Document module alias are not for re-exports

[zploskey]

This is based on a problem reported by user dvisz-inf in the Discord (github @dvisztempacct, I think) who was reporting on behalf of @scull7.

We create 3 bucklescript packages with a dependency relationship pkga -> pkgb -> pkgc.

(* pkgc/src/pkgc.ml *)
let hello name = "Hello " ^ name

(* pkgb/src/pkgb.ml *)
module C = Pkgc

(* pkga/src/demo.ml *)
let () = Js.log (Pkgb.C.hello "World!")

It fails to compile with:

> pkga@0.1.0 build /home/zach/src/bsdeps/pkga
> bsb -make-world

Duplicated package: bs-platform /home/zach/src/bsdeps/pkga/node_modules/bs-platform (chosen) vs /home/zach/src/bsdeps/pkga/node_modules/pkgb/node_modules/pkgc/node_modules/bs-platform in /home/zach/src/bsdeps/pkga/node_modules/pkgb/node_modules/pkgc 
[2/2] Building src/pkgc.mlast.d
[1/1] Building src/pkgc.cmj
Duplicated package: bs-platform /home/zach/src/bsdeps/pkga/node_modules/bs-platform (chosen) vs /home/zach/src/bsdeps/pkga/node_modules/pkgb/node_modules/bs-platform in /home/zach/src/bsdeps/pkga/node_modules/pkgb 
[2/2] Building src/pkgb.mlast.d
[1/1] Building src/pkgb.cmj
[2/2] Building src/demo.mlast.d
[1/1] Building src/demo.cmj
File "/home/zach/src/bsdeps/pkga/src/demo.ml", line 1, characters 17-29:
Error: Unbound value Pkgb.C.hello
npm ERR! code ELIFECYCLE
npm ERR! errno 2
npm ERR! pkga@0.1.0 build: `bsb -make-world`
npm ERR! Exit status 2
npm ERR! 
npm ERR! Failed at the pkga@0.1.0 build script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

Demonstration repo with instructions for commands to reproduce: https://github.com/zploskey/bsdeps
Each package only lists its direct dependency in bsconfig.json and package.json. In my testing adding pkgc to the bsconfig in pkga did not change the outcome.

I tested this on Ubuntu 18.04 x86_64 with w/ JS target (OCaml 4.02.3 + bucklescript) running on Node.

[dvisztempacct]

Thanks for filing the issue @zploskey !

In many statically typed languages, it is basically a requirement to recursively include types of all dependencies before a program can be compiled. There are ways around it, but they are usually tantamount to type aliasing IMHO.

I am understanding if bucklescript doesn't want to recursively include types for every single sub-dependency when compiling a particular unit because this can be a performance limitation for incremental builds where types are not needed for a particular module.

The trouble seems to be simply the user experience here because the error message claims Error: Unbound value Pkgb.C.hello which conflates two types of conditions:

1. that the value really isn't bound
2. that the value has "been removed" by the compiler because it would require type information from a module that wasn't available at this step in the incremental build

Note: It's possible I am severely misreading the situation, so please don't read my post and assume this is what's happening! If you know better, please correct me :)

[bobzhang]

This is actually a leaky abstraction of module alias, I will comment on why it does not work as expected later..

[dvisztempacct]

Awaiting your comment @bobzhang :)

[prophet1906]

Is there any update on this issue?

[romanschejbal]

I'd also be interested to know why this is the case and where it is headed. We encountered this issue with our monorepo setup and the compiler errors are (only in some cases, "module x is dangling" is the most obvious one but doesn't always show up) not that great so it took us quite some hard time to find out that one of the consumer packages needs to list dependencies recursively.

[bobzhang]

@romanschejbal I will write an article about it. The main thing is that module alias is not transitive, it is not designed for re-exporting

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.