Successfully invalidated the Validation Alternative

either.cabal

@@ -2,7 +2,7 @@ name:          either
 category:      Control, Monads
 version:       4.5
 license:       BSD3
-cabal-version: >= 1.6
+cabal-version: >= 1.8
 license-file:  LICENSE
 author:        Edward A. Kmett
 maintainer:    Edward A. Kmett <ekmett@gmail.com>
@@ -47,3 +47,14 @@ library
                    Data.Either.Validation
   ghc-options: -Wall
   hs-source-dirs: src
+
+test-suite tests
+  ghc-options: -Wall
+  type: exitcode-stdio-1.0
+  main-is: Main.hs
+  hs-source-dirs:     tests
+
+  build-depends:       base,
+                       either,
+                       hedgehog
+  default-language:    Haskell2010

stack.yaml

@@ -0,0 +1,66 @@
+# This file was automatically generated by 'stack init'
+#
+# Some commonly used options have been documented as comments in this file.
+# For advanced use and comprehensive documentation of the format, please see:
+# https://docs.haskellstack.org/en/stable/yaml_configuration/
+
+# Resolver to choose a 'specific' stackage snapshot or a compiler version.
+# A snapshot resolver dictates the compiler version and the set of packages
+# to be used for project dependencies. For example:
+#
+# resolver: lts-3.5
+# resolver: nightly-2015-09-21
+# resolver: ghc-7.10.2
+# resolver: ghcjs-0.1.0_ghc-7.10.2
+# resolver:
+#  name: custom-snapshot
+#  location: "./custom-snapshot.yaml"
+resolver: lts-9.11
+
+# User packages to be built.
+# Various formats can be used as shown in the example below.
+#
+# packages:
+# - some-directory
+# - https://example.com/foo/bar/baz-0.0.2.tar.gz
+# - location:
+#    git: https://github.com/commercialhaskell/stack.git
+#    commit: e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+# - location: https://github.com/commercialhaskell/stack/commit/e7b331f14bcffb8367cd58fbfc8b40ec7642100a
+#   extra-dep: true
+#  subdirs:
+#  - auto-update
+#  - wai
+#
+# A package marked 'extra-dep: true' will only be built if demanded by a
+# non-dependency (i.e. a user package), and its test suites and benchmarks
+# will not be run. This is useful for tweaking upstream packages.
+packages:
+- .
+# Dependency packages to be pulled from upstream that are not in the resolver
+# (e.g., acme-missiles-0.3)
+# extra-deps: []
+
+# Override default flag values for local packages and extra-deps
+# flags: {}
+
+# Extra package databases containing global packages
+# extra-package-dbs: []
+
+# Control whether we use the GHC we find on the path
+# system-ghc: true
+#
+# Require a specific version of stack, using version ranges
+# require-stack-version: -any # Default
+# require-stack-version: ">=1.6"
+#
+# Override the architecture used by stack, especially useful on Windows
+# arch: i386
+# arch: x86_64
+#
+# Extra directories used by stack for building
+# extra-include-dirs: [/path/to/dir]
+# extra-lib-dirs: [/path/to/dir]
+#
+# Allow a newer minor version of GHC than the snapshot specifies
+# compiler-check: newer-minor

tests/Main.hs

@@ -0,0 +1,54 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Main where
+
+import           Control.Applicative
+import           Data.Either.Validation
+import           Data.Monoid (Sum(..))
+
+import           Data.Functor (void)
+import           Hedgehog
+import qualified Hedgehog.Gen as Gen
+import qualified Hedgehog.Range as Range
+
+-- -- empty is a neutral element
+-- empty <|> u  =  u
+-- u <|> empty  =  u
+-- -- (<|>) is associative
+-- u <|> (v <|> w)  =  (u <|> v) <|> w
+
+genValidation :: Gen a -> Gen b -> Gen (Validation a b)
+genValidation ga gb = do
+  a <- ga
+  b <- gb
+  Gen.choice [return $ Failure a, return $ Success b]
+
+identity :: (Eq a, Show a) => (a -> a -> a) -> a -> Gen a -> PropertyT IO ()
+identity f i gen = do
+  x <- forAll gen
+  f x i === x
+  f i x === x
+
+assoc :: (Eq a, Show a) => (a -> a -> a) -> Gen a -> PropertyT IO ()
+assoc f gen = do
+  x <- forAll gen
+  y <- forAll gen
+  z <- forAll gen
+
+  let xy = f x y
+      yz = f y z
+
+  f x yz === f xy z
+
+prop_alternative :: Property
+prop_alternative = property $ do
+  let genSumInt = Sum <$> Gen.int (Range.linear 0 maxBound)
+      genVal = genValidation genSumInt genSumInt
+  identity (<|>) empty genVal
+  assoc (<|>) genVal
+
+main :: IO ()
+main =
+  void $ checkParallel $ Group "Test.Either" [
+      ("prop_alternative", prop_alternative)
+    ]