, respectively:

• Test.LeanCheck.Function.Listable
• Test.LeanCheck.Function.Show

The Show instance will work for all functions whose return types are instances of ShowFunction from Test.LeanCheck.Function.ShowFunction.