Composition

DRAFT

Why use compositions?

Performance

There is a tiny performance advantage, as a composition dispenses with the almost-negligible cost of putting a lambda on the evaluation stack. (But there are use cases in which every bit helps.)

Replace explicit iterations

E.g. write (f g h::)each x rather than f each g each h each.

Avoid nesting lambdas

Nesting lambdas has a cost: default arguments x, y, and z have different referents inside and outside the nested lambda. Compositions help avoid this.

For example in

{x+{f g h x}each y}

x in the inner lambda refers to y in the outer lambda. Instead, write

{x+(f g h::)each y}

Replace references with values

A composition binds in data values as they are at the time of composition, so there are no references to follow.

Abstraction

The Compose function lets you compose a ‘pipeline’ of functions at run time.

    q)fns:({x*x};1000-;count;til)
    q)cases: 1 0 3 2  / result of some test or calculation
    q)foo:('[;])over fns cases
    q)foo "abc"
    1000 999 996

Apply or Identity?

FIXME Compare composing using @ and ::.

Higher-rank compositions

FIXME Compositions with rank > 1