precursive

The other thing we are missing to have a proper language is some way to repeat the execution of some code.

The functional paradigm doesn't really like things like the loops we have in Typescript but that is not a big deal, because we can use recursive functions instead.

Wait a second!

Don't we need to reference the same function we are defining in order to make it recursive?

How do we do that if we need what we are defining while defining it?

Turns out someone else already came up with a solution for that so that we don't have to.

That solution is the Y combinator (actually we'll use the Z combinator but whatever).

note

We'll not go in the details on how it works, but if you are a curious one here's a great article that explains the Y combinator in javascript terms

All you need to know is that it allows functions to have themselves as parameters, and this solves everything!

In plu-ts there is a special typescript function that makes plu-ts functions recursive, and it's named, you guessed it, precursive.

All precursive requires to make a plu-ts function recursive is that we pass the function as the first parameter, and then we can do whatever we want with it.

So let's try to define a plu-ts function that calculates the factorial of a positive number:

const pfactorial = precursive(
    pfn([
        // remember that the first argument is the function itself?
        // for this reason as first type we specify
        // what will be the final type of the function
        // because what we have here IS the function itself
        lam( int, int ),
        int
    ],  int)
    (( self, n ) =>
        pif( int ).$(
            n.ltEq( 1 )
        )
        .then( 1 )
        .else(
            // here we are applying this same function
            // this is the equivalent of writing 
            // `pfactorial.$( n.sub( 1 ) )`
            self.$( n.sub( 1 ) )
            .mult( n )
        )
    )
)

Now we can use pfactorial just like a normal function; this is because precursive takes care of passing the first argument, so that the actual type of pfactorial is just lam( int, int )

The next step is to learn how to evaluate expressions so that we can be sure that pfactorial is working as we expect.