Require types to start with an uppercase letter

[byorgey]

Closes #1547 , which was caused by misspelled/nonexistent types being interpreted as type variables. In #1550 I proposed one solution to the problem, namely, to stop implicitly quantifying types and require explicit forall on any polymorphic type. This PR represents an alternative solution: to keep implicit quantification but require all types to start with an uppercase letter, as in Haskell. I think I like this one better but I'm open to feedback.

Specifically, with this PR:

• All built-in type constructors (Int, Cmd, Unit, etc.) must start with a capital letter
• Type variables:
  • Must start with a lowercase letter or underscore
  • May not be named a lowercase version of a type constructor, e.g. int
    • This is important so that old code with lowercase types is not silently accepted by parsing the former types as implicitly quantified type variables; even in cases where old code no longer typechecks, this will give better error messages.
• Term variables:
  • May start with upper- or lowercase
  • May be named lowercase versions of type names, e.g. let f : Int -> Int = \int. int + 1 is fine

This PR obviously represents a bigger breaking change. Once we merge this we might consider adding an option to swarm format to be able to parse old code with lowercase types and then reformat it with uppercase, to aid in porting code.

Once this is merged I will also regenerate the wiki pages that mention types.

Closes #1550.

[xsebek]

The types look more readable to me 👍

Just to check, uppercase is parsed as a concrete type or fails if it does not exist, right? Apologies if I overlooked a test for it.

[byorgey]

Just to check, uppercase is parsed as a concrete type or fails if it does not exist, right? Apologies if I overlooked a test for it.

That is correct. Wait, I think it doesn't actually do that (but it should)! I will add some more tests, and fix that behavior if I got it wrong.

[xsebek]

Btw. I think that if you showed data from Swarm 0.4 and read and printed them by 0.5, that would work as that tool.

It could look like:

swarm-0.4 format --out=Haskell old_code.sw | swarm-0.5 format --in=Haskell --stdin

[byorgey]

Btw. I think that if you showed data from Swarm 0.4 and read and printed them by 0.5, that would work as that tool.

That's true, but (1) that would require adding --out and --in flags, (2) it would be kind of annoying to tell users that in order to upgrade they needed to keep around an old binary instead of just replacing that old binary with the newer one, as usual.

Edit: actually, the big problem right now is that currently none of these approaches would preserve comments; see #1467 .

Edit 2: since I personally would really like a tool to do the type case conversion automatically on all my .sw files, I have started working on #1467, and from there I can make format work as the upgrade tool I envision, with some kind of --compat flag perhaps. Hopefully it won't be terribly hard to get something reasonable working. My initial attempts are in the branch feature/preserve-comments.

[byorgey]

@kostmo I've resurrected this PR. When you get a chance, take a look and let me know what you think.

I was going to give swarm format the ability to convert code from the old format to the new. I may still do that but I don't think it has to be in this PR.