add tuple_type to the Tuple manipulation API

[tpapp]

We have tuple_type_head and tuple_type_tail, but it would be nice to have something that is just equivalent to

tuple_type(T) = (tuple_type_head(T), tuple_type_tail(T)...)

Currently one needs to extract T.parameters, convert from the svec, deal with Vararg tuples manually, etc.

[JeffBezanson]

Why is there a need to convert from svec? I also don't see how this would eliminate the need to handle Vararg tuples.

[tpapp]

For type calculations, it would be useful to have an interface which extracts (Int, Float64, Char) from Tuple{Int, Float64, Char}. Using .parameters always feels like I am using an internal interface.

You are right, Vararg is an orthogonal issue.

[cstjean]

+1. By analogy with fieldname/fieldnames and fieldtype, it could be called fieldtypes.

[tpapp]

fieldtypes(T::Type{<: Tuple}) = ntuple(i -> fieldtype(T, i), nfields(T))

does what I want.

But I am still wondering why there is no API for this: is it something I am not supposed to do, and use some kind of lispy recursion instead with the existing head/tail API?

[vtjnash]

Should be countfields not nfields, since the number may not be a known (might be Vararg), and so this might fail. But as a definition, that seems to me like a more sound definition than tuple_type_head / tuple_type_tail / tuple_type_cons. At lot of these usages seem to have been designed before the type system was able to express and handle many subtyping questions. c.f. #22051