Partial function application

When a plu-ts function takes more than one argument, like the pTwoIntegersList we built explaining pfn, it is possible to get new functions from the first by passing only some of the parameters.

Since the type of pTwoIntegersList was something like int -> int -> list( int ), pTwoIntegersList expects 2 arguments; however if we pass only 1 the result will be a valid Term of type int -> list( int ); which is another plu-ts function!

 // this is a Term from PInt to PList<PInt>!
const pListWith42First = pTwoIntegersList.$( 42 );

In particular, the new function we get behaves just like the first but with the arguments already passed that are fixed and can't be changed.

// equivalent to pTwoIntegersList.$( 42 ).$( 69 )
const niceList = pListWith42First.$( 69 );

This not only reduces the number of new functions you need to create but is also more efficient than wrapping the first function inside of a new lambda.

// bad practice;
const pInefficientListWith42First = plam( int, list( int ) )
    ( int2 =>
        pTwoIntegersList.$( 42 ).$( int2 )
    );

Even if the compiler is smart enough to optimize some trivial cases, it is still best practice to avoid doing this.