I’m trying to understand two of the ABI documents,
<https://github.com/apple/swift/blob/master/docs/ABI.rst> and
<https://github.com/apple/swift/blob/master/docs/ABIStabilityManifesto.md>,
with respect to a strong type-alias idea I just posted:
retype IDENTIFIER: IMPLEMENTATION-TYPE (“,” PROTOCOL)* {
// New methods and computed properties
// Maybe type-level stored properties
// Optionally export symbols from the original type to this one
}
I think we would need a new ABI entry for re-types. I think the implementation
type would have to be a value type (structs, enums, tuples, and possibly
someday fixed-sized arrays); I couldn’t get how to make a retype work with a
class’s witness information block (which are customized to fit Objective-C
compatibility) and what would it mean to shadow something that is a pointer to
the real information anyway. This also means that retypes are always value
types themselves.
Looking at the ABI Stability Manifesto:
* A retype has the same data layout as its implementation type. It has the same
trivial and bitwise-movable status as its implementation type.
* The value metadata for a retype needs a pointer to its implementation type’s
value metadata. I don’t know if a retype should share its witness table with
the implementation type, or have a separate table that has a pointer to the
original’s table.
Looking at the ABI doc:
* A retype has the same layout as its implementation type.
* The common metadata layout of the type metadata has a new kind value for
retypes.
* In addition to the common metadata layout, a retype’s metadata has a nominal
type descriptor, a reference to the parent metadata record, a reference to the
implementation type’s metadata record, then possibly a generic parameter vector.
* The nominal type descriptor needs a new kind for retypes. I’m not sure if it
needs any field/case/payload data.
* I don’t know if new mangling operators are needed for retypes, or if they can
reuse their implementation type’s codes.
Other:
* I guess methods and computed property getters & setters of a type are listed
with the general functions, with some marker of where the “self” pointer is.
New methods of a retype would need new function entries, but I don’t know if
exported methods would just reuse the implementation type’s method entry, or
would new functions be created that just forward to the implementation type’s
versions.
—
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT mac DOT com
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