Schema delegation to contained dict is incomplete

[RossBoylan]

The comment in schema.jl says "A Schema behaves for all intents and purposes like an immutable Dict" and the user documentation says something similar. But it's not really true. The comment goes on to note

, and delegates the constructor as well as getindex, get, merge!, merge,
keys, and haskey to the wrapped Dict.

which translates to "behaves like a Dict if you call those methods and otherwise not."

I noticed this because about a year ago, with v 0.6.5 I was able to use values to pull out members of the schema, but now this produces an undefined function error. https://discourse.julialang.org/t/statsmodels-values-schema-no-longer-works/46659 provides more background. As noted there it is not clear to me why it stopped working, because even the old version did not define values.

My particular problem can be solved by defining

Base.values(schema::Schema) = values(schema.schema)

but that doesn't solve the general problem that Schema does not act like a Dict in many ways (e.g., my sample code on discourse also shows a failure because iterate is not defined, triggered by the use of foreach).

I don't understand the design goals well enough to know whether implementing the full Dict protocol is a good idea, but the current implementation seems to be at an uncomfortable part-way there point.

[kleinschmidt]

Yeah it's spotty. At one point the schema really was just a Dict but that lead to too many method ambiguity errors so now there's a special type for it.

I'd be open to a PR which implements the missing methods, they can just be passed through like you noted. I just have never needed it internally but if it's useful to you than I don't object!

[RossBoylan]

Would it be better to implement the Dict protocol (I'm not entirely sure how to discover what that is; it doesn't seem to be explicitly documented in https://docs.julialang.org/en/v1/base/collections/#Base.Dict), e.g., keys and values, or use domain-specific names like abstractTerms and concreteTerms?

Do you have any ideas why values was working before?

[kleinschmidt]

See #103: before that, there was no Schema, just a Dict.

I think it's better to implement the "normal" methods for Dict rather than creating our own special ones...