2009/12/2 Richard Shann <richard.sh...@virgin.net>: > I am stuck on one of those symbol/variable-name-in-a-string things > again: > > (define mything "display") > (display (eval-string mything)) > > that's fine. But can I test that the string in mything is the name of a > variable before doing the eval-string and finding out the hard way? I've > been doing (symbol? mything) etc, and going witless. Do I have to do all > that catch stuff?
If I understand you correctly, no. That's because the thing going into eval-string can be arbitrarily complex -- e.g. (eval-string "(define foo (lambda (x) (+ x y))) (define (bar z y) (foo (z)) (bar 2 2)") and there's no particular way to know if evaluating that won't throw some error. -- really, you have to evaluate it to find out. If you want to find out if some particular variable is bound to a proceedure, you can try (proceedure? thing) see e.g. http://www.gnu.org/software/guile/manual/html_node/Procedure-Properties.html If you want to know if a variable is bound, try defined? e.g. guile> (defined? 'x) #f guile> (defined? (string->symbol "x")) #f guile> (define x 42) guile> (defined? 'x) #t guile> (defined? (string->symbol "x")) #t --linas --linas
