Unknown tags and roundtripping

[braingram]

This issue is to discuss handling of unknown tags.

The specification has some description of implementation handling of unknown tags (either from unsupported asdf core schema versions or from custom extensions):
https://www.asdf-format.org/projects/asdf-standard/en/1.5.0/versioning.html#handling-version-mismatches
and near the end of:
https://www.asdf-format.org/projects/asdf-standard/en/1.5.0/tree.html#tags

Using the following example file:

#ASDF 1.0.0
#ASDF_STANDARD 1.6.0
%YAML 1.1
%TAG ! tag:stsci.edu:asdf/
--- !core/asdf-1.1.0
dict: !<asdf://example.com/foo/bar-1.0.0>
 key: 1
...

that contains a key "dict" with tag "asdf://example.com/foo/bar-1.0.0". Let's assume this tag comes from a custom extension registered by the user.

Reading the file using the current commit on main (3d5ef0c) with the following:

using ASDF
af = load("example.asdf")

produces an error:

ERROR: could not determine a constructor for the tag 'asdf://example.com/foo/bar-1.0.0'

Modifying the code to pass "extensions=true"

using ASDF
af = load("example.asdf"; extensions=true)

the file opens.

However is the tag lost?

If I write the file back out:

save("roundtrip.asdf", af)

the file no longer contains the tag:

#ASDF 1.0.0
#ASDF_STANDARD 1.2.0
# This is an ASDF file <https://asdf-standard.readthedocs.io/>
%YAML 1.1
%TAG ! tag:stsci.edu:asdf/
---
!core/asdf-1.1.0
dict:
  key: 1
asdf/library: !core/software-1.0.0
  name: "ASDF.jl"
  author: "Erik Schnetter <schnetter@gmail.com>"
  homepage: "https://github.com/JuliaAstro/ASDF.jl"
  version: "2.0.0"
...
#ASDF BLOCK INDEX
%YAML 1.1
---
[]
...

What would be required to:

• present unknown (or known) tags to users (if not already supported)?
• preserve unknown tags on read->write?

[icweaver]

Oh, good point. I've added an approach to warn about unknown tags and write them back to file in the parent PR #40 (1424c3d)