What is Expressiveness in a Computer Language

Chris Uppal chris.uppal at metagnostic.REMOVE-THIS.org
Wed Jun 21 11:39:15 CEST 2006

David Hopwood wrote:

> When people talk
> about "types" being associated with values in a "latently typed" or
> "dynamically typed" language, they really mean *tag*, not type.

I don't think that's true.  Maybe /some/ people do confuse the two, but I am
certainly a counter-example ;-)

The tag (if any) is part of the runtime machinery (or, if not, then I don't
understand what you mean by the word), and while that is certainly a reasonably
approximation to the type of the object/value, it is only an approximation,
and -- what's more -- is only an approximation to the type as yielded by one
specific (albeit abstract, maybe even hypothetical) type system.

If I send #someMessage to a proxy object which has not had its referent set
(and assuming the default value, presumably some variant of nil, does not
understand #someMessage), then that's just as much a type error as sending
#someMessage to a variable holding a nil value.  If I then assign the referent
of the proxy to some object which does understand #someMessage, then it is not
a type error to send #someMessage to the proxy.  So the type has changed, but
nothing in the tag system of the language implementation has changed.

    -- chris

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