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elm-verify-examples Build Status

Verify examples in your docs.

ℹ️ This was formerly known as elm-doc-test.

⚠️ This is not a replacement for tests, this tool should be used for improving your documentation.


$ npm i elm-test -g
$ npm i elm-verify-examples -g
$ elm-test init


$ touch tests/elm-verify-examples.json

elm-verify-examples.json contains information on which files contain verified examples and where to find them.

  "root": "../src",
  "tests": ["Mock", "Mock.Foo.Bar.Moo", "./"]

It's recommended to add ./tests/VerifyExamples to your .gitignore.

Writing Verified Examples

Verified examples look like normal code examples in doc-comments.
Code needs to be indented by 4 spaces. You can specify the expected result of an expression, by adding a comment --> (the > is important) and an expected expression.

{-| returns the sum of two int.

    -- You can write the expected result on the next line,

    add 41 1
    --> 42

    -- or on the same line.

    add 3 3 --> 6


add : Int -> Int -> Int
add =

Multiline Examples

You can write examples on multiple lines.

{-| reverses the list

        [ 41
        , 1
    --> [ 1
    --> , 41
    --> ]

    rev [1, 2, 3]
        |> toString
        |> String.concat
    --> "321"


rev : List a -> List a
rev =


You can specify imports, if you want to use a module or a special test util.


    import Dict

    myWeirdFunc (Dict.fromList [(1, "a"), (2, "b")]) [2, 1]
    --> "ba"


Intermediate Definitions

You can use intermediate definitions in your example. :information: Unused functions don't get added to the test. This is useful if you wanna add incomplete examples to your docs. ⚠️ Intermediate definitions need a type signature!


    isEven : Int -> Bool
    isEven n =
        remainderBy 2 n == 0

    List.Extra.filterNot isEven [1,2,3,4] --> [1,3]


filterNot : (a -> Bool) -> List a -> List a

Types in Examples

You can define union types and type aliases in your examples.

{-| With a union type in the example.
type Animal
= Dog
| Cat

    double Dog
    --> (Dog, Dog)


double : a -> ( a, a )
double a =
    ( a, a )
{-| With a type alias in the example.

    customTypeAlias defaultUser "?"
    --> "?Luke"

    type alias User =
        { id: Int -- ID
        , name: String

    defaultUser : User
    defaultUser =
        { id = 1
        , name = "Luke"

    customTypeAlias defaultUser "_"
    --> "_Luke"


customTypeAlias : { a | name : String } -> String -> String
customTypeAlias { name } prefix =
    prefix ++ name

Examples in markdown files

You can also verify code example in markdown files (such as your README). To do so, add the file's path to your elm-verify-examples.json file and write your example using the sames rules as above (no need for 4-space indentation here).

This is my README!
It explains how the `Documented` module works:

import Documented

Documented.two --> 2

Verify Examples

elm-verify-examples converts your verify-examples into elm-tests, and optionally runs them using elm-test. To only generate the test files in tests/VerifyExamples/:

$ elm-verify-examples

This is useful if you want to run your tests using different runner than elm-test, e.g. elm-coverage. If you also want to run the generated tests:

$ elm-verify-examples --run-tests

Note that this way the test files will be removed after they are ran.

By default, this command looks for the config file at tests/elm-verify-examples.json. If you want it to load a specific config file use the --config argument (e.g. elm-verify-examples --config my/custom/path/elm-verify-examples.json will read the config from my/custom/path/elm-verify-examples.json).

You can run elm-verify-examples for one or more modules explicitly. They don't have to be specified in tests/elm-verify-examples.json.

$ elm-verify-examples ./src/Foo.elm ./src/Foo/Bar.elm

You can pass a custom path to elm-test if necessary.

$ elm-verify-examples --elm-test=./node_modules/.bin/elm-test
$ # or add it to your elm-verify-examples.json `elmTest: "../node....`
$ # you can also pass arguments to elm-test with --elm-test-args

It will use the elm-test installed with this package.


You can run the examples using:

npm start