FavourNestedFunctions: implement rule

Implemented FavourNestedFunctions rule. Added rule text message
to Text.resx.

Fixes #638

Author: webwarrior-ws

src/FSharpLint.Core/Rules/Conventions/FavourNestedFunctions.fs

@@ -7,8 +7,63 @@ open FSharpLint.Framework.Rules
 open FSharpLint.Framework
 open FSharpLint.Framework.Suggestion
 
+let private (|FunctionDeclaration|_|) (declaration: SynModuleDecl) = 
+    match declaration with
+    | SynModuleDecl.Let(_, [ SynBinding(_, _, _, _, _, _, _, headPat, _, expr, _, _) ], _) ->
+        match headPat with
+        | SynPat.LongIdent(LongIdentWithDots([ident], _), _, _, _, accessibility, _) ->
+            Some(ident, expr, accessibility)
+        | _ -> None
+    | _ -> None
+
 let runner (args: AstNodeRuleParams) =
-    failwith "Not yet implemeted"    
+    match args.AstNode with
+    | AstNode.ModuleOrNamespace(SynModuleOrNamespace(_, _, _kind, declarations, _, _, _, _)) ->
+        let privateFunctionIdentifiers = 
+            declarations
+            |> Seq.choose 
+                (fun declaration ->
+                    match declaration with
+                    | FunctionDeclaration(ident, _body, Some(SynAccess.Private)) -> 
+                        Some ident
+                    | _ -> None)
+            |> Seq.toArray
+
+        match args.CheckInfo with
+        | Some checkInfo when privateFunctionIdentifiers.Length > 0 ->
+            let otherFunctionBodies =
+                declarations
+                |> List.choose 
+                    (fun declaration ->
+                        match declaration with
+                        | FunctionDeclaration(ident, body, _) 
+                            when not(Array.exists (fun (each: Ident) -> each.idText = ident.idText) privateFunctionIdentifiers) -> 
+                            Some body
+                        | _ -> None)
+        
+            privateFunctionIdentifiers
+            |> Array.choose
+                (fun currFunctionIdentifier ->
+                    match ExpressionUtilities.getSymbolFromIdent args.CheckInfo (SynExpr.Ident currFunctionIdentifier) with
+                    | Some symbolUse ->
+                        let numberOfOtherFunctionsCurrFunctionIsUsedIn =
+                            otherFunctionBodies
+                            |> Seq.filter (fun funcBody -> 
+                                checkInfo.GetUsesOfSymbolInFile symbolUse.Symbol
+                                |> Array.exists (fun usage -> ExpressionUtilities.rangeContainsOtherRange funcBody.Range usage.Range))
+                            |> Seq.length
+                        if numberOfOtherFunctionsCurrFunctionIsUsedIn = 1 then
+                            Some {
+                                Range = currFunctionIdentifier.idRange
+                                WarningDetails.Message = Resources.GetString "RulesFavourNestedFunctions"
+                                SuggestedFix = None
+                                TypeChecks = List.Empty
+                            }
+                        else
+                            None
+                    | None -> None)
+        | _ -> Array.empty
+    | _ -> Array.empty
 
 let rule =
     { Name = "FavourNestedFunctions"

src/FSharpLint.Core/Text.resx

@@ -372,4 +372,7 @@
   <data name="RulesFavourAsKeyword" xml:space="preserve">
     <value>Prefer using the 'as' pattern to match a constant and bind it to a variable.</value>
   </data>
+  <data name="RulesFavourNestedFunctions" xml:space="preserve">
+    <value>Prefer using local functions over private module-level functions</value>
+  </data>
 </root>