Clojurians Log v2
malli 2026-02-19
I'm reading the malli documentation, and I'm kinda confused on the difference between mr/composite-registry and just mergeing schemas into a registry. There are lots of examples that just merge schemas into a registry like this:
(def registry (merge (m/default-schemas) (mu/schemas)))mr/composite-registry like this:
(mr/set-default-registry!
(mr/composite-registry
(m/default-schemas)
(met/schemas)))mu/schemas for :merge schema into the default registry, what should I use and why?
If the registries you are merging are maps, there is no difference. Internally, mr/registry is always called coercing to Registry. mr/composite-registry, mr/set-default-registry! both do this.
The one case you should be careful about when using merge on non-map registries like (merge (mr/composite-registry ..) (mr/composite-registry ..)). This won't work.
> I want to include mu/schemas for :merge schema into the default registry, what should I use and why?
Either will work, because mu/schemas is a map, and set-default-registry! coerces to Registry internally.
It's worth noting that merge has a performance advantage over composite-registry. The latter iterates over multiple lookups instead of pouring them into the same map.
It seems that circular schema references stop working when 'resetting' the default registry, poc:
(malli-registry/set-default-registry!
(malli/default-schemas))
(def sample-tree-schema
[:map
[:value :int]
[:children {:optional true}
[:vector
[:ref #'sample-tree-schema]]]])
(malli/assert
sample-tree-schema
{:value 1
:children
[{:value 2
:children [{:value 4}]}
{:value 3}]})
=> :malli.core/invalid-ref
Any ideas on how to improve the documentation?
@joel.kaasinen I brainstormed some here, just pushed what I had https://github.com/metosin/malli/pull/1261/changes
Guessing you need to add var-registry to the default registry https://github.com/metosin/malli?tab=readme-ov-file#var-registry
i.e., (composite-registry (default-schemas) (var-registry))
so malli/default-registry is not malli/default-schemas by default
kinda confusing
yeah check out the default default registry https://github.com/metosin/malli/blob/69366e440cfda6b402cc84a6e67d8694a89fa12c/src/malli/core.cljc#L3060
Under the hood, m/default-registry is a map of schemas, but mr/var-registry is a Registry. It's a good example of the difference between using merge and composite-registry here, merge wouldn't work.
It's a leaky abstraction but it might be a little less confusing in practice knowing that.
Leaky because default-registry doesn't document whether it's a map or a Registry, but we always assume it's a map just because it's more convenient (merge, assoc, select-keys).
thanks again for the explanation, I too found out its a documentation issue