doctest runner against Elm-lang source files
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doctest runner against Elm-lang source files


npm install elm-doctest

It depends on elm and assumes that elm-make and elm-repl are available either via systemwide installation or npm module installation. Make sure elm-make succeeds with your elm source files.

how does it work?

It utilizes elm-repl for expression evaluation and compare the values against the expected value. (It does not comapre stringified values like haskell doctest does via GHCi outputs.)

It only evaluates the expressions that follows -- >>> (i.e. Elm comment symbol followed by space and three LT chars until end of the line) and the expression on the next line after -- .

For example, if the comment states:

-- >>> x =
-- >>>   1 + 2
-- >>> x * 2
-- 6

Then, elm-doctest asks elm-repl to evaluate the actual code section in the source file and effectively following expression:

((1 + 2) * 2) == (6)

If value reported by elm-repl is True then test passes, fail otherwise.


Usage: elm-doctest [--watch] [--help] [--elm-repl-path PATH]
                   [--elm-make-path PATH]
                   [--pretest CMD] FILES...
  run doctest against given Elm files

Available options:
  -h,--help             Show this help text
  --pretest CMD         command to run before doc-test
  --elm-repl-path PATH  Path to elm-repl executable
  --elm-make-path PATH  Path to elm-make executable
  -w,--watch            Watch and run tests when target files get updated



module ModuleTobeTested exposing(..)

-- |
-- >>> add 3 5
-- 8
-- >>> removeZeros [0, 1, 2, 3, 0]
-- [1, 2, 3]
-- >>> greetingTo "World"
-- "Konnichiwa World"
add : Int -> Int -> Int
add x y = x + y

greetingTo : String -> String
greetingTo x = "Hello " ++ x

removeZeros : List Int -> List Int
removeZeros = List.filter (\x -> x /= 0)

evaluation elm-doctest ModuleTobeTested.elm outputs

Starting elm-doctest ...

 processing: test/TestData/TestFail.elm
### Failure in test/TestData/TestFail.elm:10: expression
  greetingTo "World"
expected: "Konnichiwa World"
 but got: "Hello World"
Examples: 3  Failures: 1


As it utilizes elm-repl, the script must be runnable inside elm-repl. For example, code which imports elm-lang/navigation@1.0.0 module cannot be tested. Since the stdout is used to evaluate the test result, avoid using Debug.log.

Also, make sure elm-make runs without error. You can auto run elm-make by using --pretest command-line option.