module.builtin() is not documented

[mootari]

Compiled Observable notebooks with file attachments call main.builtin() to register a FileAttachment builtin that is local to the module:

const main = runtime.module();
const fileAttachments = new Map([["chinook.db","https://static.observableusercontent.com/files/b3711cfd9bdf50cbe4e74751164d28e907ce366cd4bf56a39a980a48fdc5f998c42a019716a8033e2b54defdd97e4a55ebe4f6464b4f0678ea0311532605a115"]]);
main.builtin("FileAttachment", runtime.fileAttachments(name => fileAttachments.get(name)));

This is currently the only way to define local builtins, as the second argument of the Module constructor is not exposed via runtime.module(). Unfortunately the method is not documented.

[mootari]

(As an aside, runtime.fileAttachments isn't documented either.)

[mbostock]

Somewhat a mistake that module.builtin was made public. A private setter was added in 2d4da0adeedc88bd61246dbf135550100d62b81e to fix a bug, then made public in e804a8682c98268c83b510bb5698e99d484e4ce6 (without documentation). But we should have preserved the original design where the builtins are specified up-front when the module is constructed. But that would require changing the generated code to look something like this:

const fileAttachments = new Map([["chinook.db","https://static.observableusercontent.com/files/b3711cfd9bdf50cbe4e74751164d28e907ce366cd4bf56a39a980a48fdc5f998c42a019716a8033e2b54defdd97e4a55ebe4f6464b4f0678ea0311532605a115"]]);
const main = runtime.module({FileAttachment: runtime.fileAttachments(name => fileAttachments.get(name))});

And I don’t want to spend the effort to create a new compiler version, so I think we are stuck with this (for now). I guess I’ll document the method but also the caveat that you need to call module.builtin before defining any variables (or else).