Testing on the Haskell track

Learn how to test your Haskell exercises on Exercism

Running Tests

Execute the following command in your exercise directory to run the tests:

stack test

To run the tests on every file save, run this command instead:

stack test --file-watch

If you get an error message like this...

No .cabal file found in directory

You are probably running an old stack version and need to upgrade it.

Otherwise, if you get an error message like this...

No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...

Just do as it says and it will download and install the correct compiler version:

stack setup

Solving the exercise

A solution to an exercise is a Haskell module. The module's name is defined in an import statement in the test suite, usually at the beginning:

import Test.Hspec        (Spec, describe, it, shouldBe)
import Test.Hspec.Runner (configFailFast, defaultConfig, hspecWith)

import ModuleName (someFunc)

main :: IO ()
main ...

In this example, the test file imports, in line 4, a module named ModuleName, that exports a function named someFunc. This means you have to create a file named ModuleName.hs that would be something like this:

module ModuleName (someFunc) where

someFunc :: ...
someFunc ...

You'll a find a file with this name already in place in src/, which you can use as a starting point for your solution.

Just keep in mind that this stub is there just for you to get started. Feel free to change it completely if you think it is the right thing to do.

Using packages

If you want to use some packages to write a more elegant solution, first add the packages to your solution's dependencies in package.yaml:

library:
  exposed-modules: ModuleName
  source-dirs: src
  dependencies:
    - foo
    - bar

While working locally, you can use any packages you like. The online test runner, however, has access to only a limited set of packages. The available packages are

  • the ones that come pre-installed with GHC (most notably base, containers, array, time, mtl, deepseq, exceptions, bytestring), plus
  • those pre-built for the test runner, listed at haskell-test-runner/pre-compiled/package.yaml.

If you submit a solution that uses a package that is not available, then the test runner will run into its lack of internet access and produce an error that looks like this:

Cabal file info not found for unavailable-package-1.0@..., updating
Selected mirror https://hackage.haskell.org/
Downloading root
HttpExceptionRequest Request {
  host                 = "hackage.haskell.org"
  ...
}
 (ConnectionFailure ...)

Running GHCi

If you want to play with your solution in GHCi, just run the command:

stack ghci

HLint (optional)

It's highly recommended that you run hlint on your solution to see if it catches something you are missing:

hlint ModuleName.hs

You don't have to accept all the suggestions given by hlint, but usually they will help you learn how to write beautiful, idiomatic Haskell code.