fix: add State monad

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

scan.bat

@@ -0,0 +1,3 @@
+@echo off
+
+busybox find . -type f -name "*\.go" | busybox xargs gopls check

state/eq.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2023 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package state
+
+import (
+	EQ "github.com/IBM/fp-go/eq"
+	G "github.com/IBM/fp-go/state/generic"
+)
+
+// Constructs an equal predicate for a [State]
+func Eq[S, A any](w EQ.Eq[S], a EQ.Eq[A]) func(S) EQ.Eq[State[S, A]] {
+	return G.Eq[State[S, A]](w, a)
+}
+
+// FromStrictEquals constructs an [EQ.Eq] from the canonical comparison function
+func FromStrictEquals[S, A comparable]() func(S) EQ.Eq[State[S, A]] {
+	return G.FromStrictEquals[State[S, A]]()
+}

state/generic/eq.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2023 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package generic
+
+import (
+	EQ "github.com/IBM/fp-go/eq"
+	P "github.com/IBM/fp-go/pair"
+)
+
+// Constructs an equal predicate for a [State]
+func Eq[GA ~func(S) P.Pair[A, S], S, A any](w EQ.Eq[S], a EQ.Eq[A]) func(S) EQ.Eq[GA] {
+	eqp := P.Eq(a, w)
+	return func(s S) EQ.Eq[GA] {
+		return EQ.FromEquals(func(l, r GA) bool {
+			return eqp.Equals(l(s), r(s))
+		})
+	}
+}
+
+// FromStrictEquals constructs an [EQ.Eq] from the canonical comparison function
+func FromStrictEquals[GA ~func(S) P.Pair[A, S], S, A comparable]() func(S) EQ.Eq[GA] {
+	return Eq[GA](EQ.FromStrictEquals[S](), EQ.FromStrictEquals[A]())
+}

state/generic/monad.go

@@ -0,0 +1,88 @@
+// Copyright (c) 2024 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package generic
+
+import (
+	"github.com/IBM/fp-go/internal/applicative"
+	"github.com/IBM/fp-go/internal/functor"
+	"github.com/IBM/fp-go/internal/monad"
+	"github.com/IBM/fp-go/internal/pointed"
+	P "github.com/IBM/fp-go/pair"
+)
+
+type statePointed[GA ~func(S) P.Pair[A, S], S, A any] struct{}
+
+type stateFunctor[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], S, A, B any] struct{}
+
+type stateApplicative[GB ~func(S) P.Pair[B, S], GAB ~func(S) P.Pair[func(A) B, S], GA ~func(S) P.Pair[A, S], S, A, B any] struct{}
+
+type stateMonad[GB ~func(S) P.Pair[B, S], GAB ~func(S) P.Pair[func(A) B, S], GA ~func(S) P.Pair[A, S], S, A, B any] struct{}
+
+func (o *statePointed[GA, S, A]) Of(a A) GA {
+	return Of[GA](a)
+}
+
+func (o *stateApplicative[GB, GAB, GA, S, A, B]) Of(a A) GA {
+	return Of[GA](a)
+}
+
+func (o *stateMonad[GB, GAB, GA, S, A, B]) Of(a A) GA {
+	return Of[GA](a)
+}
+
+func (o *stateFunctor[GB, GA, S, A, B]) Map(f func(A) B) func(GA) GB {
+	return Map[GB, GA](f)
+}
+
+func (o *stateApplicative[GB, GAB, GA, S, A, B]) Map(f func(A) B) func(GA) GB {
+	return Map[GB, GA](f)
+}
+
+func (o *stateMonad[GB, GAB, GA, S, A, B]) Map(f func(A) B) func(GA) GB {
+	return Map[GB, GA](f)
+}
+
+func (o *stateMonad[GB, GAB, GA, S, A, B]) Chain(f func(A) GB) func(GA) GB {
+	return Chain[GB, GA](f)
+}
+
+func (o *stateApplicative[GB, GAB, GA, S, A, B]) Ap(fa GA) func(GAB) GB {
+	return Ap[GB, GAB, GA](fa)
+}
+
+func (o *stateMonad[GB, GAB, GA, S, A, B]) Ap(fa GA) func(GAB) GB {
+	return Ap[GB, GAB, GA](fa)
+}
+
+// Pointed implements the pointed operations for [Writer]
+func Pointed[GA ~func(S) P.Pair[A, S], S, A any]() pointed.Pointed[A, GA] {
+	return &statePointed[GA, S, A]{}
+}
+
+// Functor implements the functor operations for [Writer]
+func Functor[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], S, A, B any]() functor.Functor[A, B, GA, GB] {
+	return &stateFunctor[GB, GA, S, A, B]{}
+}
+
+// Applicative implements the applicative operations for [Writer]
+func Applicative[GB ~func(S) P.Pair[B, S], GAB ~func(S) P.Pair[func(A) B, S], GA ~func(S) P.Pair[A, S], S, A, B any]() applicative.Applicative[A, B, GA, GB, GAB] {
+	return &stateApplicative[GB, GAB, GA, S, A, B]{}
+}
+
+// Monad implements the monadic operations for [Writer]
+func Monad[GB ~func(S) P.Pair[B, S], GAB ~func(S) P.Pair[func(A) B, S], GA ~func(S) P.Pair[A, S], S, A, B any]() monad.Monad[A, B, GA, GB, GAB] {
+	return &stateMonad[GB, GAB, GA, S, A, B]{}
+}

