Lambda Calculus via C# (6) If Logic, And Reduction Strategies

[Obsolete] See latest version - [Lambda Calculus]

The if logic is already built in Church Booleans.

The first If

So naturedly, This is the first implementation of if based on Church Boolean:

public static partial class ChurchBoolean
{
    // If1 = condition => then => @else => condition(then, @else)
    public static Func<T, Func<T, T>> If1<T>
        (Boolean condition) => then => @else =>
            (T)condition
                (then)
                (@else);
}

Straightforward:

  • When condition is True, if returns then
  • When condition is False, If returns @else.

It can be applied like this:

ChurchBoolean
    .If1<Boolean>(True)
        (True.And(True))
        (True.Or(False));

Running this code will show a problem - And and Or are both triggered. However, when condition is either True or False, only one branch is expected to trigger. Here it is  True.And(False) to be triggered, since condition is True.

Reduction strategies

How does If work? There are 3 arguments to be applied: If(arg1)(arg2)(arg3).

The first application will be a beta-reduction:

  If (arg1) (arg2) (arg3)
≡ (condition => then => @else => condition (then) (@else)) (True) (arg2) (arg3)
≡ (then => @else => True (then) (@else)) (arg2) (arg3)

Since the second reduction, it becomes tricky. Because now both lambda expression and arg2 can be reduced.

Normal order

If the lambda expression is reduced before the arguments:

  (then => @else => True (then) (@else)) (arg2) (arg3)
≡ (then => @else => then) (arg2) (arg3).
≡ (@else => arg2) (arg3)
≡ arg2
≡ True.And(False)
≡ False

Eventually only arg2 need to be reduced. This is called normal order. The unreduced arguments are used for function reduction.

Applicative order

However, C# has a different reduction strategy called applicative order. C# always first reduces a function's arguments, then use those reduced arguments to reduces the function itself:

  (then => @else => True (then) (@else)) (arg2) (arg3)
≡ (then => @else => True (then) (@else)) (True.And(False)) (arg3)
≡ (then => @else => True (then) (@else)) (False) (arg3)
≡ (@else => True (False) (@else)) (arg3)
≡ (@else => True (False) (@else)) (True.Or(False))
≡ (@else => True (False) (@else)) (True)
≡ True (False) (True)
≡ False

This is why both And and Or are triggered. This is an example that reduction order matters.

Make If lazy

Under the C# reduction order, can If function be lazy, and works just like the first reduction order above? In the above version of If, both then and @else are of type T. In C# the easiest to think about is, changing both parameters from T into a function - the simplest will be Func<T>, so that after the condition returns one of those 2 functions, then the returned Func<T> function can be applied to return a T value.

public static partial class ChurchBoolean
{
    // If2 = condition => then => @else => condition(then, @else)()
    public static Func<Func<T>, Func<Func<T>, T>> If2<T>
        (Boolean condition) => then => @else =>
            ((Func<T>)condition
                (then)
                (@else))();
}

The application becomes:

ChurchBoolean
    .If2<Boolean>(False)
        (() => True.And(True))
        (() => True.Or(False));

Now in If, only 1 “branch” will be applied. However, in lambda calculus, a lambda expression without variable - λ.E (corresponding to Func<T>) - does not exist. This is easy to resolve - just make up a variable for lambda expression/a parameter for C# function. So If can be refactored to:

public static partial class ChurchBoolean
{
    public static Func<Func<Func<T, T>, T>, Func<Func<Func<T, T>, T>, T>> If<T>
        (Boolean condition) => then => @else => 
            ((Func<Func<T, T>, T>)condition
                (then)
                (@else))(_ => _);
}

And the application is almost the same:

ChurchBoolean
    .If<Boolean>(True)
        (_ => True.And(True))
        (_ => True.Or(False));

In lambda calculus, If is much cleaner without type information:

If := λc.λt.λf.c t f (λx.x)

Unit tests

The following unit test verifies If’s correctness and laziness:

[TestMethod()]
public void IfTest()
{
    Assert.AreEqual(
        true ? true && false : true || false,
        ChurchBoolean.If<Boolean>(True)(_ => True.And(False))(_ => True.Or(False))._Unchurch());
    Assert.AreEqual(
        false ? true && false : true || false,
        ChurchBoolean.If<Boolean>(False)(_ => True.And(False))(_ => True.Or(False))._Unchurch());

    bool isTrueBranchExecuted = false;
    bool isFalseBranchExecuted = false;
    ChurchBoolean.If<object>(True)
                    (_ => { isTrueBranchExecuted = true; return null; })
                    (_ => { isFalseBranchExecuted = true; return null; });
    Assert.IsTrue(isTrueBranchExecuted);
    Assert.IsFalse(isFalseBranchExecuted);

    isTrueBranchExecuted = false;
    isFalseBranchExecuted = false;
    ChurchBoolean.If<object>(False)
                    (_ => { isTrueBranchExecuted = true; return null; })
                    (_ => { isFalseBranchExecuted = true; return null; });
    Assert.IsFalse(isTrueBranchExecuted);
    Assert.IsTrue(isFalseBranchExecuted);
}

Finally, If is successfully encoded in lambda calculus, and it’s C# implementation is as lazy as real “if”.

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