public static EitherMonad <R2> Bind <R, R2>(EitherMonad <R> a, Func <R, EitherMonad <R2> > func)
{
if (a.IsLeft)
{
return(EitherMonad <R2> .Left(a.LeftValue));
}
return(func(a.RightValue));
}
// SelectMany implementation to allow EitherMonad<R> to be used with Linq query compositions
public static EitherMonad <C> SelectMany <A, B, C>(this EitherMonad <A> a, Func <A, EitherMonad <B> > func, Func <A, B, C> select)
{
var b = a.Bind(func);
if (b.IsLeft)
{
return(EitherMonad <C> .Left(b.LeftValue));
}
return(EitherMonad <C> .Return(select(a.RightValue, b.RightValue)));
}
public EitherMonad <R2> Bind <R2>(Func <R, EitherMonad <R2> > func)
{
return(EitherMonad <R2> .Bind(this, func));
}
// Monadic methods
public static EitherMonad <R> Return <R>(R value)
{
return(EitherMonad <R> .Right(value));
}
static void Main(string[] args)
{
var value1 = Maybe <int> .Some(10);
var value2 = Maybe <int> .Some(20);
var none = Maybe <int> .None();
/*
* Demonstrates using a Linq query comprehension with the Maybe<T> monad (facilitated by
* Maybe<T>'s implementation of SelectMany).
*
*
* [Haskell corresponding with this code]
*
* do
* v1 <- value1
* v2 <- value2
* return (v1 + v2)
*/
var tempValue1 = from v1 in value1
from v2 in value2
select v1 + v2;
Console.WriteLine(tempValue1);
/*
* The following two examples demonstrate the Linq query comprehension short-circuiting the
* "None" values through to the end of the computation (in either position).
*/
var tempValue2 = from v1 in none
from v2 in value2
select v1 + v2;
Console.WriteLine(tempValue2);
var tempValue3 = from v1 in value1
from v2 in none
select v1 + v2;
Console.WriteLine(tempValue3);
/*
* Just another demonstration with more Maybe<T> values in the Linq query comprehenshio.
*/
var tempValue4 = from v1 in value1
from v2 in value2
from v3 in Maybe <int> .Some(2)
select(v1 + v2) * v3;
Console.WriteLine(tempValue4);
/*
* Another longer Linq query comprehension with multiple "None" values.
*
*
* [corresponding Haskell code (using Haskell's equivalent to Linq query comprehension—"do" notation)]
*
* do
* v1 <- value1
* v2 <- value2
* v3 <- Some(2)
* v4 <- none
* return (v1 + v2) * v3 / v4
*/
var tempValue5 = from v1 in value1
from v2 in none
from v3 in Maybe <int> .Some(2)
from v4 in Maybe <int> .None()
select(v1 + v2) * v3 / v4;
Console.WriteLine(tempValue5);
/*
* A demonstration using the Bind operation instead of Linq query comprehsion syntax. Behind the scenes, the
* compiler converts Linq query comprehensions into SelectMany calls (as far as these examples are concerned)—
* where SelectMany loosely corresponds with the Monadic "Bind" operation.
*
*
* [corresponding Haskell code (but using the bind operator, >>=, directly instead of "do" notation—which the
* Haskell compiler converts to bind operators)]
*
* value1 >>= \v1 ->
* value2 >>= \v2 ->
* Some(2) >>= \v3 ->
* none >>= \v4 ->
* return (v1 + v2) * v3 / v4
*/
var tempValue6 = value1.Bind(v1 =>
value2.Bind(v2 =>
Maybe <int> .Some(2).Bind(v3 =>
none.Bind(v4 =>
Maybe <int> .Return((v1 + v2) * v3 / v4)
)
)
)
);
Console.WriteLine(tempValue6);
/*
* Some examples using EitherMonad<R> (i.e., the monadic wrapper around Either<L, R>) instead of Maybe<T>.
*/
var tempValue7 = from v1 in EitherMonad <int> .Right(10)
from v2 in EitherMonad <int> .Right(20)
select v1 + v2;
Console.WriteLine(tempValue7);
var tempValue8 = from v1 in EitherMonad <int> .Right(10)
from v2 in EitherMonad <int> .Left("error-1")
from v3 in EitherMonad <int> .Left("error-2")
select v1 + v2 + v3;
Console.WriteLine(tempValue8);
Console.WriteLine();
Console.WriteLine("<Press enter to continue.>");
Console.ReadLine();
}
