another frustrating learner question, CLOS
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From: Günther Thomsen on 24 Feb 2010 20:19 On Feb 24, 3:08 pm, ccc31807 <carte...(a)gmail.com> wrote: > Learning Lisp is like pulling teeth, except more painful! > Learning something new should be fun. I at least enjoyed it very so far, but then, I enjoyed also learning Forth, C and Pascal back when. Even C++ and Java for a while. Oh, and in Lisp, I stayed so far away from OOP - I just didn't have a need for it. If it's painful, chances are you're doing it wrong. Keep in mind that for virtually all problems there are multiple solutions. Pick an easy one, go with the flow.
From: Tamas K Papp on 25 Feb 2010 01:23 On Wed, 24 Feb 2010 15:08:02 -0800, ccc31807 wrote: > Learning Lisp is like pulling teeth, except more painful! Either Lisp is not a good match for you, or you are not ready for it yet. In either case, you are not likely to benefit from learning Lisp for now. Regarding your code: as others have said, it should work fine. You must be making some mistake when you are running/compiling your program. Tamas
From: Kaz Kylheku on 25 Feb 2010 02:45 On 2010-02-24, ccc31807 <cartercc(a)gmail.com> wrote: > Learning Lisp is like pulling teeth, except more painful! > > Here is a code snippet copied directly from Figure 11.2, page 178, of > Graham's 'ANSI Common Lisp': > > (defclass rec () > (height width)) > (defclass cir () > (radius)) > (defmethod area ((x rec)) > (* (slot-value x 'height) (slot-value x 'width ))) > (defmethod area ((x cir)) > (* pi (expt (slot-value x 'radius) 2))) > > Here is the error message gives when I try to compile this: > NO-APPLICABLE-METHOD: When calling #<GENERIC-FUNCTION FIND-METHOD> > with arguments (#<COMPILED-CLOSURE > AREA> > NIL (REC) NIL), no method is applicable. > [Condition of type METHOD-CALL-TYPE-ERROR] The error message is impossible from compiling /just/ the above, and nothing more. It's a message about an erroneous call of the AREA generic function; but your example has no call at all, only method specializations defining two flavors of AREA. It's puzzling because it seems to be complaining that there is no AREA specialization which takes a REC, which you have in fact defined. Why don't you post minimal program (all in one file, if possible) which reproduces the error message, along with steps to reproduce. Which Lisp implementation are you invoking, in what manner, etc.
From: ccc31807 on 25 Feb 2010 09:17 On Feb 25, 2:45 am, Kaz Kylheku <kkylh...(a)gmail.com> wrote: > The error message is impossible from compiling /just/ the above, and nothing > more. It's a message about an erroneous call of the AREA generic function; > but your example has no call at all, only method specializations > defining two flavors of AREA. I sometimes think that the programming gods get together at night for code reviews, and sometimes where there has been a shedding of blood in an amount sufficient to atone for programming sins, graciously extend mercy and forgiveness, even though it's neither earned nor merited by the programmer. Yesterday, I had emacs and slime open all day hacking with Lisp during free moments, and at the end of the day was reviewing Ch 11 of 'ANSI Common Lisp' when I had the problem. I don't (usually) post to a news group about an error unless I am at my wit's end, which I was yesterday evening. This morning, with a fresh slate, I opened the file, compiled it, and it ran perfectly. I don't know what I was doing wrong, and certainly didn't do anything to fix it, so the only explanation I can give is the remission of programming sins by the programming gods. CC.
From: ccc31807 on 25 Feb 2010 09:25
On Feb 25, 1:23 am, Tamas K Papp <tkp...(a)gmail.com> wrote: > On Wed, 24 Feb 2010 15:08:02 -0800, ccc31807 wrote: > > Learning Lisp is like pulling teeth, except more painful! > > Either Lisp is not a good match for you, or you are not ready for it > yet. In either case, you are not likely to benefit from learning Lisp > for now. I strongly disagree. I don't believe that people are 'matched' to programming languages any more than they are 'matched' to human languages. A good programmer can program in any language whether he knows it or not, and a poor programmer can't program in any language whether he know it or not. As to 'being ready' this is like lifting weights. The only way you can be ready to lift heavy weights is to start by lifting light weights and build yourself up. How could anyone 'be ready' to learn any language if he never exercised himself by attempting to learn the language? > Regarding your code: as others have said, it should work fine. You > must be making some mistake when you are running/compiling your > program. See my other post this morning. Obviously, I made some mistake, which was probably related to fooling around all day and corrupting the environment in some kind of way. I had probably defined a version of the 'area' function earlier in the day which the environment still 'remembered' and gave the error message. In a perfect world, the error message would have been something like this: "Hey, you dummy, you already defined an area function, so you need to give this one a different name." Thanks, CC. |