Add export directive

[byorgey]

Adds a new export construct for better control over module exports.

• export "foo" in ... imports the module "foo" (just like import) and also re-exports it in its entirety. This is actually parsed as an import with a special "re-export" flag set.
• export x in ... causes the name x to be exported (which must be in scope).

This PR also replaces the generic strict Pair with a custom data type ModuleExports.

Related features which we could either leave to a subsequent PR, or possibly address here if there is a consensus on a nice design:

• A way to re-export tydefs in addition to defs. Currently export x only works on value-level names.
• A way to hide things from being exported. Right now, every def in a module is exported by default.

Closes #2664.

[xsebek]

Great! 👍

I don't like export in that much, it feels like forward declarations in C, but backwards. Could we have a syntax for def/tydef?

export def foo = 1 end
export tydef Bar b = (b,b) end

to desugar to:

def foo = 1 end; 
export foo;
tydef Bar b = (b,b) end;
export Bar;

EDIT: Ah, after reading the comment in go I see that is not how it works. Maybe the name should be reexport?

[byorgey]

@xsebek Yeah, the export ... in is kind of an artifact of the way we encode it in the AST; I imagine no one will ever actually use that syntax, but will instead write export blah; foo. Maybe we should not even allow the export ... in ... syntax?

In any case, def foo = 1 end already does mean

def foo = 1 end; 
export foo;

That is, every def is already exported by default. Adding an export foo would be allowed but would be redundant.

I think export is still an accurate name: saying export foo makes foo be exported. It just so happens that every local definition is exported by default, so the only places where you would need to say export are when you are re-exporting something from an imported module.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-03-16 00:21 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking main (2cc271e) and #2697 together #2700
• ✅ Merged — 2026-03-16 00:21 UTC · at 22a296c31a5a5de5f5c05e51cd38da592bbf8748

This pull request spent 11 seconds in the queue, including 1 second running CI.

Required conditions to merge

• #approved-reviews-by >= 1 [🛡 GitHub branch protection]
  • Add export directive #2697
• #changes-requested-reviews-by = 0 [🛡 GitHub branch protection]
  • Add export directive #2697

[byorgey]

I went ahead and merged this but happy to still entertain ideas for making it better!