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Mônadas como um padrão de reutilização de código


Em um artigo anterior, discutimos por que a programação funcional não é nada do que é PR e que ela não contradiz o OOP, de modo que até Fowler escreve sobre um bom design de FP que gera um bom design de programa de OOP (e vice-versa).


Agora, quero lhe dizer o que realmente são as mônadas , como elas são úteis para um desenvolvedor praticante comum e darei exemplos de por que o suporte insuficiente em idiomas comuns leva a copiar e colar e soluções não confiáveis.


Mas existem literalmente centenas de artigos na Internet sobre FP e mônadas na Internet, por que escrever outro?


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var hashSet = CreateSomethingOfInts<HashSet>();
// …
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//    Array<int>, Nullable<int>, LinkedList<int>, ...

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public extension EnumerableFunctor of IEnumerable : Functor<IEnumerable>
{
    public static IEnumerable<B> Map<A, B>(IEnumerable<A> source, Func<A, B> map) =>
        source.Select(map);
}

//   
var range = Enumerable.Range(1, 10);
Console.WriteLine(Map(range, x => x).SequenceEquals(x)) //  True

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Nullable. :


public extension NullableFunctor of Nullable : Functor<Nullable>
{
    public static B? Map<A, B>(A? source, Func<A, B> map) =>
        source is A notNullSource ? map(notNullSource) : default(B?);
}

//   
int? nullableTen = 10;
int? nullableNull = null;
Console.WriteLine(Map(nullableTen, x => x) == nullableTen); //  True
Console.WriteLine(Map(nullableNull, x => x) == nullableNull); //  True


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public shape Applicative<T> where T : <>
{
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}

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public extension EnumerableApplicative of IEnumerable : Applicative<IEnumerable>
{
    static IEnumerable<A> Pure<A>(A a) => new[] { a };
    static IEnumerable<C> LiftA2<A, B, C>(IEnumerable<A> ta, 
                                          IEnumerable<B> tb, 
                                          Func<A, B, C> map2) =>
            ta.SelectMany(a => tb.Select(b => map2(a, b)));
}

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Nullable? :


public extension NullableApplicative of IEnumerable : Applicative<Nullable>
{
    static A? Pure<A>(A a) => a;
    static C? LiftA2<A, B, C>(A? ta, B? tb, Func<A, B, C> map2) =>
        (ta, tb) switch {
            (A a, B b) => map2(a, b), //    null -  
            _ => default(C?)          // -  -  null
        };
}

" ", , " , , ...". , , , . ZipList:


public class ZipList<T> : IEnumerable<T>
{
    private IEnumerable<T> _inner;

    // ..    IEnumerable<T>   
}

public extension ZipListApplicative of ZipList : Applicative<IEnumerable>
{
    static IEnumerable<A> Pure<A>(A a) =>
        //        'a'
        Enumerable.Repeat(a, int.MaxValue); 
    static IEnumerable<C> LiftA2<A, B, C>(IEnumerable<A> ta, 
                                          IEnumerable<B> tb, 
                                          Func<A, B, C> map2) =>
        ta.Zip(tb, map2);
}

, ZipList , . Applicative , Zip.


, , - ? ? , — - , - , , - . , , , : (F<A>, F<B>) -> F<(A, B)>. :


public static T<(A, B)> Combine(T<A> ta, T<B> tb) =>
    LiftA2(ta, tb, (a, b) => (a, b));

var eta = Enumerable.Range(3, 2);
var etb = Enumerable.Range(15, 4);

int? nta = 10;
int? ntb = null;

Combine(eta, etb) // [(3, 15), (3, 16), (3, 17), (3, 18), (4, 15), (4, 16), (4, 17), (4, 18)]
Combine(nta, nta) // (10, 10)
Combine(nta, ntb) // Null
Combine(new ZipList<int>(eta), new ZipList<int>(etb)) //  [(3, 15), (4, 16)]


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,


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static T<B> MapAnyFunctor<T, A, B>(T<A> source, Func<A, B> map) where T : Applicative =>
    LiftA2(source, Pure(0), (a, _) => map(a));

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public shape Applicative<T> : Functor<T> where T : <>
{
    static T<A> Pure<A>(A a);
    static T<C> LiftA2<A, B, C>(T<A> ta, T<B> tb, Func<A,B,C> map2);
}

