Facilitation.md

Monads (2/2)

Learning Goals

• Discover other monads than `Option

Connect - Limits

What is the limit of this code snippet?

public record Account(string Number, decimal Balance)
{
    public Option<Account> WithDraw(decimal amount)
        => amount >= Balance
            ? None
            : this with {Balance = Balance - amount};
}

Correction

• In terms of signature this method is pretty transparent: decimal -> Option<Account>
  • It says explicitly that the operation could fail
  • But does not explain why it could fail -> None only
• Sometimes in our business system we need to make more sense of this kind of code
  • To apply recover logic for example

Concepts

Try

Try is a monadic container which represents a computation that may either throw an exception or successfully completes .

var random = new Random();
var robots = new[] {"Tars", "Kipp", "Case"};

Try<string> RandomRobot() =>
    () => random.Next(10) > 5
        ? throw new InvalidProgramException("Plenty of slaves for my robot colony")
        : robots[random.Next(3)];

RandomRobot().Map(_ => _.ToUpperInvariant()); // Failure -> InvalidProgramException : "Plenty of slaves for my robot colony"
RandomRobot().Map(_ => _.ToUpperInvariant()); // "KIPP"
RandomRobot().Map(_ => _.ToUpperInvariant()); // "CASE"
RandomRobot().Map(_ => _.ToUpperInvariant()); // Failure -> InvalidProgramException : "Plenty of slaves for my robot colony"

Either

Either Holds one of two values 'Left' or 'Right’.

• Left is considered wrong or in error
• Right is right

When the Either is in a Left state, it cancels computations like bind or map, etc.

public record Account(string Number, decimal Balance)
{
    public Either<Error, Account> WithDraw(decimal amount)
        => amount >= Balance
            ? Error.New("Insufficient Balance")
            : this with {Balance = Balance - amount};
}

new Account("Account1", 900).WithDraw(1000); // Left("Insufficient Balance")
new Account("Account2", 900).WithDraw(100); // Right(Account { Balance = 800 })

Error is a useful struct available through Language-Ext.

Concrete Practice

• Do the TryExercises
• Then the EitherExercises

Step-by-step solution available here

Conclusion - When?

Think about when to use it. List some usages.

Usually we use Try only when we run unpredictable code like:

• calling an API
• storing data in a database
• sending an email
• ...

We prefer using Either or a Result for every error we have under management in our business process