Re: Who discovered the type erasure technique in C++?
Roshan Naik wrote:
No doubt the idiom was used for a while, but assigning the right names
to idioms is important too. I recall the first time I heard "type
erasure" in context with the C++ way of doing it was in a messageby Dave
Abrahams, I think on Boost.
Andrei
Although I too have been using the term "type erasure" to refer to this
feature, I have wondered many times, about how the term came to be used in
C++. I am unclear if it is an appropriate name for it. The reason is as
follows :
The terms "type erasure" and "type reconstruction" have their origins in
lambda calculus. Currently my understanding of this calclus is
insufficient to figure out what those folks in the functional community
mean exactly. The book "Types and programming languages" by Benjamin
Pierce makes some very brief attempts at explaining the terms. My reading
of Benjamin's wordings is : Type Erasure is basically about getting rid of
all type information. Which seems (to me) more closer to the usage of this
term in context of Java.
Java has little to do with it, although its generics are a good example
of type erasure at work. Type erasure is somewhat loosely defined. It
basically means that operational semantics of a language are independent
of the static types used, so in essence you can first typecheck, then
remove all type annotations in the program, then execute the untyped
resulting program without a hitch.
The simplest example of type-dependent semantics is overloading, where a
function is selected depending on the static types of its arguments. ML
and other languages that focus on type erasure are adamant on
disallowing overloading. I recall ML did cheat a bit with its +
operator, but I might be wrong.
The origin of my confusion came about when I submitted some stuff to one
of the SIGPLAN conferences. One of the reviewers pointed out that my usage
of term "type erasure" for this was incorrect. I was unable to get much
details from him though since reviews are anonymous. And since the review
committee was made up some of prominent names from the functional world, I
had some reason to believe that the comment had merrit.
If this term already exists in Computer Science for something that is a
little different, then it may not be a good name in C++. To me "External
Polmorphism" sounded like a appropriate name for this feature, as coined
in the ACE paper.
In C++ the idiom involves "erasing" the static type information received
during construction and keeping it in a dynamic form (e.g. a vtable). I
think it is a great concise description, and I knew what it means the
first time I saw the phrase in boost. I hadn't connected the dots, and
connecting the dots between two well-researched pools of knowledge is an
important thing.
External polymorphism is just an invented term that associates a name
with a particular method, without putting the latter in the larger
context. Our community has been, and sadly still is, guilty of inventing
provincial and confusing terminology for concepts that are either well
understood or actively researched in the Programming Languages
community. I think this is causing a fair amount of damage.
Andrei
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