Capability — model

Functions as values

A function is a value: you can pass it, store it, return it.

LISP made functions ordinary data in 1958, and everything downstream — closures, callbacks, higher-order methods, the lambda — descends from that one move. If a function is a value, you can compose programs instead of writing them: pass behaviour into a sort, return behaviour from a factory, store it in a table.

It also changed what a program looks like. In a language where functions are values, iteration is a function you hand a behaviour to; collection methods are not language features so much as functions everyone happens to agree on.

The trade-off

First-class functions cost allocation and identity: a closure needs a home, and two closures created from the same source are not the same object.
They also make control flow harder to follow — the classic bug is a callback that runs after the thing it was meant to update is gone.

Where it came from

LISP, 1958 — lambda as an object, quotable and storable like any other datum.

in practicejavascript

const twice = (f) => (x) => f(f(x));
const inc = (n) => n + 1;

twice(inc)(5);   //=> 7  — behaviour passed around

Who carries it

canonical 8

present 13

partial 11