+ (Value)Task.mapError

Author: gusty

src/FSharpPlus/Extensions/Task.fs

@@ -376,6 +376,27 @@ module Task =
         let source = nullArgCheck (nameof source) source
 
         tryWith (fun () -> source) (mapper >> Task.FromResult)
+
+    /// <summary>Maps the exception of a faulted task to another exception.</summary>
+    /// <param name="mapping">Mapping function from exception to exception.</param>
+    /// <param name="source">The source task.</param>
+    /// <returns>The resulting task.</returns>
+    let mapError (mapping: exn -> exn) (source: Task<'T>) : Task<'T> =
+        let source = nullArgCheck (nameof source) source
+
+        if source.IsCompleted then
+            match source with
+            | Succeeded _ -> source
+            | Faulted exn -> FromExceptions (AggregateException (mapping exn))
+            | Canceled    -> source
+        else
+            let tcs = TaskCompletionSource<'T> TaskCreationOptions.RunContinuationsAsynchronously
+            let k = function
+                | Succeeded r -> tcs.SetResult r
+                | Faulted aex -> tcs.SetException (AggregateException (mapping aex)).InnerExceptions
+                | Canceled    -> tcs.SetCanceled ()
+            source.ConfigureAwait(false).GetAwaiter().UnsafeOnCompleted (fun () -> k source)
+            tcs.Task
 
     /// <summary>Creates a Task that's completed unsuccessfully with the specified exception.</summary>
     /// <param name="exn">The exception to be raised.</param>

src/FSharpPlus/Extensions/ValueTask.fs

@@ -346,6 +346,25 @@ module ValueTask =
     let inline recover ([<InlineIfLambda>]mapper: exn -> 'T) (source: ValueTask<'T>) : ValueTask<'T> =
         tryWith (mapper >> ValueTask.FromResult) (fun () -> source)
 
+    /// <summary>Maps the exception of a faulted task to another exception.</summary>
+    /// <param name="mapping">Mapping function from exception to exception.</param>
+    /// <param name="source">The source task.</param>
+    /// <returns>The resulting task.</returns>
+    let mapError (mapping: exn -> exn) (source: ValueTask<'T>) : ValueTask<'T> =
+        if source.IsCompleted then
+            match source with
+            | Succeeded _ -> source
+            | Faulted exn -> FromExceptions (AggregateException (mapping exn))
+            | Canceled    -> source
+        else
+            let tcs = TaskCompletionSource<'T> TaskCreationOptions.RunContinuationsAsynchronously
+            let k = function
+                | Succeeded r -> tcs.SetResult r
+                | Faulted aex -> tcs.SetException (AggregateException (mapping aex)).InnerExceptions
+                | Canceled    -> tcs.SetCanceled ()
+            source.ConfigureAwait(false).GetAwaiter().UnsafeOnCompleted (fun () -> k source)
+            ValueTask<'T> tcs.Task
+
     /// <summary>Creates a ValueTask that's completed unsuccessfully with the specified exception.</summary>
     /// <param name="exn">The exception to be raised.</param>
     /// <returns>A ValueTask that is completed unsuccessfully with the specified exception.</returns>