fix: Writer monad

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

writer/bind_test.go

@@ -19,6 +19,7 @@ import (
 	"testing"
 
 	A "github.com/IBM/fp-go/array"
+	EQ "github.com/IBM/fp-go/eq"
 	F "github.com/IBM/fp-go/function"
 	"github.com/IBM/fp-go/internal/utils"
 	M "github.com/IBM/fp-go/monoid"
@@ -28,6 +29,8 @@ import (
 var (
 	monoid = A.Monoid[string]()
 	sg     = M.ToSemigroup(monoid)
+
+	eq = Eq(A.Eq[string](EQ.FromStrictEquals[string]()), EQ.FromStrictEquals[string]())
 )
 
 func getLastName(s utils.Initial) Writer[[]string, string] {
@@ -47,7 +50,7 @@ func TestBind(t *testing.T) {
 		Map[[]string](utils.GetFullName),
 	)
 
-	assert.Equal(t, res(), Of[string](monoid)("John Doe")())
+	assert.True(t, eq.Equals(res, Of[string](monoid)("John Doe")))
 }
 
 func TestApS(t *testing.T) {
@@ -59,5 +62,5 @@ func TestApS(t *testing.T) {
 		Map[[]string](utils.GetFullName),
 	)
 
-	assert.Equal(t, res(), Of[string](monoid)("John Doe")())
+	assert.True(t, eq.Equals(res, Of[string](monoid)("John Doe")))
 }

writer/generic/writer.go

@@ -18,12 +18,20 @@ package generic
 import (
 	F "github.com/IBM/fp-go/function"
 	C "github.com/IBM/fp-go/internal/chain"
+	FC "github.com/IBM/fp-go/internal/functor"
 	IO "github.com/IBM/fp-go/io/generic"
 	M "github.com/IBM/fp-go/monoid"
 	SG "github.com/IBM/fp-go/semigroup"
 	T "github.com/IBM/fp-go/tuple"
 )
 
+func Tell[GA ~func() T.Tuple3[any, W, SG.Semigroup[W]], W any](s SG.Semigroup[W]) func(W) GA {
+	return F.Flow2(
+		F.Bind13of3(T.MakeTuple3[any, W, SG.Semigroup[W]])(nil, s),
+		IO.Of[GA],
+	)
+}
+
 func Of[GA ~func() T.Tuple3[A, W, SG.Semigroup[W]], W, A any](m M.Monoid[W]) func(A) GA {
 	return F.Flow2(
 		F.Bind23of3(T.MakeTuple3[A, W, SG.Semigroup[W]])(m.Empty(), M.ToSemigroup(m)),
@@ -132,3 +140,14 @@ func MonadListens[GA ~func() T.Tuple3[A, W, SG.Semigroup[W]], GAB ~func() T.Tupl
 func Listens[GA ~func() T.Tuple3[A, W, SG.Semigroup[W]], GAB ~func() T.Tuple3[T.Tuple2[A, B], W, SG.Semigroup[W]], FCT ~func(W) B, W, A, B any](f FCT) func(GA) GAB {
 	return F.Bind2nd(MonadListens[GA, GAB, FCT], f)
 }
+
+func MonadFlap[FAB ~func(A) B, GFAB ~func() T.Tuple3[FAB, W, SG.Semigroup[W]], GB ~func() T.Tuple3[B, W, SG.Semigroup[W]], W, A, B any](fab GFAB, a A) GB {
+	return FC.MonadFlap(
+		MonadMap[GB, GFAB, func(FAB) B],
+		fab,
+		a)
+}
+
+func Flap[FAB ~func(A) B, GFAB ~func() T.Tuple3[FAB, W, SG.Semigroup[W]], GB ~func() T.Tuple3[B, W, SG.Semigroup[W]], W, A, B any](a A) func(GFAB) GB {
+	return FC.Flap(Map[GB, GFAB, func(FAB) B], a)
+}

writer/writer.go

@@ -26,6 +26,11 @@ import (
 
 type Writer[W, A any] IO.IO[T.Tuple3[A, W, S.Semigroup[W]]]
 
+// Tell appends a value to the accumulator
+func Tell[W any](s S.Semigroup[W]) func(W) Writer[W, any] {
+	return G.Tell[Writer[W, any]](s)
+}
+
 func Of[A, W any](m M.Monoid[W]) func(A) Writer[W, A] {
 	return G.Of[Writer[W, A]](m)
 }
@@ -87,21 +92,29 @@ func Evaluate[W, A any](fa Writer[W, A]) A {
 }
 
 // MonadCensor modifies the final accumulator value by applying a function
-func MonadCensor[FCT ~func(W) W, W, A any](fa Writer[W, A], f FCT) Writer[W, A] {
+func MonadCensor[A any, FCT ~func(W) W, W any](fa Writer[W, A], f FCT) Writer[W, A] {
 	return G.MonadCensor[Writer[W, A]](fa, f)
 }
 
 // Censor modifies the final accumulator value by applying a function
-func Censor[FCT ~func(W) W, W, A any](f FCT) func(Writer[W, A]) Writer[W, A] {
+func Censor[A any, FCT ~func(W) W, W any](f FCT) func(Writer[W, A]) Writer[W, A] {
 	return G.Censor[Writer[W, A]](f)
 }
 
 // MonadListens projects a value from modifications made to the accumulator during an action
-func MonadListens[FCT ~func(W) B, W, A, B any](fa Writer[W, A], f FCT) Writer[W, T.Tuple2[A, B]] {
+func MonadListens[A any, FCT ~func(W) B, W, B any](fa Writer[W, A], f FCT) Writer[W, T.Tuple2[A, B]] {
 	return G.MonadListens[Writer[W, A], Writer[W, T.Tuple2[A, B]]](fa, f)
 }
 
 // Listens projects a value from modifications made to the accumulator during an action
-func Listens[FCT ~func(W) B, W, A, B any](f FCT) func(Writer[W, A]) Writer[W, T.Tuple2[A, B]] {
+func Listens[A any, FCT ~func(W) B, W, B any](f FCT) func(Writer[W, A]) Writer[W, T.Tuple2[A, B]] {
 	return G.Listens[Writer[W, A], Writer[W, T.Tuple2[A, B]]](f)
 }
+
+func MonadFlap[W, B, A any](fab Writer[W, func(A) B], a A) Writer[W, B] {
+	return G.MonadFlap[func(A) B, Writer[W, func(A) B], Writer[W, B]](fab, a)
+}
+
+func Flap[W, B, A any](a A) func(Writer[W, func(A) B]) Writer[W, B] {
+	return G.Flap[func(A) B, Writer[W, func(A) B], Writer[W, B]](a)
+}