Re: [boost] [mpl] Metafunction terminology - revival?

From: Topher Cooper (topher_at_[hidden])
Date: 2006-07-28 09:47:30

At 08:11 AM 7/28/2006, Dave Abrahams wrote:
> > On Tue, 25 Jul 2006 10:44:14 -0400, David Abrahams
> > <dave_at_[hidden]> wrote:
> >
> >>> So, I will use "metafunctor" and "metafunctoid"
> >>
> >>Those terms don't intimidate junior colleagues?  They intimidate me!
> >
> > Nah, don't let them do. They are just thin wrappers around Greek
> > terminology ;-)
>
>Do "...or" and "...oid" really connote distinct meanings?  That's the
>problem I have with them

As near as I can tell, the terms are being applied pretty 
arbitrarilly -- they have almost no useful a-priori connotation at all.

According to Miriam-Webster on-line: the ending "-or" means "one who 
does a (specified) thing" so, for example, a "grantor" is one who 
"grants".  An alternative meaning is "condition" or "activity".

 From the same source the ending "-oid" means either "something 
resembling a (specified) object" or "having a (specified) 
quality".  Metafunctoid does fit this somewhat but its implicit 
meaning is pretty broad to be introduced to draw fine distinctions.

As for "functor" it comes from Category Theory and is something that 
maps one category (broad mathematical structure, like the general 
idea of a "group") to another while preserving certain 
properties.  With or without the "meta-" it doesn't seem at all relevant.

It probably entered this area of discussion by way of Prolog which 
used the term to designate the atom that started a structure.  The 
probable intention of this terminology, though, was "function" 
meaning "purpose" -- the "functor" was the thing that specified the 
purpose, or meaning, of the structure.  So in (father-of jim tom) the 
functor is father-of.  To be useful (interpretable) use of a 
particular functor implies that the structure has a particular 
"arity" with each position having an implied meaning and therefore 
implicit type (essentially -- an expectation that only certain atoms 
or structures with certain functors will appear at that position).  A 
"functor" in Prolog pretty much corresponds to "class" in C++.

Since Prolog, "functors" as things that tag a list implying the 
number and types of the elements has entered a number of 
languages/data systems (e.g., Eclipse).  Structures representing 
"function invocations" are certainly applicable and the pressure of a 
false etymology certainly encourages that specific narrowed 
usage.  But it is a false etymology and, I would say, is best avoided.

Topher