add function defined?

[ssqq]

I think newlisp  should add function *defined?*, for all not defined variable is *nil*.

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> (defined? 'var) ; --> nil
> (set 'var nil)
> (defined? 'var) ; --> true
> (delete 'var)
> (defined? 'var) ; --> nil



[TedWalther]

Or perhaps a "strict" mode where unbound variables aren't auto-defined?  With a new debug message, "symbol FOO is undefined"



The behavior where (++ foo) gives 1, is confusing if newLisp is trying to keep "nil" and "0" distinct from each other.  Although (++ foo) => 1 is nice behavior and I like it.  I'd like to know more about the need to separate nil from 0.

[hartrock]

Code Select Expand



> (defined? 'var) ; --> nil
> (set 'var nil)
> (defined? 'var) ; --> true
> (delete 'var)
> (defined? 'var) ; --> nil



This would not work, because using the symbol 'var defines it before calling the function defined?.



But there is:

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> ;; check for var 'var
> (sym "var" (context) nil)
nil
> ;; -> not there
> ;; now define it:
> var
nil
> ;; check
> (sym "var" (context) nil)
var
> ;; -> now it exists
>

The flag nil in (sym "var" (context) nil) is important, since it suppresses creating the 'var symbol.

[TedWalther]

Wonder if that would work as (define-macro (defined? a) ...)

[hartrock]

Just started with using emacros (expansion macros) here a simple solution (with limitations):

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> (macro (defined? V) (sym V (context) nil))
(lambda-macro (V) (expand '(sym V (context) nil)))
> (defined? "var")
nil
> var
nil
> (defined? "var")
var
> ; but:
> (defined? "V")
V
> ; -> because it is used as symbol by the macro
> (defined? 'v2)
v2
> ; -> because it is defined *before* calling defined?
A more general variant would allow to choose another as the current and/or all contexts to search for the sym (using at least one more emacro parameter variable then).

A naming convention for such general-purpose emacros could be, to only use one-letter parameter names for such emacros:

[*] this avoids confusion with contexts, which may be named starting uppercase, too (but are usually multi-letter); and

[*] reduces sym pollution of MAIN context to a minimum.

[/list]
Note: starting parameter names with uppercase is needed for emacros.