Spago does not emit an error when bundling an app's main module that does not have a `main` function

[JordanMartinez]

Steps to reproduce:

mkdir explore-bundle-no-main
cd explore-bundle-no-main
spago init
# edit the `src/Main.purs` file to remove the "main" function
spago bundle -m Main -t dist/main.js

Spago doesn't fail with an error that Main.purs does not have a function called main. Thus, the output of the bundle is this:

// Generated by purs bundle 0.12.5
var PS = {};
PS["Main"].main();

One won't realize that the source of their issue is the compiled output until after exploring a few things more.

I only came across this because I used the wrong module name in the spago bundle command.

[f-f]

Good catch! This is interesting because spago bundle just shells out to purs bundle, which is happy to generate that malformed js.

Pulp however implements this check, and fails with a nice error message:

$ pulp build -m Main -t dist/main.js
* Building project in /Users/fabrizio/code/explore-bundle-no-main
* Build successful.
* Bundling JavaScript...
* Main cannot be used as an entry point module because it
* does not export a `main` value.
*
* If you need to create a JavaScript bundle without an entry point, use
* the --skip-entry-point flag.
*
* ERROR: Failed entry point check for module Main

Since that looks like a bunch of code, this makes me wonder if it should be instead purs to implement the check instead.
We could implement it here, but the compiler is a heavy dependency that we've been trying to avoid by using the externs files as pulp does

EDIT: it looks like there's an entire PureScript package dedicated to this check: externs-check

[f-f]

Ping @hdgarrood: thoughts on adding this check to purs?

[hdgarrood]

This is something we previously decided is best not done inside purs bundle (since purs bundle only has access to the generated js). See purescript/purescript#2086

[f-f]

@hdgarrood thanks for the context! It makes sense, I'll look into using the externs files for the check 👍

[hdgarrood]

Then again I suppose since purs bundle is unlikely to be run against any JS files which aren’t output from purs compile, perhaps it would make more sense to just point purs bundle at the compiler’s output directory, so that it would have the information it needed to be able to implement this check inside purs.

[hdgarrood]

It is certainly less than ideal that pulp is depending on the externs files format, since these are not part of the compiler’s public API.

[f-f]

Indeed, but if it's necessary to rely on them and other tools (e.g. spago) start doing that too then it could make sense to include them in the public API? (though I'd be more for purs checking on this by itself)

[hdgarrood]

Yeah what I’m suggesting is that we modify purs bundle to have access to externs files as well as the js so that it can do this check.