fix: implement Sequence for Pair

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

v2/pair/sequence.go

@@ -0,0 +1,48 @@
+package pair
+
+import "github.com/IBM/fp-go/v2/function"
+
+func MonadSequence[L, A, HKTA, HKTPA any](
+	mmap func(HKTA, Kleisli[L, A, A]) HKTPA,
+	fas Pair[L, HKTA],
+) HKTPA {
+	return mmap(Tail(fas), FromHead[A](Head(fas)))
+}
+
+func MonadTraverse[L, A, HKTA, HKTPA any](
+	mmap func(HKTA, Kleisli[L, A, A]) HKTPA,
+	f func(A) HKTA,
+	fas Pair[L, A],
+) HKTPA {
+	return mmap(f(Tail(fas)), FromHead[A](Head(fas)))
+}
+
+func Sequence[L, A, HKTA, HKTPA any](
+	mmap func(Kleisli[L, A, A]) func(HKTA) HKTPA,
+) func(Pair[L, HKTA]) HKTPA {
+	fh := function.Flow2(
+		Head[L, HKTA],
+		FromHead[A, L],
+	)
+	return func(fas Pair[L, HKTA]) HKTPA {
+		return mmap(fh(fas))(Tail(fas))
+	}
+}
+
+func Traverse[L, A, HKTA, HKTPA any](
+	mmap func(Kleisli[L, A, A]) func(HKTA) HKTPA,
+) func(func(A) HKTA) func(Pair[L, A]) HKTPA {
+	fh := function.Flow2(
+		Head[L, A],
+		FromHead[A, L],
+	)
+	return func(f func(A) HKTA) func(Pair[L, A]) HKTPA {
+		ft := function.Flow2(
+			Tail[L, A],
+			f,
+		)
+		return func(fas Pair[L, A]) HKTPA {
+			return mmap(fh(fas))(ft(fas))
+		}
+	}
+}