Add DefaultPatternPosition rule

README.md

@@ -18,6 +18,7 @@ You can read the slides for my [presentation](http://slides.com/jeroenengels/elm
 
 ## Rules
 
+- [DefaultPatternPosition](rules/DefaultPatternPosition.md) - Enforce the default pattern to always appear first or last.
 - [NoConstantCondition](rules/NoConstantCondition.md) - Forbid the use of expressions in an If condition whose value are always the same.
 - [NoDebug](rules/NoDebug.md) - Forbid the use of `Debug` before it goes into production.
 - [NoDuplicateImports](rules/NoDuplicateImports.md) - Forbid importing the same module several times in a file.

elm-package.json

@@ -9,6 +9,7 @@
     ],
     "exposed-modules": [],
     "dependencies": {
+        "elm-community/list-extra": "5.0.1 <= v < 6.0.0",
         "elm-lang/core": "5.0.0 <= v < 6.0.0",
         "elm-lang/html": "2.0.0 <= v < 3.0.0",
         "jfmengels/elm-ast": "1.0.1 <= v < 2.0.0"

example/Main.elm

@@ -14,6 +14,7 @@ import Regex exposing (regex, escape)
 
 -- Rules
 
+import DefaultPatternPosition
 import NoConstantCondition
 import NoDebug
 import NoDuplicateImports
@@ -32,7 +33,8 @@ type Msg
 
 rules : List (String -> List Types.Error)
 rules =
-    [ NoConstantCondition.rule
+    [ DefaultPatternPosition.rule { position = DefaultPatternPosition.Last }
+    , NoConstantCondition.rule
     , NoDebug.rule
     , NoDuplicateImports.rule
     , NoExposingEverything.rule

example/elm-package.json

@@ -10,6 +10,7 @@
     ],
     "exposed-modules": [],
     "dependencies": {
+        "elm-community/list-extra": "5.0.1 <= v < 6.0.0",
         "elm-lang/core": "5.0.0 <= v < 6.0.0",
         "elm-lang/html": "2.0.0 <= v < 3.0.0",
         "jfmengels/elm-ast": "1.0.0 <= v < 2.0.0"

rules/DefaultPatternPosition.elm

@@ -0,0 +1,94 @@
+module DefaultPatternPosition exposing (rule, PatternPosition(..))
+
+import Ast.Expression exposing (..)
+import Html.Attributes exposing (pattern)
+import Lint exposing (doNothing, lint)
+import List.Extra exposing (findIndex)
+import Regex
+import Set exposing (Set)
+import Types exposing (Direction(..), Error, LintRule)
+
+
+type alias Context =
+    {}
+
+
+type PatternPosition
+    = First
+    | Last
+
+
+type alias Configuration =
+    { position : PatternPosition
+    }
+
+
+rule : Configuration -> String -> List Error
+rule config input =
+    lint input (implementation config)
+
+
+implementation : Configuration -> LintRule Context
+implementation configuration =
+    { statementFn = doNothing
+    , typeFn = doNothing
+    , expressionFn = expressionFn configuration
+    , moduleEndFn = (\ctx -> ( [], ctx ))
+    , initialContext = Context
+    }
+
+
+error : String -> Error
+error =
+    Error "DefaultPatternPosition"
+
+
+{-| TODO Share this in a util file, already defined in NoUselessPatternMatching
+-}
+isVariable : String -> Bool
+isVariable =
+    Regex.contains (Regex.regex "^[_a-z][\\w\\d]*$")
+
+
+isDefaultPattern : ( Expression, Expression ) -> Bool
+isDefaultPattern pattern =
+    case Tuple.first pattern of
+        Variable names ->
+            if isVariable (String.join "." names) then
+                True
+            else
+                False
+
+        _ ->
+            False
+
+
+findDefaultPattern : List ( Expression, Expression ) -> Maybe Int
+findDefaultPattern =
+    findIndex isDefaultPattern
+
+
+expressionFn : Configuration -> Context -> Direction Expression -> ( List Error, Context )
+expressionFn config ctx node =
+    case node of
+        Enter (Case expr patterns) ->
+            case findDefaultPattern patterns of
+                Nothing ->
+                    ( [], ctx )
+
+                Just index ->
+                    case config.position of
+                        First ->
+                            if index /= 0 then
+                                ( [ error "Expected default pattern to appear first in the list of patterns" ], ctx )
+                            else
+                                ( [], ctx )
+
+                        Last ->
+                            if index /= (List.length patterns) - 1 then
+                                ( [ error "Expected default pattern to appear last in the list of patterns" ], ctx )
+                            else
+                                ( [], ctx )
+
+        _ ->
+            ( [], ctx )

