Add docs+example for withExtraFilesModuleVisitor

src/Review/Rule.elm

@@ -1903,6 +1903,8 @@ doesn't analyze by default.
 
 The visitor function will be called with all the files matching the file patterns.
 
+REPLACEME
+
 The following example rule reads a project's `.css` files to extract all the mentioned CSS classes,
 then finds calls to `Html.Attributes.class` in the Elm code (such as `Html.Attributes.class "big-red-button"`)
 and reports errors when the classes given as argument are unknown.
@@ -2497,7 +2499,96 @@ withElmJsonModuleVisitor visitor (ModuleRuleSchema schema) =
     ModuleRuleSchema { schema | elmJsonVisitor = Just (combineContextOnlyVisitor visitor schema.elmJsonVisitor) }
 
 
-{-| REPLACEME
+{-| Add a visitor to the [`ModuleRuleSchema`](#ModuleRuleSchema) to visit files that `elm-review`
+doesn't analyze by default.
+
+The visitor function will be called with all the files matching the file patterns.
+
+The following example rule reads a project's `.css` files to extract all the mentioned CSS classes,
+then finds calls to `Html.Attributes.class` in the Elm code (such as `Html.Attributes.class "big-red-button"`)
+and reports errors when the classes given as argument are unknown.
+
+    import Dict exposing (Dict)
+    import Elm.Syntax.Expression as Expression exposing (Expression)
+    import Elm.Syntax.Node as Node exposing (Node)
+    import Elm.Syntax.Range exposing (Range)
+    import Regex exposing (Regex)
+    import Review.FilePattern as FilePattern
+    import Review.Rule as Rule exposing (Rule)
+    import Set exposing (Set)
+
+    rule : Rule
+    rule =
+        Rule.newModuleRuleSchema "Css.NoUnknownCssClasses" initialContext
+            |> Rule.withExtraFilesModuleVisitor cssFilesVisitor
+                [ FilePattern.include "**/*.css" ]
+            |> Rule.withExpressionEnterVisitor expressionVisitor
+            |> Rule.fromModuleRuleSchema
+
+    type alias Context =
+        { knownCssClasses : Set String
+        }
+
+    initialContext : Context
+    initialContext =
+        { knownCssClasses = Set.empty
+        }
+
+    cssClassRegex : Regex
+    cssClassRegex =
+        Regex.fromString "\\.([\\w-_]+)"
+            |> Maybe.withDefault Regex.never
+
+    cssFilesVisitor : Dict String String -> Context -> Context
+    cssFilesVisitor files context =
+        { knownCssClasses =
+            files
+                |> Dict.values
+                |> List.concatMap (\cssSource -> Regex.find cssClassRegex cssSource)
+                |> List.map (\m -> String.dropLeft 1 m.match)
+                |> Set.fromList
+        }
+
+    expressionVisitor : Node Expression -> Context -> ( List (Rule.Error {}), Context )
+    expressionVisitor node context =
+        case Node.value node of
+            Expression.Application [ function, firstArg ] ->
+                case Node.value function of
+                    Expression.FunctionOrValue [ "Html", "Attributes" ] "class" ->
+                        case Node.value firstArg of
+                            Expression.Literal stringLiteral ->
+                                ( stringLiteral
+                                    |> String.split " "
+                                    |> List.filterMap (checkForUnknownCssClass context.knownCssClasses (Node.range firstArg))
+                                , context
+                                )
+
+                            _ ->
+                                ( [], context )
+
+                    _ ->
+                        ( [], context )
+
+            _ ->
+                ( [], context )
+
+    checkForUnknownCssClass : Set String -> Range -> String -> Maybe (Rule.Error {})
+    checkForUnknownCssClass knownCssClasses range class =
+        if Set.member class knownCssClasses then
+            Nothing
+
+        else
+            Just
+                (Rule.error
+                    { message = "Unknown CSS class " ++ class
+                    , details =
+                        [ "This CSS class does not appear in the project's `.css` files."
+                        , "Could it be that you misspelled the name of the class, or that the class recently got removed?"
+                        ]
+                    }
+                    range
+                )
+
 -}
 withExtraFilesModuleVisitor :
     (Dict String String -> moduleContext -> moduleContext)