fix: implement filter and filtermap

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

v2/internal/filterable/types.go

@@ -0,0 +1,15 @@
+package filterable
+
+import (
+	"github.com/IBM/fp-go/v2/option"
+	"github.com/IBM/fp-go/v2/pair"
+)
+
+type (
+	Option[A any]       = option.Option[A]
+	Separated[A, B any] = pair.Pair[A, B]
+
+	FilterType[A, HKTA any] = func(func(A) bool) func(HKTA) HKTA
+
+	FilterMapType[A, B, HKTA, HKTB any] = func(func(A) Option[B]) func(HKTA) HKTB
+)

v2/internal/witherable/filter.go

@@ -0,0 +1,27 @@
+package witherable
+
+import (
+	"github.com/IBM/fp-go/v2/function"
+	"github.com/IBM/fp-go/v2/internal/filterable"
+	"github.com/IBM/fp-go/v2/internal/functor"
+)
+
+func Filter[A, HKT_G_A, HKT_F_HKT_G_A any](
+	fmap functor.MapType[HKT_G_A, HKT_G_A, HKT_F_HKT_G_A, HKT_F_HKT_G_A],
+	ffilter filterable.FilterType[A, HKT_G_A],
+) func(func(A) bool) func(HKT_F_HKT_G_A) HKT_F_HKT_G_A {
+	return function.Flow2(
+		ffilter,
+		fmap,
+	)
+}
+
+func FilterMap[A, B, HKT_G_A, HKT_G_B, HKT_F_HKT_G_A, HKT_F_HKT_G_B any](
+	fmap functor.MapType[HKT_G_A, HKT_G_B, HKT_F_HKT_G_A, HKT_F_HKT_G_B],
+	ffilter filterable.FilterMapType[A, B, HKT_G_A, HKT_G_B],
+) func(func(A) Option[B]) func(HKT_F_HKT_G_A) HKT_F_HKT_G_B {
+	return function.Flow2(
+		ffilter,
+		fmap,
+	)
+}

v2/internal/witherable/partition.go

@@ -0,0 +1 @@
+package witherable

v2/internal/witherable/type.go

@@ -0,0 +1,7 @@
+package witherable
+
+import "github.com/IBM/fp-go/v2/option"
+
+type (
+	Option[A any] = option.Option[A]
+)