Program representations

Crochet differentiates delayed programs created through the lazy special syntax and other partial or delayed programs created either through holes or the curly-brackets syntax. The primary reason for this is that they really do behave differently under the hood, and unifying them would make things more confusing.

Thunks

A lazy delayed program (created through the lazy special syntax) is a Thunk, and it’s associated with the thunk type. The force special syntax, which executes a lazy delayed program if no value for it has been computed before, is the primary way of interacting with these programs.

Functions

Every other delayed or partial program is a Function—a representation of a (maybe partial) program that can be executed at will through the Program() syntax.

Functions are classified by the number of requirements they have. Also called their “arity”. For example, the function { 1 + 2 } has no requirements, and thus arity 0 (or nullary). It is represented by the type function-0. Whereas the function { A, B in A + B } and the function _ + _ both have two requirements, and thus arity 2 (or binary). These are represented by the type function-2.

Functions can be executed by providing values to all of their requirements:

let Add = { A, B in A + B };
Add(1, 2);

It’s not immediately obvious by looking at this syntax if all of the requirements have been fulfilled, but for Crochet it’s an error if the number of values we provide differs from the number of requirements.

Partials

A Partial is a special kind of function that’s created when commands are partially applied. For example, the expression _ + 1 creates a value of the type partial-1. Partials can be used anywhere functions of the same arity are expected.