<title>feat(nix): add nix.tag, a way of discriminating via tagged unions</title>
<updated>2021-01-30T15:41:08+00:00</updated>
<author>
<name>Profpatsch</name>
<email>mail@profpatsch.de</email>
...</author>
<published>2021-01-23T16:23:39+00:00</published>
<link rel="alternate" type="text/html" href="http://code.tvl.fyi/commit/?id=e4a7704583812786d90ddc1f3aa639b2866e18e2"/>
<id>urn:sha1:e4a7704583812786d90ddc1f3aa639b2866e18e2</id>
<content type="text">
Introduces the concept of a “tag”, a single-keyed attrset which
annotates a nix value with a name.
This can be used to implement tagged unions (by implying the list of
possible tags is well-known), which has some overlap with how
`nix.yants` does it.
However, the more fascinating use-case is in concert with a
so-called discriminator, `match` and hylomorphisms.
The discriminator can take a nix value, and add tags to it based on
some predicate.
With `match`, we can then use that information to convert the
discriminated values again.
With `hylo`, we can combine both the “constructive” discriminator step
with the “destructive” match step to recursively walk over a nix data
structure (based on a description of how to recurse, e.g. through attrset
values or list values), and then apply a transformation in one go.
Change-Id: Ia335ca8b0881447fbbcb6bcd80f49feb835f1715
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2434
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
</content>