🚀 go-pugleaf

In article <6090@pdn.paradyne.com> alan@oz.paradyne.com (Alan Lovejoy) writes:
>In article <1941@etive.ed.ac.uk> db@lfcs.ed.ac.uk (Dave Berry) writes:
>You cannot rightfully divorce "interpreter activity" from "program activity."
>The point is that there is nothing inherent in the SEMANTICS of a function
>that dictates whether its evaluation is necessarily part of the language
>interpretation process or part of the program execution process.

I don't see how this is relevant to the discussion of the semantics of
a language, as opposed to a function written in that language.  Yes, if
you use denotational semantics as your formal model you can describe
each construct of the language by a function, but that function is then
a function of the underlying formal system, not of the language being
described.

>In other words, different languages can
>choose completely disjoint sets of "built-in" or "primitive" functions.
>A language that left EVERYTHING to the programmer as an exercise would be
>the null language which ("permits" | "requires") the programmer to write
>his own interpreter/compiler and define his own syntax.

Which would be implementing a different language.  The null language would
not provide assignment or recursion or any other approach.  In fact you
couldn't write an interpreter or compiler using it at all.

>Different implementions of different languages make different decisions 
>about what functions exist, and at what time they are evaluated.

For example, some have assignment, and some don't.

>I assert that the choice of "primitives" to be provided
>by a programming language is like the choice of primitive dimensions:
>completely arbitrary. You can change the expression of the definition of
>a dimension and/or a data type by choosing a different set of primitives,
>but you haven't changed what it actually *IS*.

Again, this seems to be arguing that you can implement a function in
several different languages.  This doesn't show that the languages are the
same, or that they should be considered to be the same (except in the
rather boring aspect of equal expressive power).

The difference between physics primitives and programming language primitives
is that programming languages are hierarchic; if you implement a language L'
in terms of an existing language L you are not extending L, you are using
it as an implementation tool.

>>It seems to me that I could use your argument to show that the archetypal
>>toy imperative language used in semantics textbooks doesn't have the idea
>>of state, because I can implement it in the lambda calculus.
>
>The true nature of a thing is best comprehended as the UNION of its possible
>definitions, not the INTERSECTION.

This seems more like an argument for my side, in that a comparison of two
languages should include the differences between them as well as a statement
of their expressive power (i.e. that they can all be modelled by a universal
Turing machine).

>This (functional languages must be implemented using Turing machines) is one of
>of those things that cannot be proven--only disproven.

Any model capable of implementing functional languages must be as powerful
as a Turing machine, but it need not necessarily be a Turing machine.

Dave Berry, Laboratory for Foundations      db%lfcs.ed.ac.uk@nsfnet-relay.ac.uk
    of Computer Science, Edinburgh Uni.	    <Atlantic Ocean>!mcvax!ukc!lfcs!db

 "It's a cute castle, but why did they build it right next to the railway?"