fix: add more mostly-adequate examples and solutions

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

either/traverse.go

@@ -27,13 +27,13 @@ HKTRB = HKT<Either[B]>
 HKTA = HKT<A>
 HKTB = HKT<B>
 */
-func traverse[E, A, B, HKTA, HKTB, HKTRB any](
+func traverse[E, A, B, HKTB, HKTRB any](
-	_of func(Either[E, B]) HKTRB,
+	mof func(Either[E, B]) HKTRB,
-	_map func(HKTB, func(B) Either[E, B]) HKTRB,
+	mmap func(func(B) Either[E, B]) func(HKTB) HKTRB,
 ) func(Either[E, A], func(A) HKTB) HKTRB {
 
-	left := F.Flow2(Left[B, E], _of)
+	left := F.Flow2(Left[B, E], mof)
-	right := F.Bind2nd(_map, Right[E, B])
+	right := mmap(Right[E, B])
 
 	return func(ta Either[E, A], f func(A) HKTB) HKTRB {
 		return MonadFold(ta,
@@ -43,27 +43,21 @@ func traverse[E, A, B, HKTA, HKTB, HKTRB any](
 	}
 }
 
-func Traverse[E, A, B, HKTA, HKTB, HKTRB any](
+// Traverse converts an [Either] of some higher kinded type into the higher kinded type of an [Either]
-	_of func(Either[E, B]) HKTRB,
+func Traverse[A, E, B, HKTB, HKTRB any](
-	_map func(HKTB, func(B) Either[E, B]) HKTRB,
+	mof func(Either[E, B]) HKTRB,
+	mmap func(func(B) Either[E, B]) func(HKTB) HKTRB,
 ) func(func(A) HKTB) func(Either[E, A]) HKTRB {
-	delegate := traverse[E, A, B, HKTA](_of, _map)
+	delegate := traverse[E, A, B](mof, mmap)
 	return func(f func(A) HKTB) func(Either[E, A]) HKTRB {
 		return F.Bind2nd(delegate, f)
 	}
 }
 
-/*
+// Sequence converts an [Either] of some higher kinded type into the higher kinded type of an [Either]
-*
-We need to pass the members of the applicative explicitly, because golang does neither support higher kinded types nor template methods on structs or interfaces
-
-HKTRA = HKT<Either[A]>
-HKTA = HKT<A>
-HKTB = HKT<B>
-*/
 func Sequence[E, A, HKTA, HKTRA any](
-	_of func(Either[E, A]) HKTRA,
+	mof func(Either[E, A]) HKTRA,
-	_map func(HKTA, func(A) Either[E, A]) HKTRA,
+	mmap func(func(A) Either[E, A]) func(HKTA) HKTRA,
 ) func(Either[E, HKTA]) HKTRA {
-	return Fold(F.Flow2(Left[A, E], _of), F.Bind2nd(_map, Right[E, A]))
+	return Fold(F.Flow2(Left[A, E], mof), mmap(Right[E, A]))
 }

either/traverse_test.go

@@ -30,9 +30,9 @@ func TestTraverse(t *testing.T) {
 		}
 		return O.None[int]()
 	}
-	trav := Traverse[string, int, int, O.Option[Either[string, int]]](
+	trav := Traverse[int](
 		O.Of[Either[string, int]],
-		O.MonadMap[int, Either[string, int]],
+		O.Map[int, Either[string, int]],
 	)(f)
 
 	assert.Equal(t, O.Of(Left[int]("a")), F.Pipe1(Left[int]("a"), trav))
@@ -44,7 +44,7 @@ func TestSequence(t *testing.T) {
 
 	seq := Sequence(
 		O.Of[Either[string, int]],
-		O.MonadMap[int, Either[string, int]],
+		O.Map[int, Either[string, int]],
 	)
 
 	assert.Equal(t, O.Of(Right[string](1)), seq(Right[string](O.Of(1))))

option/sequence.go

@@ -19,13 +19,22 @@ import (
 	F "github.com/IBM/fp-go/function"
 )
 