tests/DefaultPatternPositionTest.elm

@@ -0,0 +1,91 @@
+port module DefaultPatternPositionTest exposing (all)
+
+import Expect
+import Test exposing (describe, test, Test)
+import DefaultPatternPosition exposing (rule, PatternPosition(First, Last))
+import Types exposing (Error)
+
+
+error : String -> Error
+error position =
+    Error "DefaultPatternPosition" ("Expected default pattern to appear " ++ position ++ " in the list of patterns")
+
+
+tests : List Test
+tests =
+    [ test "should not report when default pattern is at the expected position (first)" <|
+        \() ->
+            """a = case b of
+              _ -> 1
+              Bar -> 1
+              Foo -> 1
+            """
+                |> rule { position = First }
+                |> Expect.equal []
+    , test "should not report when default pattern is at the expected position (last)" <|
+        \() ->
+            """a = case b of
+              Foo -> 1
+              Bar -> 1
+              _ -> 1
+            """
+                |> rule { position = Last }
+                |> Expect.equal []
+    , test "should not report when there is no default pattern (first)" <|
+        \() ->
+            """a = case b of
+              Foo -> 1
+              Bar -> 1
+            """
+                |> rule { position = First }
+                |> Expect.equal []
+    , test "should not report when there is no default pattern (last)" <|
+        \() ->
+            """a = case b of
+              Foo -> 1
+              Bar -> 1
+            """
+                |> rule { position = Last }
+                |> Expect.equal []
+    , test "should report an error when the default pattern is not at the expected position (first) (opposite expected position)" <|
+        \() ->
+            """a = case b of
+              Foo -> 1
+              Bar -> 1
+              _ -> 1
+            """
+                |> rule { position = First }
+                |> Expect.equal [ error "first" ]
+    , test "should report an error when the default pattern is not at the expected position (first) (somewhere in the middle)" <|
+        \() ->
+            """a = case b of
+              Foo -> 1
+              _ -> 1
+              Bar -> 1
+            """
+                |> rule { position = First }
+                |> Expect.equal [ error "first" ]
+    , test "should report an error when the default pattern is not at the expected position (last) (opposite expected position)" <|
+        \() ->
+            """a = case b of
+              _ -> 1
+              Foo -> 1
+              Bar -> 1
+            """
+                |> rule { position = Last }
+                |> Expect.equal [ error "last" ]
+    , test "should report an error when the default pattern is not at the expected position (last) (somewhere in the middle)" <|
+        \() ->
+            """a = case b of
+              Foo -> 1
+              _ -> 1
+              Bar -> 1
+            """
+                |> rule { position = Last }
+                |> Expect.equal [ error "last" ]
+    ]
+
+
+all : Test
+all =
+    describe "DefaultPatternPosition" tests

tests/Main.elm

@@ -3,6 +3,7 @@ port module Tests exposing (..)
 import Test exposing (describe, Test)
 import Test.Runner.Node exposing (run)
 import Json.Encode exposing (Value)
+import DefaultPatternPositionTest
 import NoConstantConditionTest
 import NoDebugTest
 import NoDuplicateImportsTest
@@ -25,7 +26,8 @@ port emit : ( String, Value ) -> Cmd msg
 all : Test
 all =
     describe "Visitors"
-        [ NoConstantConditionTest.all
+        [ DefaultPatternPositionTest.all
+        , NoConstantConditionTest.all
         , NoDebugTest.all
         , NoDuplicateImportsTest.all
         , NoImportingEverythingTest.all

tests/elm-package.json

@@ -11,6 +11,7 @@
     "exposed-modules": [],
     "dependencies": {
         "elm-community/elm-test": "3.1.0 <= v < 4.0.0",
+        "elm-community/list-extra": "5.0.1 <= v < 6.0.0",
         "elm-lang/core": "5.0.0 <= v < 6.0.0",
         "elm-lang/html": "2.0.0 <= v < 3.0.0",
         "jfmengels/elm-ast": "1.0.1 <= v < 2.0.0",