This reading is also available in PDF.
Summary: In this reading, we consider one of Scheme's
central kinds of values, symbols.
Contents:
While your initial exercises in Scheme have been numeric, Scheme is not
limited to numerical computation, but can also operate on pure symbols.
Scheme's ancestor, Lisp, was originally developed to aid in experiments
in artificial intelligence. At the time, a leading theory suggested that
intelligence emphasizes symbolic manipulation. Hence, it is sensible
that Lisp and Scheme include symbols as a basic type. Evidence also shows
that many programs most appropriately work on abstract symbolic value.
So, what is a symbol? A symbol is simply a word (usually) that we use
to denote only itself. Unlike a variable, it has no associated value.
Symbols are also atomic, we cannot split them apart (as we might
a sequence of letters). The primary operation we perform on symbols is
comparison (determining whether two symbols are the same).
When we want to refer to something as a value involved in a computation,
rather than as the name of some other value, we put an apostrophe
(usually pronounced quote
) in front of it. In effect, by quoting
the symbol, we're telling Scheme to take it literally and without further
interpretation or evaluation:
We can also create symbols using the quote operation.
So, what can you do with symbols? Not a whole lot. You can determine
if a value is a symbol using the symbol? procedure and
you can determine if two symbols are the same using the eq?
procedure. Note that Scheme uses #t for yes
on
#f for no
. We'll return to those value when we
begin to consider conditionals.
> (symbol? 'sample)
#t
> (symbol? 23)
#f
> (eq? 'sample 'elpmas)
#f
> (eq? 'sample 'sample)
#t
Note that 'sample (with the quote) is very different
from sample (without the quote). In the first case,
Scheme interprets it as a symbol (an atomic value). In the second,
Scheme interprets it as a name for another value (e.g., something
defined with define. At first, you may find the distinction
a bit confusing. However, as you get used to programming
in Scheme, the distinction will become natural.
> (define sample 85)
> 'sample
sample
> sample
85
> (symbol? 'sample)
#t
> (symbol? sample)
#f
> (eq? sample 'sample)
#f
The problem becomes even worse when you use a name as a symbol, and the
name has not been defined.
> (symbol? 'elpmas)
#t
> (symbol? elpmas)
reference to undefined identifier: elpmas
Note that quote does not create a symbol. Rather, it tells Scheme to
take something literally. Hence, you can can quote many different things.
For example, you can even quote Scheme expressions.
> '(+ 2 3)
(+ 2 3)
> (quote (* 2 3))
(* 2 3)
Although you can use quote in a variety of ways, I prefer that you
limit your use to quoting symbolic values, at least for the first few
weeks of class. My experience shows that those who quote other kinds of
values early in the course careers end up with confusing results later
in the course.
;