-// HKTA = HKT<A>
+// Sequence converts an [Option] of some higher kinded type into the higher kinded type of an [Option]
-// HKTOA = HKT<Option<A>>
-//
-// Sequence converts an option of some higher kinded type into the higher kinded type of an option
 func Sequence[A, HKTA, HKTOA any](
-	_of func(Option[A]) HKTOA,
+	mof func(Option[A]) HKTOA,
-	_map func(HKTA, func(A) Option[A]) HKTOA,
+	mmap func(func(A) Option[A]) func(HKTA) HKTOA,
 ) func(Option[HKTA]) HKTOA {
-	return Fold(F.Nullary2(None[A], _of), F.Bind2nd(_map, Some[A]))
+	return Fold(F.Nullary2(None[A], mof), mmap(Some[A]))
+}
+
+// Traverse converts an [Option] of some higher kinded type into the higher kinded type of an [Option]
+func Traverse[A, B, HKTB, HKTOB any](
+	mof func(Option[B]) HKTOB,
+	mmap func(func(B) Option[B]) func(HKTB) HKTOB,
+) func(func(A) HKTB) func(Option[A]) HKTOB {
+	onNone := F.Nullary2(None[B], mof)
+	onSome := mmap(Some[B])
+	return func(f func(A) HKTB) func(Option[A]) HKTOB {
+		return Fold(onNone, F.Flow2(f, onSome))
+	}
 }

record/generic/record.go

@@ -186,6 +186,13 @@ func MapRefWithIndex[M ~map[K]V, N ~map[K]R, K comparable, V, R any](f func(K, *
 	return F.Bind2nd(MonadMapRefWithIndex[M, N, K, V, R], f)
 }
 
+func MonadLookup[M ~map[K]V, K comparable, V any](m M, k K) O.Option[V] {
+	if val, ok := m[k]; ok {
+		return O.Some(val)
+	}
+	return O.None[V]()
+}
+
 func Lookup[M ~map[K]V, K comparable, V any](k K) func(M) O.Option[V] {
 	n := O.None[V]()
 	return func(m M) O.Option[V] {

record/record.go

@@ -102,6 +102,11 @@ func Lookup[V any, K comparable](k K) func(map[K]V) O.Option[V] {
 	return G.Lookup[map[K]V](k)
 }
 
+// MonadLookup returns the entry for a key in a map if it exists
+func MonadLookup[V any, K comparable](m map[K]V, k K) O.Option[V] {
+	return G.MonadLookup[map[K]V](m, k)
+}
+
 // Has tests if a key is contained in a map
 func Has[K comparable, V any](k K, r map[K]V) bool {
 	return G.Has(k, r)

samples/mostly-adequate/chapter09_monadiconions_test.go

@@ -18,10 +18,14 @@ package mostlyadequate
 import (
 	"fmt"
 	"path"
+	"regexp"
 
 	A "github.com/IBM/fp-go/array"
+	E "github.com/IBM/fp-go/either"
+	"github.com/IBM/fp-go/errors"
 	F "github.com/IBM/fp-go/function"
 	"github.com/IBM/fp-go/io"
+	IOE "github.com/IBM/fp-go/ioeither"
 	O "github.com/IBM/fp-go/option"
 	S "github.com/IBM/fp-go/string"
 )
@@ -104,6 +108,21 @@ var (
 
 	// pureLog :: String -> IO ()
 	pureLog = io.Logf[string]("%s")
+
+	// addToMailingList :: Email -> IOEither([Email])
+	addToMailingList = F.Flow2(
+		A.Of[string],
+		IOE.Of[error, []string],
+	)
+
+	// validateEmail :: Email -> Either error Email
+	validateEmail = E.FromPredicate(Matches(regexp.MustCompile(`\S+@\S+\.\S+`)), errors.OnSome[string]("email %s is invalid"))
+
+	// emailBlast :: [Email] -> IO ()
+	emailBlast = F.Flow2(
+		A.Intercalate(S.Monoid)(","),
+		IOE.Of[error, string],
+	)
 )
 
 func Example_street() {
@@ -147,3 +166,21 @@ func Example_solution09B() {
 	// Output:
 	// ch09.md
 }
+
+func Example_solution09C() {
+
+	// // joinMailingList :: Email -> Either String (IO ())
+	joinMailingList := F.Flow4(
+		validateEmail,
+		IOE.FromEither[error, string],
+		IOE.Chain(addToMailingList),
+		IOE.Chain(emailBlast),
+	)
+
+	fmt.Println(joinMailingList("sleepy@grandpa.net")())
+	fmt.Println(joinMailingList("notanemail")())
+
+	// Output:
+	// Right[<nil>, string](sleepy@grandpa.net)
+	// Left[*errors.errorString, string](email notanemail is invalid)
+}

samples/mostly-adequate/chapter10_applicativefunctor_test.go

@@ -23,16 +23,65 @@ import (
 	R "github.com/IBM/fp-go/context/readerioeither"
 	H "github.com/IBM/fp-go/context/readerioeither/http"
 	F "github.com/IBM/fp-go/function"
+	IOO "github.com/IBM/fp-go/iooption"
+	N "github.com/IBM/fp-go/number"
+	O "github.com/IBM/fp-go/option"
+	M "github.com/IBM/fp-go/record"
+	T "github.com/IBM/fp-go/tuple"
 )
 
