Stringify expressions

[1ec5]

mbgl should have a mechanism for converting an mbgl::style::expression::Expression to a JSON structure or string. Perhaps it could live in stringify.hpp. This would be useful for implementing the style property getters on iOS/macOS for #8074: instead of working with the specialized C++ types, the getter code could transform JSON-like structures, just as the setter code produces JSON-like structures via a converter.

/ref #7563
/cc @anandthakker

[1ec5]

This mechanism or one that converts to mbgl::Value would also enable the SDKs to expose alternative setters and getters in JSON format. Studio just added an “Edit property as JSON” option that these setters and getters could tie into, and React Native Mapbox GL would also benefit from not having to reverse-engineer the native SDKs’ expression APIs.

On the iOS/macOS side, we’re still going to prefer the NSExpression syntax, as it would require far fewer @s in code, but an +[NSExpression mgl_expressionForJSONObject:] and -[NSExpression mgl_jsonObject] would be a sensible alternative for certain workflows.

/cc @nitaliano @davidtheclark

[anandthakker]

👍 this shouldn't be too hard to add.

Perhaps it could live in stringify.hpp

Might be easier for this to be a member function on each mbgl::style::expression::Expression subclass.

[ChrisLoer]

Implemented in #11156.