[pro] Phylosophical question on "syntactic" constructors

[Steve Haflich]

I see a couple potential issues.  (Better terms for whether the reader
macro evaluates subforms might be "evaluating" and "quoting".)

If the elements inside the syntax are permitted to be non-self-evaluating
objects such as symbols or conses, then under the evaluating option one
would need to write [1 'x (+ 40 2)].

Second, as I'm sure you realize, the evaluation model at read time is very
different from regular evaluation in that it always happens in the
read-time dynamic environment and the null lexical environment.  This
doesn't matter if the computations are just simple arithmetic, but it makes
read-time evaluation feel messy to me, especially if someone might later
use it with more-complex forms.

You might also consider inside your reader macro borrowing the syntax and
meaning of the backquote comma:
[ 1 x ,(+ 40 2) ].  This might make the meaning and environment more
evident to experienced CL programmers (although comma forms evaluate at run
time in the current lexical environment).


On Mon, Dec 3, 2012 at 5:12 AM, Marco Antoniotti <
antoniotti.marco at disco.unimib.it> wrote:

> Dear all,
>
> I was fooling around with reader macros to implement some - let's say -
> "tuple" syntax.
>
> Now, I am just curious about the general thinking on this.
>
> First of all, let's note that the problem here in the conflating of the
> "constructor" with the "printed representation" of an item.  I.e., in
> Matlab you say
>
> >> [1, 2, 40 + 2]
> ans =
> 1 2 42
>
>
> In CL, doing the simple thing, you get
>
> cl-prompt> [1 2 (+ 40 2)]
> [1 2 42]
>
> but
>
> cl-prompt> #(1 2 (+ 40 2))
> #(1 2 (+ 40 2))
>
> So, suppose you have your [ … ] reader macro, would you have it work
> "functionally" or "quoting-ly" (for want of a better word)?  I am curious.
>  Note that the Matlab-style version would not break referential
> transparency if you did not have mutations.
>
> best
>
> Marco Antoniotti
>
>
>
>
>
>
>
> --
> Marco Antoniotti, Associate Professor tel. +39 - 02 64 48 79 01
> DISCo, Università Milano Bicocca U14 2043 http://bimib.disco.unimib.it
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