Add NoNestedLet rule

README.md

@@ -23,6 +23,8 @@ You can read the slides for my [presentation](http://slides.com/jeroenengels/elm
 - [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.
 - [NoExposingEverything](rules/NoExposingEverything.md) - Forbid exporting everything in your modules `module Main exposing (..)`, to make your module explicit in what it exposes.
+- [NoImportingEverything](rules/NoImportingEverything.md) - Forbid importing everything from your module. This can especially be confusing to newcomers when the exposed functions and types are unknown to them.
+- [NoNestedLet](rules/NoNestedLet.md) - Forbid nesting let expressions directly.
 - [NoUnannotatedFunction](rules/NoUnannotatedFunction.md) - Ensure every top-level function declaration has a type annotation.
 - [NoUnusedVariables](rules/NoUnusedVariables.md) - Reports variables that are declared but never used.
 - [NoUselessIf](rules/NoUselessIf.md) - Reports when both paths of an If expression result will lead to the same value.

example/Main.elm

@@ -20,6 +20,7 @@ import NoDebug
 import NoDuplicateImports
 import NoExposingEverything
 import NoImportingEverything
+import NoNestedLet
 import NoUnannotatedFunction
 import NoUnusedVariables
 import NoUselessIf
@@ -40,6 +41,7 @@ rules =
     , NoDuplicateImports.rule
     , NoExposingEverything.rule
     , NoImportingEverything.rule
+    , NoNestedLet.rule
     , NoUnannotatedFunction.rule
     , NoUnusedVariables.rule
     , NoUselessIf.rule

rules/NoNestedLet.elm

@@ -0,0 +1,39 @@
+module NoNestedLet exposing (rule)
+
+import Lint exposing (lint, doNothing)
+import Types exposing (LintRule, Error, Direction(..))
+import Ast.Expression exposing (..)
+
+
+type alias Context =
+    {}
+
+
+rule : String -> List Error
+rule input =
+    lint input implementation
+
+
+implementation : LintRule Context
+implementation =
+    { statementFn = doNothing
+    , typeFn = doNothing
+    , expressionFn = expressionFn
+    , moduleEndFn = (\ctx -> ( [], ctx ))
+    , initialContext = Context
+    }
+
+
+error : Error
+error =
+    Error "NoNestedLet" "Do not nest Let expressions directly"
+
+
+expressionFn : Context -> Direction Expression -> ( List Error, Context )
+expressionFn ctx node =
+    case node of
+        Enter (Let declarations (Let _ _)) ->
+            ( [ error ], ctx )
+
+        _ ->
+            ( [], ctx )

tests/Main.elm

@@ -8,6 +8,7 @@ import NoConstantConditionTest
 import NoDebugTest
 import NoDuplicateImportsTest
 import NoImportingEverythingTest
+import NoNestedLetTest
 import NoUnannotatedFunctionTest
 import NoUnusedVariablesTest
 import NoUselessIfTest
@@ -32,6 +33,7 @@ all =
         , NoDebugTest.all
         , NoDuplicateImportsTest.all
         , NoImportingEverythingTest.all
+        , NoNestedLetTest.all
         , NoUnannotatedFunctionTest.all
         , NoUnusedVariablesTest.all
         , NoUselessIfTest.all

tests/NoNestedLetTest.elm

@@ -0,0 +1,46 @@
+port module NoNestedLetTest exposing (all)
+
+import Expect
+import Test exposing (describe, test, Test)
+import NoNestedLet exposing (rule)
+import Types exposing (Error)
+
+
+error : Error
+error =
+    Error "NoNestedLet" "Do not nest Let expressions directly"
+
+
+tests : List Test
+tests =
+    [ test "should not report single let expression" <|
+        \() ->
+            """a = let b = 1
+                   in b
+            """
+                |> rule
+                |> Expect.equal []
+    , test "should report let expression inside the body of an other let expression" <|
+        \() ->
+            """a = let b = 1
+                   in let c = 2
+                      in c
+            """
+                |> rule
+                |> Expect.equal [ error ]
+    , test "should not report let expression indirectly inside another " <|
+        \() ->
+            """a = let b = 1
+                   in if foo then
+                        let c = 2 in c
+                      else
+                        3
+            """
+                |> rule
+                |> Expect.equal []
+    ]
+
+
+all : Test
+all =
+    describe "NoNestedLet" tests