diff --git a/ioeither/http/builder/builder.go b/ioeither/http/builder/builder.go
index fb8d381..270693c 100644
--- a/ioeither/http/builder/builder.go
+++ b/ioeither/http/builder/builder.go
@@ -82,6 +82,11 @@ var (
 		O.Of[IOE.IOEither[error, []byte]],
 		Body.Set,
 	)
+	// WithBytes creates a [BuilderBuilder] for a request body using bytes
+	WithBytes = F.Flow2(
+		IOE.Of[error, []byte],
+		WithBody,
+	)
 	// WithContentType adds the [H.ContentType] header
 	WithContentType = WithHeader(H.ContentType)
 	// WithAuthorization adds the [H.Authorization] header
diff --git a/lambda/y.go b/lambda/y.go
index ef413c4..1074f77 100644
--- a/lambda/y.go
+++ b/lambda/y.go
@@ -15,16 +15,12 @@
 
 package lambda
 
-type (
-	// RecFct is the function called recursively
-	RecFct[T, R any] func(T) R
-
-	internalCombinator[T, R any] func(internalCombinator[T, R]) RecFct[T, R]
-)
-
 // Y is the Y-combinator based on https://dreamsongs.com/Files/WhyOfY.pdf
-func Y[TRFRM ~func(RecFct[T, R]) RecFct[T, R], T, R any](f TRFRM) RecFct[T, R] {
-	g := func(h internalCombinator[T, R]) RecFct[T, R] {
+func Y[TRFRM ~func(RecFct) RecFct, RecFct ~func(T) R, T, R any](f TRFRM) RecFct {
+
+	type internalCombinator[RecFct ~func(T) R, T, R any] func(internalCombinator[RecFct, T, R]) RecFct
+
+	g := func(h internalCombinator[RecFct, T, R]) RecFct {
 		return func(t T) R {
 			return f(h(h))(t)
 		}
diff --git a/lambda/y_test.go b/lambda/y_test.go
index c4f4d56..bd9d7fa 100644
--- a/lambda/y_test.go
+++ b/lambda/y_test.go
@@ -16,12 +16,13 @@
 package lambda
 
 import (
-	"fmt"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestFactorial(t *testing.T) {
-	fct := Y(func(r RecFct[int, int]) RecFct[int, int] {
+	fct := Y(func(r func(int) int) func(int) int {
 		return func(n int) int {
 			if n <= 0 {
 				return 1
@@ -29,6 +30,5 @@ func TestFactorial(t *testing.T) {
 			return n * r(n-1)
 		}
 	})
-
-	fmt.Println(fct(10))
+	assert.Equal(t, 3628800, fct(10))
 }