linting.md

Linting Configuration

Elm Language Server has built-in support for linting. All linting rules are enabled by default. If you want to disable a rule you can disable them by placing a configuration file in your project.

The linting system was based on Elm-Analyse, and the cofiguration:

<workspace-root>/elm-analyse.json

Example Configuration

This is the default configuration, with all checks enabled:

{
  "checks": {
    "BooleanCase": true,
    "DropConcatOfLists": true,
    "DropConsOfItemAndList": true,
    "NoUncurriedPrefix": true,
    "MapNothingToNothing": true,
    "SingleFieldRecord": true,
    "UnnecessaryListConcat": true,
    "UnnecessaryPortModule": true,
    "UnusedImport": true,
    "UnusedImportAlias": true,
    "UnusedImportedVariable": true,
    "UnusedPatternVariable": true,
    "UnusedTopLevel": true,
    "UnusedTypeAlias": true,
    "UnusedValueConstructor": true,
    "UnusedVariable": true,
    "UseConsOverConcat": true,
    "MissingTypeAnnotation": true
  },
  "excludedPaths": []
}

Exclude paths

You can exclude certain files and folders from being linted by adding their path to excludedPaths in elm-analyse.json.

Example:

"excludedPaths" : [
	"src/Vendor",
	"src/App/FileThatShouldNotBeLinted.elm"
],

Checks

BooleanCase

If you case over a boolean value, it is more idiomatic to use an if-expression.

Example rule violation:

thing : Boolean -> String
thing x =
    case x of
        True ->
            "Hello"
        False ->
            "Goodbye"

DropConcatOfLists

If you concatenate two list literals [...] ++ [...], then you can merge them into one list.

Example rule violation:

foo : List Int
foo =
    [ 1, 2, 3 ] ++ [ 4, 5, 6]

DropConsOfItemAndList

If you cons an item to a literal list (x :: [1, 2, 3]), then you can just put the item into the list.

Example rule violation:

foo : List Int
foo =
    1 :: [ 2, 3, 4]

NoUncurriedPrefix

It's not needed to use an operator in prefix notation when you apply both arguments directly.

Example rule violation:

hello : String
hello =
    (++) "Hello " "World"

MapNothingToNothing

Do not map a Nothing to Nothing with a case expression. Use andThen or map instead.

Example rule violation:

greet : Maybe String -> Maybe String
greet x =
    case x of
        Nothing ->
            Nothing
        Just x ->
            Just ("Hello " ++ x)

SingleFieldRecord

Using a record is obsolete if you only plan to store a single field in it.

Example rule violation:

type Model =
    Model { input : String }

UnnecessaryListConcat

You should not use List.concat to concatenate literal lists. Just join the lists together.

Example rule violation:

foo : List Int
foo =
    List.concat [ [ 1, 2 ,3 ], [ a, b, c] ]

UnnecessaryPortModule

Don't use the port keyword if you do not need it.

Example rule violation:

port module Foo exposing (notAPort)

notAPort : Int
notAPort = 1

UnusedImport

Imports that have no meaning should be removed.

Example rule violation:

module Foo exposing (main)

import Html exposing (Html, text)
import SomeOtherModule

main : Html a
main =
    text "Hello"

UnusedImportAlias

You defined an alias for an import (import Foo as F), but it turns out you never use it.

Example rule violation:

module Foo exposing (main)

import Html as H exposing (Html, text)

main : Html a
main =
    text "Hello"

UnusedImportedVariable

When a function is imported from a module but is unused, it is better to remove it.

Example rule violation:

module Foo exposing (thing)

import Html exposing (Html, div, text)

main : Html a
main =
    text "Hello World!"

UnusedPatternVariable

Variables in pattern matching that are unused should be replaced with '_' or left out completely.

Example rule violation:

greetRecord {name, age} = "Hello " ++ name

greetTuple (name, age) = "Hello " ++ name

greetConstructor (Person name age) = "Hello " ++ name

UnusedTopLevel

Functions and values that are unused in a module and not exported are dead code.

Example rule violation:

module Foo exposing (thing)

thing : Int
thing =
    1

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

UnusedTypeAlias

You defined a type alias, but you do not use it in any signature or expose it.

Example rule violation:

module Foo exposing (main)

import Html exposing (Html, text, Html)

type alias SomeUnusedThing =
    { name : String }

main : Html a
main =
    text "Hello World"

UnusedValueConstructor

Value constructors which are not exposed and used should be eliminated.

Example rule violation:

module Greet exposing (Color, red, blue)

type Color
    = Blue
    | Red
    | Green

red : Color
red = Red

blue : Color
blue = Blue

UnusedVariable

Variables that are not used could be removed or marked as '_'.

Example rule violation:

module Foo exposing (foo)

foo : String -> Int
foo x =
    1

UseConsOverConcat

If you concatenate two lists, but the right hand side is a single element list, then you should use the cons operator ::.

Example rule violation:

foo : List String
foo =
    [ "a" ] ++ [ "b", "c" ]

MissingTypeAnnotation

Type annotations are required on top-level declarations.

Example rule violation:

module Foo exposing (foo)

foo = 1

Note: This check was called "NoTopLevelSignature" in Elm-Analyse.

Compatibility with Elm Analyse

The linter was inspired by the tool Elm-Analyse by Mats Stijlaart, and existing elm-analyse configuration should be fully compatible with ElmLS.

While Elm Language Server does not use Elm-Analyse direct, the check names and descriptions are inspired by it.

We have deliberately excluded some checks that are supported by Elm-Analyse. Either they did not fit well with ElmLS or the issues they check are automatically fixed by elm-format.

ElmLS does also not support the TriggerWords property from Elm-Analyses and will just ignore it.