Re: Terminology question: are s-exps the text or the data or both?
From: Anton van Straaten (anton_at_appsolutions.com)
Date: 10/26/03
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Date: Sun, 26 Oct 2003 09:12:22 GMT
Kenny Tilton wrote:
>
>
> Sebastian Stern wrote:
> > The concept of s-expressions is clearly explained in John McCarthy's
> > original paper on Lisp
> >
(http://www-formal.stanford.edu/jmc/recursive/node3.html#SECTION000300000000
> > 00000000), and he uses the words 'character' and 'string', so he must
means
> > the external (textual) representation.
>
> You guys are all kidding right, when you say "text"?:
>
> "We shall now define the S-expressions (S stands for symbolic). They are
> formed by using the special characters:
>
> . ( )
>
> and an infinite set of distinguishable atomic symbols. For atomic
> symbols, we shall use strings of capital Latin letters and digits with
> single imbedded blanks."
Those "special characters" and "strings of capital Latin letters and digits
with single imbedded blanks" are an example of what is usually referred to
as "text".
> What part of "S stands for symbols" is not understood?
"S stands for symbolic" refers to the expressions that are formed by the
provided rules. S-expressions are symbolic expressions which are defined in
terms of a textual representation, i.e. they are formed by a sequence of
characters, including special characters and strings of alphanumeric
characters.
There are other possible representations for S-expressions. One such
alternate representation is the Lisp objects produced by the reader.
However, in that representation, the special characters, . ( ), do not
usually exist explicitly. This is a further indication that the referenced
definition is referring to a textual representation.
Note that the S-expressions formed by the defined notation are not text, and
nor are they Lisp objects. They are a conceptual entity with multiple
possible representations, but in the above link they are defined in terms of
their textual representation.
> And then "For atomic symbols, /we shall use/ strings..." emphasis added.
> So the strings are not the symbols, they are used for them.
Right, the strings are not the symbols, they are the textual representation
of the symbols. That's the point - the definition defines how text is used
to form S-expressions.
> And if one programs in Lisp, I trust one knows that the name of a
> symbol (the string) is the name of the symbol, not the symbol itself.
>
> Tell me, why isn't read-from-string called read-from-symbol?
Because read-from-string, like READ, converts a textual representation of
S-expressions into a Lisp object representation.
> Does symbol-name return a symbol? Not. Look up sumbol in the HyperSpec.
> WEhat do you know! Name is an /attribute/ of a symbol:
>
> "The name of a symbol is a string used to identify the symbol.
> Every symbol has a name, and the consequences are undefined
> if that name is altered. The name is used as part of the external,
> printed representation of the symbol"
>
> I would say a little more rigor was required, but this does not even
> require rigor to get straight.
Then again, perhaps it does... ;)
> When you realized you did not have a good answer, you should have said.
> "Doh!" and changed your answer. The reader reads text and returns
> symbolic-expressions.
The reader reads a textual representation of symbolic expressions and
returns an object representation.
Anton
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