-type PostItem struct {
+type (
+	PostItem struct {
 		UserId uint   `json:"userId"`
 		Id     uint   `json:"id"`
 		Title  string `json:"title"`
 		Body   string `json:"body"`
+	}
+
+	Player struct {
+		Id   int
+		Name string
+	}
+
+	LocalStorage = map[string]Player
+)
+
+var (
+	playerAlbert = Player{
+		Id:   1,
+		Name: "Albert",
+	}
+	playerTheresa = Player{
+		Id:   2,
+		Name: "Theresa",
+	}
+	localStorage = LocalStorage{
+		"player1": playerAlbert,
+		"player2": playerTheresa,
+	}
+
+	// getFromCache :: String -> IO User
+	getFromCache = func(name string) IOO.IOOption[Player] {
+		return func() O.Option[Player] {
+			return M.MonadLookup(localStorage, name)
+		}
+	}
+
+	// game :: User -> User -> String
+	game = F.Curry2(func(a, b Player) string {
+		return fmt.Sprintf("%s vs %s", a.Name, b.Name)
+	})
+)
+
+func (player Player) getName() string {
+	return player.Name
 }
 
-func getTitle(item PostItem) string {
+func (player Player) getId() int {
+	return player.Id
+}
+
+func (item PostItem) getTitle() string {
 	return item.Title
 }
 
@@ -55,7 +104,7 @@ func Example_renderPage() {
 		idxToUrl,
 		H.MakeGetRequest,
 		H.ReadJson[PostItem](client),
-		R.Map(getTitle),
+		R.Map(PostItem.getTitle),
 	)
 
 	res := F.Pipe2(
@@ -71,3 +120,64 @@ func Example_renderPage() {
 	// Right[<nil>, string](<div>Destinations: [qui est esse], Events: [ea molestias quasi exercitationem repellat qui ipsa sit aut]</div>)
 
 }
+
+func Example_solution10A() {
+	safeAdd := F.Curry2(func(a, b O.Option[int]) O.Option[int] {
+		return F.Pipe3(
+			N.Add[int],
+			O.Of[func(int) func(int) int],
+			O.Ap[func(int) int](a),
+			O.Ap[int](b),
+		)
+	})
+
+	fmt.Println(safeAdd(O.Of(2))(O.Of(3)))
+	fmt.Println(safeAdd(O.None[int]())(O.Of(3)))
+	fmt.Println(safeAdd(O.Of(2))(O.None[int]()))
+
+	// Output:
+	// Some[int](5)
+	// None[int]
+	// None[int]
+}
+
+func Example_solution10B() {
+
+	safeAdd := F.Curry2(T.Untupled2(F.Flow2(
+		O.SequenceTuple2[int, int],
+		O.Map(T.Tupled2(N.MonoidSum[int]().Concat)),
+	)))
+
+	fmt.Println(safeAdd(O.Of(2))(O.Of(3)))
+	fmt.Println(safeAdd(O.None[int]())(O.Of(3)))
+	fmt.Println(safeAdd(O.Of(2))(O.None[int]()))
+
+	// Output:
+	// Some[int](5)
+	// None[int]
+	// None[int]
+}
+
+func Example_solution10C() {
+	// startGame :: IO String
+	startGame := F.Pipe2(
+		IOO.Of(game),
+		IOO.Ap[func(Player) string](getFromCache("player1")),
+		IOO.Ap[string](getFromCache("player2")),
+	)
+
+	startGameTupled := F.Pipe2(
+		T.MakeTuple2("player1", "player2"),
+		IOO.TraverseTuple2(getFromCache, getFromCache),
+		IOO.Map(T.Tupled2(func(a, b Player) string {
+			return fmt.Sprintf("%s vs %s", a.Name, b.Name)
+		})),
+	)
+
+	fmt.Println(startGame())
+	fmt.Println(startGameTupled())
+
+	// Output:
+	// Some[string](Albert vs Theresa)
+	// Some[string](Albert vs Theresa)
+}