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Task<T>:


public extension TaskApplicative of Task: Applicative<Task>
{
    public static Task<A> Pure<A>(A a) => Task.FromResult(a);

    public static async Task<C> LiftA2<A, B, C>(Task<A> ta, Task<B> tb, Func<A, B, C> map2)
    {
        await Task.WhenAll(ta, tb);
        return map2(ta.Result, tb.Result);
    }
}


Monad


. , .
, :


public shape Monad<T> where T : <>
{
    static T<A> Pure<A>(A a);
    static T<B> Bind<A, B>(T<A> ta, Func<A, T<B>> mapInner);
}

-, :


— Pure Bind, T<A> , A T<B>, T<B>

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public extension EnumerableMonad of IEnumerable : Monad<IEnumerable>
{
    static IEnumerable<B> Bind<A, B>(IEnumerable<A> ta, Func<A, IEnumerable<B>> mapInner) =>
        ta.SelectMany(mapInner);
    // Pure      
}

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let foo = do
  a <- someA
  b <- someB
  pure (doSomethingWith a b)

Bind :


var foo = Bind(someA, a => Bind(SomeB, b => Pure(DoSomethingWIth(a,b)))

. <- Bind,
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.


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var values = from x in new []{ 1, 2, 3 }
             from y in new []{ 4, 5, 6 }
             select x + y;

, do- ( ):


let values = do 
    x <- [1,2,3]
    y <- [4,5,6]
    pure (x + y)

:


var maybeA = GetMaybeA();
var maybeB = maybeA?.GetB();
var maybeC = maybeB?.GetC();
var result = maybeC;

do- Maybe ( Option, — Nullable):


let result = do
    maybeA <- getMaybeA
    maybeB <- getB maybeA
    maybeC <- getC maybeB
    pure maybeC

?


var valueA = await GetSomeA();
var valueB = await GetSomeB(valueA);
var result = valueB;

do- IO ( , , , Task ):


let result = do
    valueA <- getSomeA
    valueB <- getSomeB valueA
    pure valueB

,
: T<>
A T<B>, T<B>, A[] A -> B[], A? A -> B?,
Task<A> A -> Task<B>,… .


...




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. , , : SOLID. ? , , , :


public M<Comment> GetArticleComment<M>(int articleId) 
    where M : MonadWriter<LogMessage[]>, MonadReader<Config>, MonadHttp<AllowedSite>

MonadWriter/MonadReader/MonadHttp — SOLID , . , ( LogMessage!), ( Config!) Http ( AllowedSite!), .


, , . , , Task. return 10, Task<int>.


, Task M, — , .


, SOLID, — . Autofac/Windsor/Ninject/… " ", where , , . , , , .



, , , .


, , , . : , , - - .


class MyService 
{
    async Task<Comment> SomeBusinessLogicAsync(int commentId) {
        var comment = await this.remoteClient.GetAsync($"some/url/{commentId}");
        // .. do stuff ..
        return await DoOtherStuffAsync(comment);
    }
}

. C# ?
, , MyService remoteClient , .


- , httpClient, ( , ), act/assert .


? , ( ) Id, :


public class Id<T>
{
    public T Value { get; }

    public Id(T value)
    {
        Value = value
    }
}

public extension IdMonad of Id : Monad<Id>
{
    static IdMonad<A> Pure<A>(A a) => new Id<A>(a); //   
    static IdMonad<B> Bind<A, B>(IdMonad<A> ta, Func<A, IdMonad<B>> mapInner)  =>
        mapInner(ta.Value); //      
    //  map  liftA2  -
}

:


Task<Comment> SomeBusinessLogicAsync(int commentId);


M<Comment> SomeBusinessLogicAsync<M>(int commentId) where M : Monad =>
    this.remoteClient.Get($"some/url/{commentId}").Bind(comment => 
        // .. do stuff ..
        return DoOtherStuff(comment);
    );

do- 11, . :


var comment = await myService.SomeBusinessLogicAsync<Task>(547);

:


var comment = myService.SomeBusinessLogicAsync<Id>(547).Value;

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    -


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    "".


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    T<A>


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Mônadas são apenas uma ferramenta para poder usar. Estudar é um investimento bem-sucedido que salvará mais de um mês de vida, destruindo a causa de muitas perguntas “bem, que diabos isso não funciona, eu verifiquei tudo ” pela raiz. E embora nenhuma técnica de programação o salve de todos os problemas , é um pecado não usar uma ferramenta que resolva uma parte significativa deles.

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