出于教育原因,我试图在C#中实现F#中的Choice和Option Type。这是受到“真实世界功能编程”一书的启发。以及一些博客文章:http://bugsquash.blogspot.de/2011/08/refactoring-to-monadic-c-applicative.html和http://tomasp.net/blog/idioms-in-linq.aspx/。
我想让这个工作,但我不知道如何实现选择类型的扩展(Bind,Map,SelectMany,...):
public static void Division()
{
Console.WriteLine("Enter two (floating point) numbers:");
(
from f1 in ReadDouble().ToChoice("Could not parse input to a double.")
from f2 in ReadDouble().ToChoice("Could not parse input to a double.")
from result in Divide(f1, f2).ToChoice("Cannot divide by zero.")
select result
)
.Match(
x => Console.WriteLine("Result = {0}", x),
x => Console.WriteLine("Error: {0}", x));
}
public static Option<double> Divide(double a, double b)
{
return b == 0 ? Option.None<double>() : Option.Some(a / b);
}
public static Option<Double> ReadDouble()
{
double i;
if (Double.TryParse(Console.ReadLine(), out i))
return Option.Some(i);
else
return Option.None<double>();
}
public static Option<int> ReadInt()
{
int i;
if (Int32.TryParse(Console.ReadLine(), out i))
return Option.Some(i);
else
return Option.None<int>();
}
}
选项类型如下所示:
public enum OptionType
{
Some, None
}
public abstract class Option<T>
{
private readonly OptionType _tag;
protected Option(OptionType tag)
{
_tag = tag;
}
public OptionType Tag { get { return _tag; } }
internal bool MatchNone()
{
return Tag == OptionType.None;
}
internal bool MatchSome(out T value)
{
value = Tag == OptionType.Some ? ((Some<T>)this).Value : default(T);
return Tag == OptionType.Some;
}
public void Match(Action<T> onSome, Action onNone)
{
if (Tag == OptionType.Some)
onSome(((Some<T>)this).Value);
else
onNone();
}
public Choice<T, T2> ToChoice<T2>(T2 value)
{
if (Tag == OptionType.Some)
{
T some;
MatchSome(out some);
return Choice.NewChoice1Of2<T, T2>(some);
}
else
return Choice.NewChoice2Of2<T, T2>(value);
}
}
internal class None<T> : Option<T>
{
public None() : base(OptionType.None) { }
}
internal class Some<T> : Option<T>
{
public Some(T value)
: base(OptionType.Some)
{
_value = value;
}
private readonly T _value;
public T Value { get { return _value; } }
}
public static class Option
{
public static Option<T> None<T>()
{
return new None<T>();
}
public static Option<T> Some<T>(T value)
{
return new Some<T>(value);
}
}
public static class OptionExtensions
{
public static Option<TResult> Map<T, TResult>(this Option<T> source, Func<T, TResult> selector)
{
T value;
return source.MatchSome(out value) ? Option.Some(selector(value)) : Option.None<TResult>();
}
public static Option<TResult> Bind<T, TResult>(this Option<T> source, Func<T, Option<TResult>> selector)
{
T value;
return source.MatchSome(out value) ? selector(value) : Option.None<TResult>();
}
public static Option<TResult> Select<T, TResult>(this Option<T> source, Func<T, TResult> selector)
{
return source.Map(selector);
}
public static Option<TResult> SelectMany<TSource, TValue, TResult>(this Option<TSource> source, Func<TSource, Option<TValue>> valueSelector, Func<TSource, TValue, TResult> resultSelector)
{
return source.Bind(s => valueSelector(s).Map(v => resultSelector(s, v)));
}
}
这是选择类型实现:
public enum ChoiceType { Choice1Of2, Choice2Of2 };
public abstract class Choice<T1, T2>
{
private readonly ChoiceType _tag;
protected Choice(ChoiceType tag)
{
_tag = tag;
}
public ChoiceType Tag { get { return _tag; } }
