feat: add either.AsyncErr type & helpers (#115)

* feat: add either.AsyncErr type & helpers

* fix: comment typo

Co-authored-by: Daniel Olshansky <[email protected]>

---------

Co-authored-by: Daniel Olshansky <[email protected]>

pkg/either/errors.go

@@ -0,0 +1,35 @@
+package either
+
+// SyncErr creates an AsyncError either from a synchronous error.
+// It wraps the Error into the left field (conventionally associated with the
+// error value in the Either pattern) of the Either type. It casts the result
+// to the AsyncError type.
+func SyncErr(err error) AsyncError {
+	return AsyncError(Error[chan error](err))
+}
+
+// AsyncErr creates an AsyncError from an error channel.
+// It wraps the error channel into the right field (conventionally associated with
+// successful values in the Either pattern) of the Either type.
+func AsyncErr(errCh chan error) AsyncError {
+	return AsyncError(Success[chan error](errCh))
+}
+
+// SyncOrAsyncError decomposes the AsyncError into its components, returning
+// a synchronous error and an error channel. If the AsyncError represents a
+// synchronous error, the error channel will be nil and vice versa.
+func (soaErr AsyncError) SyncOrAsyncError() (error, chan error) {
+	errCh, err := Either[chan error](soaErr).ValueOrError()
+	return err, errCh
+}
+
+// IsSyncError checks if the AsyncError represents a synchronous error.
+func (soaErr AsyncError) IsSyncError() bool {
+	return Either[chan error](soaErr).IsError()
+}
+
+// IsAsyncError checks if the AsyncError represents an asynchronous error
+// (sent through a channel).
+func (soaErr AsyncError) IsAsyncError() bool {
+	return Either[chan error](soaErr).IsSuccess()
+}

pkg/either/types.go

@@ -0,0 +1,6 @@
+package either
+
+// AsyncError represents a value which could either be a synchronous error or
+// an asynchronous error (sent through a channel). It wraps the more generic
+// `Either` type specific for error channels.
+type AsyncError Either[chan error]