Working with Either

"The Either type represents values with two possibilities: a value of type Either<'L,'R> is either Left<'L> or Right<'R>. The Either type is sometimes used to represent a value which is either correct or an error; by convention, Left is used to hold an error value and Right holds a correct value" - Haskell documentation

Either instance

Create an Either by injecting a value into either case with Either.Left<'L,'R>('L) or Either.Right<'L,'R>('R).

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Either<int, string> left = Either.Left<int, string>(2);
Either<int, string> right = Either.Right<int, string>("rrr");

Use the FromEither<'L,'R,'T>('L->'T,'R->'T) function to extract the value from either side.

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var leftV = left.FromEither(l => l.ToString(), r => r); // "2"
var rightV = right.FromEither(l => l.ToString(), r => r); // "rrr"

Check out the API Reference for more information.

Exception and error-handling

The following snippet depicts a three step process to load, validate and launch a saved game. Each of these steps can potentially fail and raise an Exception that needs to be caught.

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void StartGame(byte[] saveGame)
{
    if(ValidateSaveGame(saveGame))
    {
        Debug.WriteLine("Play it!! I could crash here too...");
    }
    else
    {
        throw new Exception("corrupt save game");
    }
}

bool ValidateSaveGame(byte[] saveGame)
{
    return saveGame.Length > 8;
}

byte[] LoadSaveGame(string file)
{
    return File.ReadAllBytes(file);
}

try
{
    StartGame(LoadSaveGame("x.y"));
}
catch (Exception e)
{
    throw e; // Which exceptions will I catch here?!?
}

Composing Either

Using the monadic bind function of Either we can compose these steps together while preserving granular information like individual error messages.

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Either<string, Unit> StartGame(byte[] saveGame)
{
    return Either.Try(e=> "start failed: "+ e.Message,
                     ()=> Unit.Default);
}

Either<string, byte[]> ValidateSaveGame(byte[] saveGame)
{
    return Either.When(()=> saveGame.Length>8,
                       ()=> "validation failed",
                       ()=> saveGame);
}

Either<string, byte[]> LoadSaveGame(string file)
{
    return Either.Try(e=> "load failed: "+ e.Message,
                     ()=> File.ReadAllBytes(file));
}

var launch = LoadSaveGame("x.y")
             .Bind(ValidateSaveGame)
             .Bind(StartGame)
             .FromEither(s=>"e: "+s,
                         s=>"all good"); // p: FileNotFoundException

Wooga.Lambda

Working with Either

Either instance

Exception and error-handling

Composing Either