partial function application

When a plu-ts function takes more than one argument, like the pTwoIntegersList we built a moment ago, 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.$( pInt(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.$( pInt(42) ).$( pInt( 69 ) )
const niceList = pListWith42First.$( pInt( 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.

const pInefficientListWith42First = plam( int, list( int ) )
    ( int2 =>
        pTwoIntegersList.$( pInt(42) ).$( int2 ) // BAD

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