Add defaultError and zipError

Author: sep2

src/FsToolkit.ErrorHandling/Result.fs

@@ -117,6 +117,12 @@ module Result =
     | Ok x -> x
     | Error _ -> ifError
 
+  // Returns the contained value if Error, otherwise returns ifOk.
+  let defaultError ifOk result =
+      match result with
+      | Error error -> error
+      | Ok _ -> ifOk
+
   /// Returns the contained value if Ok, otherwise evaluates ifErrorThunk and
   /// returns the result.
   let defaultWith ifErrorThunk result =
@@ -178,4 +184,11 @@ module Result =
     match x1,x2 with
     | Ok x1res, Ok x2res -> Ok (x1res, x2res)
     | Error e, _ -> Error e
-    | _, Error e -> Error e
+    | _, Error e -> Error e
+
+  /// Takes two results and returns a tuple of the error pair
+  let zipError x1 x2 =
+      match x1, x2 with
+      | Error x1res, Error x2res -> Error(x1res, x2res)
+      | Ok e, _ -> Ok e
+      | _, Ok e -> Ok e

tests/FsToolkit.ErrorHandling.Tests/Result.fs

@@ -377,6 +377,18 @@ let defaultValueTests =
   ]
 
 
+let defaultErrorTests =
+  testList "defaultError Tests" [
+    testCase "defaultError returns the error value" <| fun _ ->
+      let v = Result.defaultError 43 (Error 42)
+      Expect.equal v 42 ""
+
+    testCase "defaultError returns the given value for Ok" <| fun _ ->
+      let v = Result.defaultError 43 (Ok 42)
+      Expect.equal v 43 ""
+  ]
+
+
 let defaultWithTests =
   testList "defaultWith Tests" [
     testCase "defaultWith returns the ok value" <| fun _ ->
@@ -565,6 +577,22 @@ let zipTests =
       Expect.equal actual (Error "Bad1") "Should be Error"
   ]
 
+let zipErrorTests =
+  testList "zipError tests" [
+    testCase "Ok, Ok" <| fun () ->
+      let actual = Result.zipError (Ok 1) (Ok 2)
+      Expect.equal actual (Ok (1)) "Should be ok"
+    testCase "Ok, Error" <| fun () ->
+      let actual = Result.zipError (Ok 1) (Error "Bad")
+      Expect.equal actual (Ok 1) "Should be ok"
+    testCase "Error, Ok" <| fun () ->
+      let actual = Result.zipError (Error "Bad") (Ok 1) 
+      Expect.equal actual (Ok 1) "Should be ok"
+    testCase "Error, Error" <| fun () ->
+      let actual = Result.zipError (Error "Bad1") (Error "Bad2")
+      Expect.equal actual (Error ("Bad1", "Bad2")) "Should be Error"
+  ]
+
 let allTests = testList "Result Tests" [
   resultIsOk
   resultIsError
@@ -592,6 +620,7 @@ let allTests = testList "Result Tests" [
   setErrorTests
   withErrorTests
   defaultValueTests
+  defaultErrorTests
   defaultWithTests
   ignoreErrorTests
   teeTests
@@ -601,4 +630,5 @@ let allTests = testList "Result Tests" [
   sequenceAsyncTests
   valueOrTests
   zipTests
+  zipErrorTests
 ]