state/generic/state.go

@@ -0,0 +1,131 @@
+// Copyright (c) 2024 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+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"
+	P "github.com/IBM/fp-go/pair"
+)
+
+var (
+	undefined any = struct{}{}
+)
+
+func Get[GA ~func(S) P.Pair[S, S], S any]() GA {
+	return P.Of[S]
+}
+
+func Gets[GA ~func(S) P.Pair[A, S], FCT ~func(S) A, A, S any](f FCT) GA {
+	return func(s S) P.Pair[A, S] {
+		return P.MakePair(f(s), s)
+	}
+}
+
+func Put[GA ~func(S) P.Pair[any, S], S any]() GA {
+	return F.Bind1st(P.MakePair[any, S], undefined)
+}
+
+func Modify[GA ~func(S) P.Pair[any, S], FCT ~func(S) S, S any](f FCT) GA {
+	return F.Flow2(
+		f,
+		F.Bind1st(P.MakePair[any, S], undefined),
+	)
+}
+
+func Of[GA ~func(S) P.Pair[A, S], S, A any](a A) GA {
+	return F.Bind1st(P.MakePair[A, S], a)
+}
+
+func MonadMap[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], FCT ~func(A) B, S, A, B any](fa GA, f FCT) GB {
+	return func(s S) P.Pair[B, S] {
+		p2 := fa(s)
+		return P.MakePair(f(P.Head(p2)), P.Tail(p2))
+	}
+}
+
+func Map[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], FCT ~func(A) B, S, A, B any](f FCT) func(GA) GB {
+	return F.Bind2nd(MonadMap[GB, GA, FCT, S, A, B], f)
+}
+
+func MonadChain[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], FCT ~func(A) GB, S, A, B any](fa GA, f FCT) GB {
+	return func(s S) P.Pair[B, S] {
+		a := fa(s)
+		return f(P.Head(a))(P.Tail(a))
+	}
+}
+
+func Chain[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], FCT ~func(A) GB, S, A, B any](f FCT) func(GA) GB {
+	return F.Bind2nd(MonadChain[GB, GA, FCT, S, A, B], f)
+}
+
+func MonadAp[GB ~func(S) P.Pair[B, S], GAB ~func(S) P.Pair[func(A) B, S], GA ~func(S) P.Pair[A, S], S, A, B any](fab GAB, fa GA) GB {
+	return func(s S) P.Pair[B, S] {
+		f := fab(s)
+		a := fa(P.Tail(f))
+
+		return P.MakePair(P.Head(f)(P.Head(a)), P.Tail(a))
+	}
+}
+
+func Ap[GB ~func(S) P.Pair[B, S], GAB ~func(S) P.Pair[func(A) B, S], GA ~func(S) P.Pair[A, S], S, A, B any](ga GA) func(GAB) GB {
+	return F.Bind2nd(MonadAp[GB, GAB, GA, S, A, B], ga)
+}
+
+func MonadChainFirst[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], FCT ~func(A) GB, S, A, B any](ma GA, f FCT) GA {
+	return C.MonadChainFirst(
+		MonadChain[GA, GA, func(A) GA],
+		MonadMap[GA, GB, func(B) A],
+		ma,
+		f,
+	)
+}
+
+func ChainFirst[GB ~func(S) P.Pair[B, S], GA ~func(S) P.Pair[A, S], FCT ~func(A) GB, S, A, B any](f FCT) func(GA) GA {
+	return C.ChainFirst(
+		Chain[GA, GA, func(A) GA],
+		Map[GA, GB, func(B) A],
+		f,
+	)
+}
+
+func Flatten[GAA ~func(S) P.Pair[GA, S], GA ~func(S) P.Pair[A, S], S, A any](mma GAA) GA {
+	return MonadChain[GA, GAA, func(GA) GA](mma, F.Identity[GA])
+}
+
+func Execute[GA ~func(S) P.Pair[A, S], S, A any](s S) func(GA) S {
+	return func(fa GA) S {
+		return P.Tail(fa(s))
+	}
+}
+
+func Evaluate[GA ~func(S) P.Pair[A, S], S, A any](s S) func(GA) A {
+	return func(fa GA) A {
+		return P.Head(fa(s))
+	}
+}
+
+func MonadFlap[FAB ~func(A) B, GFAB ~func(S) P.Pair[FAB, S], GB ~func(S) P.Pair[B, S], S, 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(S) P.Pair[FAB, S], GB ~func(S) P.Pair[B, S], S, A, B any](a A) func(GFAB) GB {
+	return FC.Flap(Map[GB, GFAB, func(FAB) B], a)
+}

state/monad.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2024 IBM Corp.
+// All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package state
+
+import (
+	"github.com/IBM/fp-go/internal/applicative"
+	"github.com/IBM/fp-go/internal/functor"
+	"github.com/IBM/fp-go/internal/monad"
+	"github.com/IBM/fp-go/internal/pointed"
+	G "github.com/IBM/fp-go/state/generic"
+)
+
+// Pointed implements the pointed operations for [State]
+func Pointed[S, A any]() pointed.Pointed[A, State[S, A]] {
+	return G.Pointed[State[S, A], S, A]()
+}
+
+// Functor implements the pointed operations for [State]
+func Functor[S, A, B any]() functor.Functor[A, B, State[S, A], State[S, B]] {
+	return G.Functor[State[S, B], State[S, A], S, A, B]()
+}
+
+// Applicative implements the applicative operations for [State]
+func Applicative[S, A, B any]() applicative.Applicative[A, B, State[S, A], State[S, B], State[S, func(A) B]] {
+	return G.Applicative[State[S, B], State[S, func(A) B], State[S, A]]()
+}
+
+// Monad implements the monadic operations for [State]
+func Monad[S, A, B any]() monad.Monad[A, B, State[S, A], State[S, B], State[S, func(A) B]] {
+	return G.Monad[State[S, B], State[S, func(A) B], State[S, A]]()
+}