Hi people! Nice explanation, about the difference of "special form" in Clojure, vs other Lisp.
I remember (Queinnec Lisp?): lambda: to make "normal functions" mlambda: to make "macros" (something that produce a form, and then, evaluate that result) flambda: spread, non evaluation of parameters nlambda: non-spread (all arguments in one variable), non evaluation of parameters. Usually, when I wrote a Lisp interpreter, I implement the above "words". AFAIK, Clojure doesn't have any way to build a new special form, except via macros. Why? It was a design decision? motivated by Java compilation? Angel "Java" Lopez http://www.ajlopez.com http://twitter.com/ajlopez On Mon, May 31, 2010 at 11:06 AM, Patrick Stein <p...@nklein.com> wrote: > Maybe Clojure uses the term "special form" differently than Common > Lisp does, but here's how to think of "special form" in the Common > Lisp sense. > > A form that is *NOT* a special form like (F A B C D ...) will either > evaluate: > > 1. (SYMBOL-FUNCTION F), then A, B, C, D, ... in order from left-to- > right, OR > 2. A, B, C, D, ... in order from left-to-right then (SYMBOL-FUNCTION > F) > > And, cannot change the lexical environment (add or remove symbols in > the environment). > > It is up to the implementation whether (SYMBOL-FUNCTION F) gets > evaluated first or last, but A, B, C, D, ... get evaluated from let to > right in order before F is invoked. > > Any form that does not ALWAYS obey that is a "special form". > > Examples: > > (+ 3 5 (+ 6 7) ii), the form 3 is evaluated, then 5, then (+ 6 7), > then ii, then all of these are passed to (SYMBOL-FUNCTION +) to > execute. This IS NOT a special form. > > (IF A B C) does not evaluate both B and C. Thus, it IS a special > form. > > The following IS NOT a special form (as long as XX and YY aren't > dynamic variables that we're overriding but rather just local > variables here like it looks) because it always evaluates A and B in > the expected order: > (DEFMACRO WEIRD (A B) > `(LET ((XX ,A)) > (LET ((YY ,B)) > (DO-SOMETHING (+ XX YY) (- XX YY))))) > > (WEIRD (+ 6 7) ii) > > > The following IS a special form because it does not evaluate A and B > in the expected order: > (DEFMACRO WEIRD-SPECIAL (A B) > `(LET ((YY ,B)) > (LET ((XX ,A)) > (DO-SOMETHING (+ XX YY) (- XX YY))))) > > (WEIRD-SPECIAL (+ 6 7) ii) > > The following IS also a special form since it doesn't always evaluate > A and B in the expected order: > (DEFMACRO WEIRD-MAY-BE-SPECIAL (SPECIAL A B) > `(IF ,SPECIAL > (LET ((XX ,A)) > (LET ((YY ,B)) > (DO-SOMETHING (+ XX YY) (- XX YY)))) > (LET ((YY ,B)) > (LET ((XX ,A)) > (DO-SOMETHING (+ XX YY) (- XX YY))))) > > (WEIRD-MAY-BE-SPECIAL (ZEROP (RANDOM 2)) (+ 6 7) ii) > > The term "special form" (at least in Common Lisp) has nothing to do > with whether it is built-in or user-defined. It has to do with > whether or not all of its components are evaluated in the expected > order. > > ttyl, > Patrick > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with > your first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com<clojure%2bunsubscr...@googlegroups.com> > For more options, visit this group at > http://groups.google.com/group/clojure?hl=en > -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
