Scheme Programming Language :: Why can't "define" double as "set!"?


	Why can't "define" double as "set!"? hi Is there any good reason that "define" shouldn't double as "set!"? dillog@gmail.com wrote: > Is there any good reason that "define" shouldn't double as "set!"? Yes: DEFINE defines variables, SET! changes the values of (defined) variables: (set! x 1) => wrong! (define x 0) (set! x 1) x => 1 Some interpreters may implicitly define variables when you try to set! them for the first time, thereby making DEFINE and SET! /appear/ to be the same. -- Nils M Holm <n m h @ t 3 x . o r g> -- http://t3x.org/nmh/ On Thu, 5 Apr 2007 08:47:02 +0000 (UTC), Nils M Holm <before-2007-07@online.de> wrote: >dillog@gmail.com wrote: >> Is there any good reason that "define" shouldn't double as "set!"? >Yes: DEFINE defines variables, SET! changes the values of (defined) >variables: >(set! x 1) => wrong! >(define x 0) >(set! x 1) >x => 1 >Some interpreters may implicitly define variables when you try >to set! them for the first time, thereby making DEFINE and SET! >/appear/ to be the same. Additionally, implicit defines do not work in compiled code and may work only at the top level in an interpreter. Within a function, setting a variable that hasn't been declared (at the top level using define, in a lambda list or in a let) will fail if the function is compiled and may fail if the function is interpreted. (define x 0) (define (f z) (let ((w 0)) (set! w 1) (set! x 1) (set! z 1) (set! y 1))) (f 0) set!: cannot set undefined identifier: y So, it may work in some situations, but don't count on it. George -- for email reply remove "/" from address <dillog@gmail.com> wrote: > hi > Is there any good reason that "define" shouldn't double as "set!"? Doesn't define extend the environment function with a new binding whereas set! modifies the store function?