Validator extensions, zomp vsn 0.2.0 (#5)

Co-authored-by: Ulf Wiger <ulf@wiger.net>
Reviewed-on: #5

README.md

@@ -11,6 +11,9 @@ management subsystem. It is based on JSON-Schema, and includes, among other thin
 * Caching of the user config (and schema) as persistent terms
 * Fast config lookups using key paths
 * Lookups can handle both schema defaults and user-provided defaults
+* Optional type coercion during validation
+* Optional conversion of enums to atoms
+* Optional extensions with custom validator funs
 
 ## JSON-Schema validator
 
@@ -78,3 +81,47 @@ there are surely other things that are unsupported.
 * `"merge"`   (for objects, keeps and/or updates existing values)
 * `"suggest"` (adds value if not already present)
 
+### Custom validation options
+
+Using `gmconfig_schema_utils:validate(Json, Schema, Opts)`, a few options are supported
+to further enhance the validation (`Opts` is of type `map()`):
+
+`coerce => boolean()` converts strings to integers, null and booleans when needed.
+`enums_to_atoms => boolean()` converts enum strings to atoms
+`extensions => map()` supports mapping `x-...` properties to custom validators.
+
+#### Validator extensions
+
+See the following test case:
+```erlang
+t_nested_refs() ->
+    S = read("data/nested_refs_schema.json"),
+    F = fun(Str, #{<<"tags">> := Tags}) ->
+                true = lists:any(
+                         fun(T) ->
+                                 nomatch =/= string:prefix(Str, T)
+                         end, Tags)
+        end,
+    Opts = #{extensions => #{<<"x-serialization">> => F}},
+    Vs = #{<<"tx">> => #{<<"from">> => <<"ak_good">>}},
+    Vf = #{<<"tx">> => #{<<"from">> => <<"ac_bad">>}},
+    validate(Vs, S, Opts),
+    fails(Vf, S, Opts, #{e => failing_schemas}),
+    ok.
+```
+This simulates an encoding extension, where the example fun here simply checks
+if tags specified under the `x-serialization` property are prefixes of the 
+given string value.
+
+In the test schema, we can see the following definition:
+```json
+"Pubkey": {
+    "type": "string",
+    "x-serialization": {
+        "tags": ["ak", "ct"]
+    }
+```
+
+Whenever the validator encounters an `x-...` property mapped to a validator fun,
+this fun is called with the value and the schema part of the property. The return
+value of the fun is ignored, and any normal return is treated as a validation success.