I don't like it either because it makes the function a contract violator: (define (listy l) (cond [(empty? l) ...] [else ... (first l) ...]))
(define (f x) ... (listy 5) ...) f breaks listy's contract, but it goes undetected in most of Racket and all of HtDP and listy gets blamed for breaking first's contract. A student sees an error message about first, not listy. If you had [(cons? ...) ...], then you'd get "all questions were false" inside listy, which is nicer. I understand that signatures are meant to solve this, but they aren't part of HtDP yet. On the other hand, if the contracts are there, I like else because each cond eliminates one variant and there is finally just one variant left. (Note: I only like this if it is unlikely I will add more variants. I dislike that type-case from PLAI has else, for example.) Jay On Fri, Sep 16, 2011 at 10:13 AM, Robby Findler <[email protected]> wrote: > Why is else evil? I can see how it might be pragmatic to avoid it in a > language without contracts, but I'm having trouble seeing evil. > > Robby > > On Fri, Sep 16, 2011 at 10:52 AM, Shriram Krishnamurthi <[email protected]> > wrote: >> I introduced templates today. Almost as if on cue, one student asked >> whether he could use else instead of (cons? l). I told them I was >> going to make a MORAL judgment about why it was EVIL, and spent ten >> minutes talking about all that. >> >> As class ended, one of my students came up and said, >> >> "So why doesn't the Racket language Web site agree with you?" >> >> http://docs.racket-lang.org/guide/Lists__Iteration__and_Recursion.html >> >> (Look for "[else".) >> >> Shriram >> _________________________________________________ >> For list-related administrative tasks: >> http://lists.racket-lang.org/listinfo/dev >> > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/dev -- Jay McCarthy <[email protected]> Assistant Professor / Brigham Young University http://faculty.cs.byu.edu/~jay "The glory of God is Intelligence" - D&C 93 _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/dev
