* Adds Seq module for lazy result traversal

Author: njlr

src/FsToolkit.ErrorHandling/FsToolkit.ErrorHandling.fsproj

@@ -31,6 +31,7 @@
     <Compile Include="AsyncOptionCE.fs" />
     <Compile Include="AsyncOptionOp.fs" />
     <Compile Include="List.fs" />
+    <Compile Include="Seq.fs" />
     <None Include="Script.fsx" />
   </ItemGroup>
 
@@ -39,6 +40,6 @@
     <Content Include="*.fsproj; **\*.fs" PackagePath="fable\" />
   </ItemGroup>
 
-  
+
   <Import Project="..\..\.paket\Paket.Restore.targets" />
 </Project>

src/FsToolkit.ErrorHandling/Seq.fs

@@ -0,0 +1,17 @@
+namespace FsToolkit.ErrorHandling
+
+[<RequireQualifiedAccess>]
+module Seq =
+
+  let sequenceResultM (xs : seq<Result<'t, 'e>>) : Result<'t list, 'e> =
+    let rec loop xs ts =
+      match Seq.tryHead xs with
+      | Some x ->
+        x
+        |> Result.bind (fun t ->
+          loop (Seq.tail xs) (t :: ts))
+      | None ->
+        Ok (List.rev ts)
+
+    // Seq.cache prevents double evaluation in Seq.tail
+    loop (Seq.cache xs) []

tests/FsToolkit.ErrorHandling.Tests/FsToolkit.ErrorHandling.Tests.fsproj

@@ -23,13 +23,14 @@
     <Compile Include="AsyncOption.fs" />
     <Compile Include="AsyncOptionCE.fs" />
     <Compile Include="List.fs" />
+    <Compile Include="Seq.fs" />
     <Compile Include="AsyncResult.fs" />
     <Compile Include="AsyncResultCE.fs" />
     <Compile Include="AsyncResultOption.fs" />
     <Compile Include="Validation.fs" />
     <Compile Include="ValidationCE.fs" />
     <Compile Include="Main.fs" />
   </ItemGroup>
-  
+
   <Import Project="..\..\.paket\Paket.Restore.targets" />
 </Project>

tests/FsToolkit.ErrorHandling.Tests/Main.fs

@@ -15,6 +15,7 @@ let allTests = testList "All Tests" [
   AsyncOptionTests.allTests
   AsyncOptionCETests.allTests
   ListTests.allTests
+  SeqTests.allTests
   AsyncResultTests.allTests
   AsyncResultCETests.allTests
   AsyncResultOptionTests.allTests
@@ -27,5 +28,5 @@ let main argv =
   #if FABLE_COMPILER
   Mocha.runTests allTests
   #else
-  Tests.runTestsWithArgs defaultConfig argv allTests  
+  Tests.runTestsWithArgs defaultConfig argv allTests
   #endif

tests/FsToolkit.ErrorHandling.Tests/Seq.fs

@@ -0,0 +1,52 @@
+module SeqTests
+
+
+#if FABLE_COMPILER
+open Fable.Mocha
+#else
+open Expecto
+#endif
+open SampleDomain
+open TestData
+open TestHelpers
+open System
+open FsToolkit.ErrorHandling
+
+
+
+let sequenceResultMTests =
+  testList "Seq.sequenceResultM Tests" [
+    testCase "traverseResult with an empty sequence" <| fun _ ->
+      let tweets = []
+      let expected = Ok []
+      let actual = Seq.sequenceResultM (Seq.map Tweet.TryCreate tweets)
+      Expect.equal actual expected "Should have an empty list of valid tweets"
+
+    testCase "traverseResult with a sequence of valid data" <| fun _ ->
+      let tweets = ["Hi"; "Hello"; "Hola"]
+      let expected = List.map tweet tweets |> Ok
+      let actual = Seq.sequenceResultM (Seq.map Tweet.TryCreate tweets)
+      Expect.equal actual expected "Should have a list of valid tweets"
+
+    testCase "sequenceResultM with few invalid data" <| fun _ ->
+      let tweets = [""; "Hello"; aLongerInvalidTweet] :> seq<_>
+      let actual = Seq.sequenceResultM (Seq.map Tweet.TryCreate tweets)
+      Expect.equal actual (Error emptyTweetErrMsg) "traverse the sequence and return the first error"
+
+    testCase "sequenceResultM stops after first invalid data" <| fun _ ->
+      let mutable counter = 0
+      let tweets = seq {
+        "Hi"
+        "Hello"
+        "Hola"
+        aLongerInvalidTweet
+        counter <- counter + 1
+      }
+      let actual = Seq.sequenceResultM (Seq.map Tweet.TryCreate tweets)
+      Expect.equal actual (Error longerTweetErrMsg) "traverse the sequence and return the first error"
+      Expect.equal counter 0 "evaluation of the sequence stops at the first error"
+  ]
+
+let allTests = testList "Seq Tests" [
+  sequenceResultMTests
+]