Add Option.traverseAsync and Option.sequenceAsync (#298)

* Implement Option.traverseAsync and Option.sequenceAsync and add tests

* Add examples to documentation

* Fix typo in doc

* One more doc fix

* One more doc fix

* One more doc fix

gitbook/option/sequenceAsync.md

@@ -0,0 +1,27 @@
+## Option.sequenceAsync
+
+Namespace: `FsToolkit.ErrorHandling`
+
+Function Signature:
+
+```fsharp
+Async<'a> option -> Async<'a option>
+```
+
+Note that `sequence` is the same as `traverse id`. See also [Option.traverseAsync](traverseAsync.md).
+
+See also Scott Wlaschin's [Understanding traverse and sequence](https://fsharpforfunandprofit.com/posts/elevated-world-4/).
+
+## Examples
+
+### Example 1
+
+```fsharp
+let a1 : Async<int option> =
+  Option.sequenceAsync (Some (Async.singleton 42))
+// async { return Some 42 }
+
+let a2 : Async<int option> =
+  Option.sequenceAsync None
+// async { return None }
+```

gitbook/option/traverseAsync.md

@@ -0,0 +1,45 @@
+## Option.traverseAsync
+
+Namespace: `FsToolkit.ErrorHandling`
+
+Function Signature:
+
+```fsharp
+('a -> Async<'b>) -> 'a option -> Async<'b option>
+```
+
+Note that `traverse` is the same as `map >> sequence`. See also [Option.sequenceAsync](sequenceAsync.md).
+
+See also Scott Wlaschin's [Understanding traverse and sequence](https://fsharpforfunandprofit.com/posts/elevated-world-4/).
+
+## Examples
+
+### Example 1
+
+Let's assume we have a type `Customer`:
+
+```fsharp
+type Customer = {
+  Id : int
+  Email : string
+}
+```
+
+And we have a function called `getCustomerByEmail` that retrieves a `Customer` by email address asynchronously from some external source -- a database, a web service, etc:
+
+```fsharp
+// string -> Async<Customer>
+let getCustomerByEmail email : Async<Customer> = async {
+    return { Id = 1; Email = "[email protected]" } // return a constant for simplicity
+}
+```
+
+If we have a value of type `string option` and want to call the `getCustomerByEmail` function, we can achieve it using the `traverseAsync` function as below:
+
+```fsharp
+Some "[email protected]" |> Option.traverseAsync getCustomerByEmail
+// async { return Some { Id = 1; Email = "[email protected]" } }
+
+None |> Option.traverseAsync getCustomerByEmail
+// async { return None }
+```

src/FsToolkit.ErrorHandling/Option.fs

@@ -321,6 +321,28 @@ module Option =
 
         opt
 
+    /// Converts a Option<Async<_>> to an Async<Option<_>>
+    let inline sequenceAsync (optAsync: Option<Async<'T>>) : Async<Option<'T>> =
+        async {
+            match optAsync with
+            | Some asnc ->
+                let! x = asnc
+                return Some x
+            | None -> return None
+        }
+
+    /// <summary>
+    /// Maps an Async function over an Option, returning an Async Option.
+    /// </summary>
+    /// <param name="f">The function to map over the Option.</param>
+    /// <param name="opt">The Option to map over.</param>
+    /// <returns>An Async Option with the mapped value.</returns>
+    let inline traverseAsync
+        ([<InlineIfLambda>] f: 'T -> Async<'T>)
+        (opt: Option<'T>)
+        : Async<Option<'T>> =
+        sequenceAsync ((map f) opt)
+
     /// <summary>
     /// Creates an option from a boolean value and a value of type 'a.
     /// If the boolean value is true, returns <c>Some</c> value.

tests/FsToolkit.ErrorHandling.Tests/Option.fs

@@ -184,6 +184,66 @@ let teeIfTests =
             Expect.equal foo "foo" ""
     ]
 
+let sequenceAsyncTests =
+    testList "Option.sequenceAsync Tests" [
+        testCaseAsync "sequenceAsync returns the async value if Some"
+        <| async {
+            let optAsync =
+                async { return "foo" }
+                |> Some
+
+            let! value =
+                optAsync
+                |> Option.sequenceAsync
+
+            Expect.equal value (Some "foo") ""
+        }
+
+        testCaseAsync "sequenceAsync returns None if None"
+        <| async {
+            let optAsync = None
+
+            let! value =
+                optAsync
+                |> Option.sequenceAsync
+
+            Expect.equal value None ""
+        }
+    ]
+
+let traverseAsyncTests =
+    testList "Option.traverseAsync Tests" [
+        testCaseAsync "traverseAsync returns the async value if Some"
+        <| async {
+            let optAsync = Some "foo"
+
+            let optFunc =
+                id
+                >> Async.singleton
+
+            let! value =
+                (optFunc, optAsync)
+                ||> Option.traverseAsync
+
+            Expect.equal value (Some "foo") ""
+        }
+
+        testCaseAsync "traverseAsync returns None if None"
+        <| async {
+            let optAsync = None
+
+            let optFunc =
+                id
+                >> Async.singleton
+
+            let! value =
+                (optFunc, optAsync)
+                ||> Option.traverseAsync
+
+            Expect.equal value None ""
+        }
+    ]
+
 let traverseResultTests =
     testList "Option.traverseResult Tests" [
         testCase "traverseResult with Some of valid data"
@@ -366,6 +426,8 @@ let optionOperatorsTests =
 
 let allTests =
     testList "Option Tests" [
+        sequenceAsyncTests
+        traverseAsyncTests
         traverseResultTests
         tryParseTests
         tryGetValueTests