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Summary: In this reading, we consider one of Scheme's central kinds of values, symbols.
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
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:
> 'sample sample
We can also create symbols using the
> (quote sample) sample
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
procedure. Note that Scheme uses
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
'sample (with the quote) is very different
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
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.
I usually create these pages
on the fly, which means that I rarely
proofread them and they may contain bad grammar and incorrect details.
It also means that I tend to update them regularly (see the history for
more details). Feel free to contact me with any suggestions for changes.
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