Result.mapError

Author: jmagaram

src/Core__Result.mjs

@@ -102,6 +102,17 @@ function cmp(a, b, f) {
   }
 }
 
+function mapError(r, f) {
+  if (r.TAG === /* Ok */0) {
+    return r;
+  } else {
+    return {
+            TAG: /* Error */1,
+            _0: Curry._1(f, r._0)
+          };
+  }
+}
+
 export {
   getExn ,
   mapWithDefault ,
@@ -112,5 +123,6 @@ export {
   isError ,
   eq ,
   cmp ,
+  mapError ,
 }
 /* No side effect */

src/Core__Result.res

@@ -91,3 +91,15 @@ let cmpU = (a, b, f) =>
   }
 
 let cmp = (a, b, f) => cmpU(a, b, (. x, y) => f(x, y))
+
+// If the source result is Ok, should we return that instance, or
+// create it again? In this implementation I'm returning that specific
+// instance. However this is not consistent with the implementation for
+// other functions like mapU and flatMapU, which recreate the result.
+// This is more efficient. I'm not sure why the other implementations
+// return a new instance.
+let mapError = (r, f) =>
+  switch r {
+  | Ok(_) as ok => ok
+  | Error(e) => Error(f(e))
+  }

src/Core__Result.resi

@@ -197,3 +197,17 @@ let eq: (t<'a, 'c>, t<'b, 'd>, ('a, 'b) => bool) => bool
   ```
 */
 let cmp: (t<'a, 'c>, t<'b, 'd>, ('a, 'b) => int) => int
+
+/**
+`mapError(res, f)` - If the source is `Ok`, return it. Otherwise apply 
+the provided mapping function to the `Error` value.
+
+## Examples
+  ```rescript
+
+let formatError = n => `Error code: ${n->Int.toString}`
+mapError(Error(14), formatError) // evaluates to Error("Error code: 14")
+mapError(Ok("abc"), formatError) // evaluates to Ok("abc")
+```
+*/
+let mapError: (result<'a, 'b>, 'b => 'c) => result<'a, 'c>

test/ResultTests.mjs

@@ -0,0 +1,48 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+import * as Test from "./Test.mjs";
+import * as Caml_obj from "rescript/lib/es6/caml_obj.js";
+import * as Core__Result from "../src/Core__Result.mjs";
+
+var eq = Caml_obj.equal;
+
+Test.run([
+      [
+        "ResultTests.res",
+        9,
+        20,
+        48
+      ],
+      "mapError: if ok, return it"
+    ], Core__Result.mapError({
+          TAG: /* Ok */0,
+          _0: 5
+        }, (function (i) {
+            return Math.imul(i, 3);
+          })), eq, {
+      TAG: /* Ok */0,
+      _0: 5
+    });
+
+Test.run([
+      [
+        "ResultTests.res",
+        12,
+        13,
+        42
+      ],
+      "mapError: if error, apply f"
+    ], Core__Result.mapError({
+          TAG: /* Error */1,
+          _0: 5
+        }, (function (i) {
+            return Math.imul(i, 3);
+          })), eq, {
+      TAG: /* Error */1,
+      _0: 15
+    });
+
+export {
+  eq ,
+}
+/*  Not a pure module */

test/ResultTests.res

@@ -0,0 +1,16 @@
+open RescriptCore
+
+let eq = (a, b) => a == b
+
+// ========
+// mapError
+// ========
+
+Test.run(__POS_OF__("mapError: if ok, return it"), Ok(5)->Result.mapError(i => i * 3), eq, Ok(5))
+
+Test.run(
+  __POS_OF__("mapError: if error, apply f"),
+  Error(5)->Result.mapError(i => i * 3),
+  eq,
+  Error(15),
+)

test/TestSuite.mjs

@@ -5,6 +5,7 @@ import * as TestTests from "./TestTests.mjs";
 import * as ArrayTests from "./ArrayTests.mjs";
 import * as ErrorTests from "./ErrorTests.mjs";
 import * as PromiseTest from "./PromiseTest.mjs";
+import * as ResultTests from "./ResultTests.mjs";
 
 var bign = TestTests.bign;
 
@@ -26,10 +27,10 @@ var Concurrently = PromiseTest.Concurrently;
 
 var panicTest = ErrorTests.panicTest;
 
-var eq = IntTests.eq;
-
 var $$catch = IntTests.$$catch;
 
+var eq = ResultTests.eq;
+
 export {
   bign ,
   TestError ,
@@ -41,7 +42,7 @@ export {
   Catching ,
   Concurrently ,
   panicTest ,
-  eq ,
   $$catch ,
+  eq ,
 }
 /* IntTests Not a pure module */

test/TestSuite.res

@@ -3,3 +3,4 @@ include PromiseTest
 include ErrorTests
 include ArrayTests
 include IntTests
+include ResultTests