Property Testing in Elm

Elm is perfectly suited to property testing, with its delightful data-in–data-out functions. Testing in Elm should super easy.

The tooling isn’t there yet, though. This post documents what was necessary today to get a property to run in Elm.

Step 1: elm-test

This includes an Elm library and a node module for a command-line runner. The library alone will let you create a web page of test results and look at it, but I want to run them in my build script and see results in my terminal.

Installation in an existing project:

elm package install deadfoxygrandpa/elm-test
npm install -g elm-test

The node module offers an “elm test init” functionality to put some test files in the current directory: TestRunner (which is the Main module for test runs[1]) and Tests.elm which holds actual tests. Personally, I found it necessary to follow the following steps as well.

• create a test directory (I don’t want tests in my project home), and move the TestRunner.elm and Tests.elm files there.
• add that test directory to the source directories in elm-package.json

Step 2: elm-check

The first thing to know is: which elm-check to install. You need the one from NoRedInk:

elm package install NoRedInk/elm-check

The next thing is: what to import. Where do all those methods used in the README live?

Here is a full program that lets elm-test execute the properties from the elm-check readme.
TL;DR: You need to import stuff from Check and Check.Producer for all properties; and  for the runner program, ElmTest and Check.Test and Signal, Console, and Task.

Name it test/Properties.elm and run it with

elm test test/Properties.elm

The output looks like

Successfully compiled test/Properties.elm
Running tests…
  1 suites run, containing 2 tests
  All tests passed

Here’s the full text just in case.

module Main (..) where
import ElmTest
import Check exposing (Evidence, Claim, that, is, for)
import Check.Test
import Check.Producer as Producer
import List
import Signal exposing (Signal)
import Console exposing (IO)
import Task

console : IO ()
console =
  ElmTest.consoleRunner (Check.Test.evidenceToTest evidence)

port runner : Signal (Task.Task x ())
port runner =
  Console.run console

myClaims : Claim
myClaims =
  Check.suite
    “List Reverse”
    [ Check.claim
        “Reversing a list twice yields the original list”
        `that` (\list -> List.reverse (List.reverse list))
        `is` identity
        `for` Producer.list Producer.int
    , Check.claim
        “Reversing a list does not modify its length”
        `that` (\list -> List.length (List.reverse list))
        `is` (\list -> List.length list)
        `for` Producer.list Producer.int
    ]

evidence : Evidence
evidence =
  Check.quickCheck myClaims

How to write properties is a post for another day. For now, at least this will get something running.

See also: a helpful post for running elm-check in phantom.js

[1] How does that even work? I thought modules needed the same name as their file name. Apparently this is not true of Main. You must name the module Main. You do not have to have a ‘main’ function in there (as of this writing). The command-line runner needs the ‘console’ function instead.