Lou Langevin <firstname.lastname@example.org> wrote:
| I want to check for the existence of a variable...
The bad news is that the Scheme standard *DOESN'T* define a way to do this.
The good news is that most popular implementations *DO* provide this
functionality, but because it's not in the standard, the syntax and
semantics vary from implementation to implementation. (*sigh*)
Here are a few variations:
MzScheme: (defined? 'foo) "defined?" is a function, takes a symbol as an arg
thus can be used inside other functions which are
passed symbols as args. True if symbol has a global
definition (but never true of lexically-bound vars).
SCM: (defined? foo) That is, "defined?" is a *macro*, and takes an
*unquoted* symbol, and thus mostly useful only
at the top level. Like MzScheme's, never true of
Elk: (bound? 'foo) "bound?" is a function, takes a symbol as an arg,
but is is true if the symbol has *either* a top-level
or lexical binding.
| I came across the [symbol?] command and was wondering if this was
| it's purpose.
No, "symbol?" is just one of a number of "type predicate" (others include
"boolean?", "number?", "vector?", etc.) The "symbol?" predicate may be
safely applied to *any* Scheme object, but is only true if that object
is in fact a symbol. E.g.:
> (define foo 123)
> (symbol? foo)
> (symbol? 'foo)
> (map symbol? (map string->symbol '("abc" "def")))
Rob Warnock, 8L-846 email@example.com
Applied Networking http://reality.sgi.com/rpw3/
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