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[英]throwE and catchE with ExceptT monad on the bottom of monadic stack
[英]What are monadic errors?
我讀到monadic錯誤是返回值錯誤和異常的替代方法。 有人可以解釋並舉例說明monadic錯誤如何在命令式偽語言中起作用(請不要使用功能示例)。
基本理念:
這是C#中的一個工作示例:
// A discrimated union that can be either an error of type TError,
// or a successful result of type TResult.
// IsError indicates which field has a valid value.
class Throws<TError, TResult>
{
public bool IsError;
public TError Error;
public TResult Result;
// Make a new successful reslt of type TResult.
public static Throws<TError, TResult> Success(TResult result)
{
Throws<TError, TResult> t = new Throws<TError, TResult>();
t.IsError = false;
t.Result = result;
return t;
}
// Make a new error of type TError.
public static Throws<TError, TResult> Fail(TError error)
{
Throws<TError, TResult> t = new Throws<TError, TResult>();
t.IsError = true;
t.Error = error;
return t;
}
// Composition.
public Throws<TError, TResultB> Bind<TResultB>(
Func<TResult, Throws<TError, TResultB>> f)
{
if (IsError)
{
// If this is an error, then we can short circuit the evaluation
return Throws<TError, TResultB>.Fail(Error);
}
// Otherwise, forward this result to the next computation.
return f(Result);
}
}
class Test
{
// num / demom
private static Throws<string, double> Div(double num, double denom)
{
if (denom == 0)
return Throws<string, double>.Fail("divide by zero");
return Throws<string, double>.Success(num / denom);
}
// Have the user enter a double.
private static Throws<string, double> ReadDouble(string name)
{
Console.Write("{0}: ", name);
string input = Console.ReadLine();
double result;
if (!double.TryParse(input, out result))
return Throws<string, double>.Fail(string.Format("can't parse {0}", name));
return Throws<string, double>.Success(result);
}
// Read two doubles and divide them to produce the result.
private static Throws<string, double> Interact()
{
return ReadDouble("numerator").Bind(num =>
ReadDouble("denominator").Bind(denom =>
Div(num, denom)));
}
public static void TestLoop()
{
while (true)
{
// Run a computation that asks the user for two numbers,
// divides them and then prints out the result.
Throws<string, double> t = Interact();
// Notice how the return type forces you to address the
// error if you want to get to the value.
if (t.IsError)
{
Console.WriteLine("Error: {0}", t.Error);
}
else
{
Console.WriteLine("Success: {0}", t.Result);
}
}
}
}
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