samples/mostly-adequate/chapter11_transformagain_test.go

@@ -0,0 +1,89 @@
+// 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 mostlyadequate
+
+import (
+	"fmt"
+	"regexp"
+
+	A "github.com/IBM/fp-go/array"
+	E "github.com/IBM/fp-go/either"
+	F "github.com/IBM/fp-go/function"
+	IOE "github.com/IBM/fp-go/ioeither"
+	S "github.com/IBM/fp-go/string"
+)
+
+func findUserById(id int) IOE.IOEither[error, Chapter08User] {
+	switch id {
+	case 1:
+		return IOE.Of[error](albert08)
+	case 2:
+		return IOE.Of[error](gary08)
+	case 3:
+		return IOE.Of[error](theresa08)
+	default:
+		return IOE.Left[Chapter08User](fmt.Errorf("user %d not found", id))
+	}
+}
+
+func Example_solution11A() {
+	// eitherToMaybe :: Either b a -> Maybe a
+	eitherToMaybe := E.ToOption[error, string]
+
+	fmt.Println(eitherToMaybe(E.Of[error]("one eyed willy")))
+	fmt.Println(eitherToMaybe(E.Left[string](fmt.Errorf("some error"))))
+
+	// Output:
+	// Some[string](one eyed willy)
+	// None[string]
+}
+
+func Example_solution11B() {
+	findByNameId := F.Flow2(
+		findUserById,
+		IOE.Map[error](Chapter08User.getName),
+	)
+
+	fmt.Println(findByNameId(1)())
+	fmt.Println(findByNameId(2)())
+	fmt.Println(findByNameId(3)())
+	fmt.Println(findByNameId(4)())
+
+	// Output:
+	// Right[<nil>, string](Albert)
+	// Right[<nil>, string](Gary)
+	// Right[<nil>, string](Theresa)
+	// Left[*errors.errorString, string](user 4 not found)
+}
+
+func Example_solution11C() {
+	// strToList :: String -> [Char
+	strToList := Split(regexp.MustCompile(``))
+
+	// listToStr :: [Char] -> String
+	listToStr := A.Intercalate(S.Monoid)("")
+
+	sortLetters := F.Flow3(
+		strToList,
+		A.Sort(S.Ord),
+		listToStr,
+	)
+
+	fmt.Println(sortLetters("sortme"))
+
+	// Output:
+	// emorst
+}

samples/mostly-adequate/chapter12_traversing_test.go

@@ -0,0 +1,98 @@
+// 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 mostlyadequate
+
+import (
+	"fmt"
+
+	A "github.com/IBM/fp-go/array"
+	E "github.com/IBM/fp-go/either"
+	"github.com/IBM/fp-go/errors"
+	F "github.com/IBM/fp-go/function"
+	IOE "github.com/IBM/fp-go/ioeither"
+	O "github.com/IBM/fp-go/option"
+	P "github.com/IBM/fp-go/predicate"
+	S "github.com/IBM/fp-go/string"
+)
+
+var (
+	// httpGet :: Route -> Task Error JSON
+	httpGet = F.Flow2(
+		S.Format[string]("json for %s"),
+		IOE.Of[error, string],
+	)
+
+	// routes :: Map Route Route
+	routes = map[string]string{
+		"/":      "/",
+		"/about": "/about",
+	}
+
+	// validate :: Player -> Either error Player
+	validatePlayer = E.FromPredicate(P.ContraMap(Player.getName)(S.IsNonEmpty), F.Flow2(Player.getId, errors.OnSome[int]("player %d must have a name")))
+
+	// readfile :: String -> String -> Task Error String
+	readfile = F.Curry2(func(encoding, file string) IOE.IOEither[error, string] {
+		return IOE.Of[error](fmt.Sprintf("content of %s (%s)", file, encoding))
+	})
+
+	//   readdir :: String -> Task Error [String]
+	readdir = IOE.Of[error](A.From("file1", "file2", "file3"))
+)
+
+func Example_solution12A() {
+	// getJsons :: Map Route Route -> Task Error (Map Route JSON)
+	getJsons := IOE.TraverseRecord[string](httpGet)
+
+	fmt.Println(getJsons(routes)())
+
+	// Output:
+	// Right[<nil>, map[string]string](map[/:json for / /about:json for /about])
+}
+
+func Example_solution12B() {
+	// startGame :: [Player] -> [Either Error String]
+	startGame := F.Flow2(
+		E.TraverseArray(validatePlayer),
+		E.MapTo[error, []Player]("Game started"),
+	)
+
+	fmt.Println(startGame(A.From(playerAlbert, playerTheresa)))
+	fmt.Println(startGame(A.From(playerAlbert, Player{Id: 4})))
+
+	// Output:
+	// Right[<nil>, string](Game started)
+	// Left[*errors.errorString, string](player 4 must have a name)
+}
+
+func Example_solution12C() {
+	traverseO := O.Traverse[string](
+		IOE.Of[error, O.Option[string]],
+		IOE.Map[error, string, O.Option[string]],
+	)
+
+	// readFirst :: String -> Task Error (Maybe String)
+	readFirst := F.Pipe2(
+		readdir,
+		IOE.Map[error](A.Head[string]),
+		IOE.Chain(traverseO(readfile("utf-8"))),
+	)
+
+	fmt.Println(readFirst())
+
+	// Output:
+	// Right[<nil>, option.Option[string]](Some[string](content of file1 (utf-8)))
+}