FSharpLint.Core: implement new rule

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

@@ -8,25 +8,44 @@ open FSharpLint.Framework.Ast
 open FSharpLint.Framework.Rules
 
 
-
 let runner (args:AstNodeRuleParams) =
-    printfn "AstNode: %A" args.AstNode
-    let error =
-      match args.AstNode with
-      | AstNode.Expression (SynExpr.LetOrUse (isRecursive, isUse, bindings, body, range)) ->
-           match bindings with
-           | head ->
-                ()
-           | _ ->
-               ()
-          let parents = args.GetParents 5
-          printfn "%A" parents
-          Array.empty
-          // checkRecursiveAsyncFunction args range expr parents
-      | _ -> Array.empty
-
-    printfn "----------------"
+
+    let mutable error = Array.empty
+    let mutable idents = List.empty<string>
+    match args.AstNode with
+        | AstNode.Expression (SynExpr.LetOrUse (isRecursive, isUse, bindings, body, range)) ->
+            match bindings.[0] with
+            | SynBinding(synAccessOption, synBindingKind, mustInline, isMutable, synAttributeLists, preXmlDoc, synValData, headPat, synBindingReturnInfoOption, synExpr, range, debugPointAtBinding) ->
+                match headPat with
+                | SynPat.Named(synPat, ident, isSelfIdentifier, synAccessOption, range) ->
+                    idents <- List.append idents [ident.idText]
+                | _ ->
+                    ()
+
+            match body with
+            | SynExpr.Sequential(debugPointAtSequential, isTrueSeq, synExpr, expr2, range) ->
+                match synExpr with
+                    | SynExpr.App(exprAtomicFlag, isInfix, funcExpr, argExpr, range) ->
 
+                        match argExpr with
+                            | SynExpr.Ident ident ->
+                                if (List.contains ident.idText idents) then
+                                    error <- ({
+                                        Range = range
+                                        Message = String.Format(Resources.GetString ("RulesUsedUnderscorePrefixedElements"))
+                                        SuggestedFix = None
+                                        TypeChecks = []
+                                    } |> Array.singleton) 
+                                else
+                                    ()
+                            | _ ->
+                                ()
+                    | _ ->
+                        ()
+                | _ ->
+                    ()          
+        | _ -> ()
+
     error
 
 let rule =