Ideas to improve OCaml's syntax errors

[jmid]

I've come across two ocaml/ocaml issues related to sub-optimal syntax error and thought I would add them here (before I forget). They are:

• ocaml/ocaml#5068
• ocaml/ocaml#7858

[let-def]

Thanks for listing these errors. I think #5068 already provides a decent error message, but #7858 is new to me.

I will look at both in January :)

[let-def]

More cases:

• https://github.com/ocaml/ocaml/issues/10700
• https://github.com/ocaml/ocaml/issues/11108

[jmid]

Here's another one (which might be harder though):

• https://github.com/ocaml/ocaml/issues/10990

[jmid]

Another one (for .mli files):

• https://github.com/ocaml/ocaml/issues/12055

[jmid]

Here's a recent one from Discuss also related to constructor arguments:

• https://discuss.ocaml.org/t/baffled-by-this-compile-error/11767

[jmid]

Here's another one for signatures: https://github.com/ocaml/ocaml/issues/12280

[jmid]

Here's a classic, caused by confusing the two let-forms: https://discuss.ocaml.org/t/syntax-error-on-in/12297 (if nothing else, it would be nice to confirm that it is handled nicely)

[jmid]

I came across another one, involving a trailing function semi-colon: https://github.com/ocaml/ocaml/pull/102#issuecomment-56207467

[jmid]

Another one - giving a nice error message on #require and friends: https://github.com/ocaml/ocaml/issues/12554

[let-def]

Here's a classic, caused by confusing the two let-forms: https://discuss.ocaml.org/t/syntax-error-on-in/12297 (if nothing else, it would be nice to confirm that it is handled nicely)

Nice variation that I missed. Thanks.

I came across another one, involving a trailing function semi-colon: ocaml/ocaml#102 (comment)

This is handled.

Another one - giving a nice error message on #require and friends: ocaml/ocaml#12554

This could be handled at the syntactic level, though in Merlin (a long time ago), we went another way and made it a "semantic" fail. Top-level directives were syntactically allowed in any structure and were stored in the AST (as extension nodes), and were interpreted or rejected by a later pass. This allowed Merlin (in the findlib, pre-dune, era) to behave well with ocamlscript files and gracefully handle unexpected directives.

In the short-term, I will handle that as a syntax error, but longer term it might be worth upstreaming this Merlin feature to OCaml.

[jmid]

A fresh one from Gabriel: https://github.com/ocaml/ocaml/issues/12808