Doc literals fails to parse if Unison `syntax` library is missing from namespace

[aryairani]

What's wrong with the following transcript?

```unison
README = {{
Hi
}}
```

It succeeds with a preceding builtins.mergeio. This may be a clue into #2840.

[pchiusano]

This is expected behavior: the parser translates the doc syntax into calls to functions with those names. If you have a totally empty codebase then it gives errors like this. Assuming the codebase is initialized with base (or via mergio) then it should work.

Referencing those definitions via name rather than via hash has advantages, like being able to override the syntax or upgrade what the syntax refers to even without needing to update ucm.

[aryairani]

Ah, interesting, thanks. I listed a couple of thoughts below:

• I do think it would be nice if you could write a doc in an empty codebase, though it'd be a very low priority — maybe some kind of fallback would be possible if the names were missing? It at least deserves a better message to let the user know that they need to add some library to support docs.
• I guess you wouldn't necessarily be overriding it to a different Doc type (would that be realistic?) but you could do some different pre-processing, maybe change what different syntaxes mean?
• Things can blow up if I add another term called syntax.docUntitledSection in a scratch file or codebase. To be safe, there can only be one in the namespace. I suppose the same goes for the other syntax functions, if they are needed by the scratch file. Maybe it should be a more obviously reserved name, or a config file setting with some default?

[pchiusano]

My vote would be to just have a nice error message in this situation that tells the user they should fetch base.

[iacore]

It doesn't even use base.syntax.docUntitledSection even it exists. Maybe a better way to is to look up docUntitledSection in scope?

[pchiusano]

@locriacyber any chance you have multiple versions of base floating around in your namespace tree? That's the only explanation I can think of. It will also be fixed in next release, which includes #3003.

If that's not it, can you try to put together a transcript to reproduce? Here's a guide to the syntax: https://raw.githubusercontent.com/unisonweb/unison/trunk/unison-src/transcripts/hello.output.md

[iacore]

You gave the wrong link. It should be hello.md.

Will the next release respect use statements?

It seems like the bug happens when base.syntax.docUntitledSection and builtin.syntax.docUntitledSection exist together.

The transcript should be something like this:

```ucm
.> builtins.mergeio
.> pull https://github.com/unisonweb/base:.trunk .base
```

```unison
x = {{
    Hello
}}
```

However, pull is taking a long time to finish.

[pchiusano]

@locriacyber I see. You can delete builtin namespace as a workaround for now.

There should be no need to run builtins.mergeio - all supported builtins are included in base.

A use statement should work I think, and if not that's a separate bug. Can you share what you've tried for this?

[iacore]

The bug is fixed in ucm version: release/M3 (or I can't reproduce it). I was using M2I before. I vaguely remember having base.syntax.docUntitledSection and builtin.syntax.docUntitledSection, but the following code still won't work.

plus.doc : .base.Doc
plus.doc =
  use .base syntax
  {{ Hello }}

---

One more thing: Unison don't search for docUntitledSection not inside a syntax namespace.

.> builtins.mergeio
.> rename.namespace builtin.syntax builtin.s2

use builtin.s2 docUntitledSection

plus.doc = {{
  Hello
}}