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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>
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