How to merge NamedTuples in in a macro in Crystal? - crystal-lang

Question

I'm trying to do a JSON.mapping in a class (OtherThing) based on the merge of two named tuples.
class Thing
MAPPING = {
id: {type: String, default: UUID.random.to_s},
email: {type: String, nilable: true},
}
JSON.mapping({{MAPPING}})
end
class OtherThing
MAPPING = Thing::MAPPING.merge({
address: String,
city: String,
zip: {type: String, nilable: true},
latitude: {type: Float64, nilable: true},
longitude: {type: Float64, nilable: true},
})
JSON.mapping({{MAPPING}})
end
This produces an error depending on how I organize the code.
One error:
Error expanding macro
FOR expression must be an array, hash or tuple literal, not Call:
Thing::MAPPING.merge
{% for key, value in _properties_ %}
^~~~~~~~~~~~
Another possible error:
undefined macro method 'NamedTupleLiteral#merge' in the JSON.mapping of OtherThing

Answer 1

In short answer — no, you can't interpolate constant in macro right now.
#asterite:
The problem is that the mapping macros expect a HashLiteral as an
argument, but now they receive a Path
In you case instead NamedTuple macro got the Call node. In discuss on Crystal repo https://github.com/crystal-lang/crystal/issues/2388 found workaround that in theory can solve this problem but JSON.mapping it is old method and will be removed from language in next versions, instead try to JSON::Serializable

Answer 2

Thanks to #Blacksmoke16 for his help in pointing that, you can achieve desired outcome by defining your own macro with splats.
Below code should work. Credit goes to #Blacksmoke16
macro define(*args)
JSON.mapping(
{% for t in args %}
{{t.double_splat}},
{% end %}
)
end
class Thing
MAPPING = {
id: {type: String, default: UUID.random.to_s},
email: {type: String, nilable: true},
}
define {{MAPPING}}
end
class OtherThing
MAPPING = {
address: String,
city: String,
zip: {type: String, nilable: true},
latitude: {type: Float64, nilable: true},
longitude: {type: Float64, nilable: true},
}
define {{MAPPING}}, {{Thing::MAPPING}}
end