Choose a language for a script

Goal

The right language for the job, and proof the engine accepts your file as that language.

Why bother

Picking a language by preference and finding out at deploy time that the engine rejects it is an expensive way to learn the list.

The languages

`--runtime`	Use for
`javascript`	The default choice. Full control flow, objects, the host namespaces
`lua`	Small, fast, familiar to game and embedded teams
`starlark`	Deterministic and sandboxed by construction — a good fit for untrusted logic
`expr`	A single expression, not a program
`go`	Interpreted Go, for teams already writing it
`wasm`	A precompiled module

--runtime takes a language, not an engine

The flag’s own help text suggests values like v8 and goja. Those are rejected:

$ kis script compile hello.js -r v8
compilation failed: unsupported language: v8

So is the abbreviation js. Use javascript.

Step 1 — prove it compiles

kis script compile transform.js --runtime javascript

Compiling with javascript runtime...
✓ Compilation successful (95.377µs)
Source size: 59 bytes

Step 2 — read a rejection as information

Compiling the same JavaScript file under each language is a quick way to see what each one is:

$ kis script compile hello.js -r lua
error: hello.js line:1(column:17) near '{': syntax error

$ kis script compile hello.js -r expr
error: unexpected token Identifier("main") (1:10)

$ kis script compile hello.js -r wasm
error: invalid magic number

Each failure names the real reason: Lua does not use braces that way, expr wants an expression rather than a function declaration, and wasm expects a binary rather than text.

Starlark accepting a JavaScript-shaped function is not a licence to write JavaScript in it — the syntaxes overlap far enough to compile and diverge quickly after that.

Where the choice is recorded

A language chosen at the terminal is not a decision anyone else can see. Put it where it runs.

Flow
Script

- name: transform
  script:
    language: starlark
    file: ./classify.star
    namespaces: json,string
    setvar: classified

language: is explicit even when the extension already implies it — the file may be renamed, and the flow is what actually decides. Pairing it with namespaces: is the combination worth reaching for on generated code: a language that contributes no authority, and a grant that gives it none.

kis script run classify.star --runtime starlark --namespaces none

--runtime overrides the extension. Use it while you are choosing; move the answer into the flow once you have chosen.

Choosing

The script	Language
Calls host namespaces, has real control flow	`javascript`
Is untrusted or generated, and should be constrained by construction	`starlark`
Is one condition or one calculation	`expr`
Is CPU-heavy and precompiled elsewhere	`wasm`
Is maintained by a team that already writes Lua or Go	`lua` or `go`

Verify

Compile under the language you will actually configure, not the one you assume. A file that compiles as javascript says nothing about whether it compiles as starlark.