internal bool MatchChoice1Of2(out T1 value)
{
value = Tag == ChoiceType.Choice1Of2 ? ((Choice1Of2<T1, T2>)this).Value : default(T1);
return Tag == ChoiceType.Choice1Of2;
}
internal bool MatchChoice2Of2(out T2 value)
{
value = Tag == ChoiceType.Choice2Of2 ? ((Choice2Of2<T1, T2>)this).Value : default(T2);
return Tag == ChoiceType.Choice2Of2;
}
public void Match(Action<T1> onChoice1Of2, Action<T2> onChoice2Of2)
{
if (Tag == ChoiceType.Choice1Of2)
onChoice1Of2(((Choice1Of2<T1, T2>)this).Value);
else
onChoice2Of2(((Choice2Of2<T1, T2>)this).Value);
}
}
internal class Choice1Of2<T1, T2> : Choice<T1, T2>
{
public Choice1Of2(T1 value)
: base(ChoiceType.Choice1Of2)
{
_value = value;
}
private readonly T1 _value;
public T1 Value { get { return _value; } }
}
internal class Choice2Of2<T1, T2> : Choice<T1, T2>
{
public Choice2Of2(T2 value)
: base(ChoiceType.Choice2Of2)
{
_value = value;
}
private readonly T2 _value;
public T2 Value { get { return _value; } }
}
public static class Choice
{
public static Choice<T1, T2> NewChoice1Of2<T1, T2>(T1 value)
{
return new Choice1Of2<T1, T2>(value);
}
public static Choice<T1, T2> NewChoice2Of2<T1, T2>(T2 value)
{
return new Choice2Of2<T1, T2>(value);
}
}
修改
它实际上适用于下面的扩展程序。我真正喜欢的是这个实现为Choice类型添加了特定于上下文的行为。这是因为Choice1Of2是首选,因为所有的扩展方法主要是在它上运行而不是在Choice2Of2上或两者上运行。 (但这就是消费代码实际意味着什么,所以我想这是让它运转的唯一方法。)
public static Choice<TResult, T2> Map<T1, T2, TResult>(this Choice<T1, T2> source, Func<T1, TResult> selector)
{
T1 value1;
if(source.MatchChoice1Of2(out value1))
{
return Choice.NewChoice1Of2<TResult, T2>(selector(value1));
}
T2 value2;
if (source.MatchChoice2Of2(out value2))
{
return Choice.NewChoice2Of2<TResult, T2>(value2);
}
throw new InvalidOperationException("source (:Choice) has no value.");
}
public static Choice<TResult, T2> Bind<T1, T2, TResult>(this Choice<T1, T2> source, Func<T1, Choice<TResult, T2>> selector)
{
T1 value1;
if (source.MatchChoice1Of2(out value1))
{
return selector(value1);
}
T2 value2;
if (source.MatchChoice2Of2(out value2))
{
return Choice.NewChoice2Of2<TResult, T2>(value2);
}
throw new InvalidOperationException("source (:Choice) has no value.");
}
public static Choice<TResult, T2> Select<T1, T2, TResult>(this Choice<T1, T2> source, Func<T1, TResult> selector)
{
return source.Map(selector);
}
public static Choice<TResult, T2> SelectMany<TSource, TValue, T2, TResult>(this Choice<TSource, T2> source, Func<TSource, Choice<TValue, T2>> valueSelector, Func<TSource, TValue, TResult> resultSelector)
{
return source.Bind(s => valueSelector(s).Map(v => resultSelector(s, v)));
}
答案 0 :(得分:3)
由于Choice
有两个类型参数,您需要修复第一个才能编写Select
和SelectMany
(绑定):
public abstract class Choice<T1, T2>
{
public abstract Choice<T1, T3> Select<T3>(Func<T2, T3> f);
public abstract Choice<T1, T3> SelectMany<T3>(Func<T2, Choice<T1, T3>> f);
}
Choice1Of2
的实施很简单:
public override Choice<T1, T3> Select<T3>(Func<T2, T3> f)
{
return new Choice1Of2<T1, T3>(this._value);
}
public override Choice<T1, T3> SelectMany<T3>(Func<T2, Choice<T1, T3>> f)
{
return new Choice1Of2<T1, T3>(this._value);
}
并且对于Choice2Of2
,您只需要为给定函数提供内部值:
public override Choice<T1, T3> Select<T3>(Func<T2, T3> f)
{
return new Choice2Of2<T1, T3>(f(this.Value));
}
public override Choice<T1, T3> SelectMany<T3>(Func<T2, Choice<T1, T3>> f)
{
return f(this._value);
}
您可能还需要一个BiSelect
函数来映射两个类型参数:
public abstract BiSelect<T3, T4>(Func<T1, T3> ff, Func<T2, T4> fs);
如果要将SelectMany
与linq查询语法一起使用,则需要实现另一个重载,如下所示:
public abstract Choice<T1, T4> SelectMany<T3, T4>(Func<T2, Choice<T1, T3>> f, Func<T2, T3, T4> selector);
Choice1Of2
的实现与以前类似:
public override Choice<T1, T4> SelectMany<T3, T4>(Func<T2, Choice<T1, T3>> f, Func<T2, T3, T4> selector)
{
return new Choice1Of2<T1, T4>(this._value);
}
Choice2Of2
的实施是:
public override Choice<T1, T4> SelectMany<T3, T4>(Func<T2, Choice<T1, T3>> f, Func<T2, T3, T4> selector)
{
T2 val = this._value;
var e = f(val);
return e.Select(v => selector(val, v));
}
你可以这样做:
var choice = from x in new Choice2Of2<string, int>(1)
from y in new Choice2Of2<string, int>(4)
select x + y;
答案 1 :(得分:2)
这是一个新的扩展类,使“匹配”方法适用于IEnumerable
public static class ChoiceExtensions
{
// You need this method, because code 'select result' is a LINQ expression and it returns IEnumerable
public static void Match<T1, T2>(this IEnumerable<Choice<T1, T2>> seq, Action<T1> onChoice1Of2, Action<T2> onChoice2Of2)
{
foreach (var choice in seq)
{
choice.Match(onChoice1Of2, onChoice2Of2);
}
}
// This method will help with the complex matching
public static Choice<T1, T2> Flat<T1, T2>(this Choice<Choice<T1, T2>, T2> choice)
{
Choice<T1, T2> result = null;
choice.Match(
t1 => result = t1,
t2 => result = new Choice2Of2<T1, T2>(t2));
return result;
}
}
另外,我已经改变了您的Choice类:
// Implement IEnumerable to deal with LINQ
public abstract class Choice<T1, T2> : IEnumerable<Choice<T1, T2>>
{
IEnumerator<Choice<T1, T2>> IEnumerable<Choice<T1, T2>>.GetEnumerator()
{
yield return this;
}
public IEnumerator GetEnumerator()
{
yield return this;
}
// These two methods work with your Devide function
// I think, it is good to throw an exception here, if c is not a choice of 1
public static implicit operator T1(Choice<T1, T2> c)
{
T1 val;
c.MatchChoice1Of2(out val);
return val;
}
// And you can add exception here too
public static implicit operator T2(Choice<T1, T2> c)
{
T2 val;
c.MatchChoice2Of2(out val);
return val;
}
// Your Match method returns void, it is not good in functional programming,
// because, whole purpose of the method returning void is the change state,
// and in FP state is immutable
// That's why I've created PureMatch method for you
public Choice<T1Out, T2Out> PureMatch<T1Out, T2Out>(Func<T1, T1Out> onChoice1Of2, Func<T2, T2Out> onChoice2Of2)
{
Choice<T1Out, T2Out> result = null;
Match(
t1 => result = new Choice1Of2<T1Out, T2Out>(onChoice1Of2(t1)),
t2 => result = new Choice2Of2<T1Out, T2Out>(onChoice2Of2(t2)));
return result;
}
// Continue Choice class
}
您的样本稍有不正确,因为当您写下:
from f1 in ReadDouble().ToChoice("Could not parse input to a double.")
from f2 in ReadDouble().ToChoice("Could not parse input to a double.")
from result in Devide(f1, f2).ToChoice("Cannot devide by zero.")
select result
在最后一行你实际上忽略了f1和f2。所以不可能看到解析错误。更好写:
(
from f1 in ReadDouble().ToChoice("Could not parse input to a double.")
from f2 in ReadDouble().ToChoice("Could not parse input to a double.")
from result in
f1.PureMatch(
f1value => f2.PureMatch(
f2value => Devide(f1, f2).ToChoice("Cannot devide by zero."),
f2err => f2err).Flat(),
f1err => f1err
).Flat()
select result
)
.Match(
x => Console.WriteLine("Result = {0}", x),
x => Console.WriteLine("Error: {0}", x));
你可以创建好的辅助方法来处理这些复杂的东西,比如PureMatch方法但有更多的参数