Lesson 42 · Built-in functions - Build a Programming Language

Built-in functions

Some functionality has to come from the host language - measuring a string's length, printing output. Today you add built-in functions, starting with len, by introducing a builtin object and a registry the evaluator checks before the environment.

The goal

Add a built-in function object and implement len, resolving builtins by name.

Start here - the target

TO DO

Scenario: Calling the built-in len

Giventhe source len("hello")

Whenthe evaluator evaluates the program

Thenthe result is 5

Andlen("") is 0, and len(5) yields the error argument to len not supported, got INTEGER

Background

Some things a program needs cannot be written in the language itself yet - measuring a value’s length, printing to the screen. Those come from built-in functions: a new Builtin object wrapping a native (host-language) function, kept in a small registry keyed by name. When resolving an identifier, check the environment first and fall back to this registry, so len resolves to the builtin even though no let ever bound it.

Start with len, which returns the length of a string, and set up the registry so adding more builtins is trivial - a companion puts that prints each argument and returns null gives your programs a way to show output. Builtins validate their arguments and return a clear error on misuse, like len(5) reporting an unsupported argument type - the same error object you built earlier. Arrays, next, will make len and friends far more useful.

Make it work

type Builtin struct { Fn func(args ...Object) Object }
func (b *Builtin) Type() string { return "BUILTIN" }
var builtins = map[string]*Builtin{
  "len": {Fn: func(args ...Object) Object { /* one String arg -> its length */ }},
  // "puts" prints each arg's Inspect and returns NULL
}
// resolving an identifier: check builtins if the name is not in the environment

CheckpointDONE

The built-in len works, and the builtin mechanism is ready for more. Commit.