Merge pull request #97 from upenn-cis1xx/esinx/neq-physical

Rule to prefer structural inequality `<>` over referential equality `==`

Author: esinx

lib/style/checkers.ml

@@ -4,6 +4,7 @@ let expr_checks = [
 ; Equality.EqOption.name
 ; Equality.EqBool.name
 ; Equality.EqPhysical.name
+; Equality.NeqPhysical.name
 ; Match.MatchBool.name
 ; Match.MatchInt.name
 ; Match.MatchListVerbose.name

lib/style/equality.ml

@@ -67,4 +67,19 @@ module EqPhysical : EXPRCHECK = struct
       | _ -> ()
     end
   let name = "EqPhysical", check
+end
+
+(** ------------------ Checks rules: (_ != _)  ----------------------- *)
+module NeqPhysical : EXPRCHECK = struct
+  type ctxt = Parsetree.expression_desc Pctxt.pctxt
+  let fix = "using `<>` to evaluate structural inequality"
+  let violation = "using `!=` when structural equality is intended"
+  let check st (E {location; source; pattern}: ctxt) =
+    begin match pattern with
+      | Pexp_apply (application, [(_,_); (_,_)]) ->
+        if application =~ "!=" then
+          st := Hint.mk_hint location source fix violation :: !st
+      | _ -> ()
+    end
+  let name = "NeqPhysical", check
 end

test/examples/equality.ml

@@ -24,4 +24,6 @@ let b4 = false = bfalse
 
 (* Physical equality (should raise issues) *)
 let g = 1 == 1
-let h = "ocaml" == "ocaml"
+let h = "ocaml" == "ocaml"
+let i = 1 != 1
+let j = "ocaml" != "ocaml"

test/test.ml

@@ -87,6 +87,24 @@ let%expect_test _ =
   let to_lint = to_ast file in
   lint_and_hint to_lint;
   [%expect{|
+    (* ------------------------------------------------------------------------ *)
+    File ./examples/equality.ml, line 29, columns: 8-26
+    Warning:
+    	using `!=` when structural equality is intended
+    You wrote:
+    	 "ocaml" != "ocaml"
+    Consider:
+    	using `<>` to evaluate structural inequality
+
+    (* ------------------------------------------------------------------------ *)
+    File ./examples/equality.ml, line 28, columns: 8-14
+    Warning:
+    	using `!=` when structural equality is intended
+    You wrote:
+    	 1 != 1
+    Consider:
+    	using `<>` to evaluate structural inequality
+
     (* ------------------------------------------------------------------------ *)
     File ./examples/equality.ml, line 27, columns: 8-26
     Warning: