melange.emit: (emit_stdlib true) can compile unnecessary modules

[anmonteiro]

There's currently a limitation in the melange rules + dune integration where the Melange runtime/stdlib is emitted to JS, in its entirety, for every melange.emit stanza.

This happens for a few reasons:

• melange.emit is "total": one melange (target emit-folder) is self-contained and (hopefully) contains everything needed to run that application
  • this is by design and makes both rule implementation easier and adheres to the principle of least surprise
• The Dune / ocamldep level of granularity for external dependencies are currently libraries, which means
  • There have been a few attempts in the past Refining dependencies ocaml/dune#7498 but AFAIK there are no currently ongoing efforts to fix this

An alternative workaround for this issue is #620:

• to get around the emission time for melange.emit with (emit_stdlib true), we could:
  1. publish e.g. a @melange/melange NPM package
  2. use emit_stdlib false in more situations
  3. consume the melange runtime/stdlib from NPM
  • the tradeoff here is that it might become a bit harder to consume dev versions of melange between releases (the opam package / runtime npm package must be in sync)