-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 27-06-12 15:56, Matthias Felleisen wrote: > > > 1. You got trapped in a strange corner of the language: > > -- to ask whether some value is a a Vcat you should use Vcat? -- > BSL should not really allow you to say 'Vcat' all by itself.
Actually the program below says `Vchat' which isn't defined in the code. Roelof, is the below program really what you tried (it seems to also be missing a parenthesis)? Disregarding that typo the problem seems to be that the program is creating a Vanimal out of either a Vcat or a Vcham and then asking whether the Vanimal object/instance is `equal?' to Vc(h)at (the struct descriptor?), which then naturally results in a negative result. Even if the program first got a hold of the animal member of Vanimal you cannot expect that Vcat or Vcham object to be `equal?' to something that you don't know what it is. > [I need to investigate this problem. Sadly it is not a bug.] > > > 2. Try to understand this: > > (define-struct Vcat [x happiness]) ;; A Vcat is (make-Vcat Number > Number) > > (define-struct Vcham [x happiness]) ;; A Vcham is (make-Vcat Number > Number) > > > ;; An Animal is one of: ;; -- (make-Vcat Number Number) ;; -- > (make-Vcham Number Number) > > > ;; Animal -> Number ;; compute the sum of the x location and the > degree of happiness > > (check-expect (happiness-quotient (make-Vcham 10 20)) 30) > (check-expect (happiness-quotient (make-Vcat 10 100)) 110) > > (define (happiness-quotient a) (cond [(Vcat? a) (+ (Vcat-x a) > (Vcat-happiness a))] [(Vcham? a) (+ (Vcham-x a) (Vcham-happiness > a))])) > > > > On Jun 27, 2012, at 5:08 AM, Roelof Wobben wrote: > >> Hello, >> >> I try to figure out how I can check if a struct is a Vcat or a >> Vcham. So i tried this: >> >> ; – a VCham (define-struct Vanimal (animal)) >> >> >> ; a struct has two parts ( the x-coordinate of a cat and the >> happiness of the cat) (define-struct Vcat [x happiness]) ; make >> Vcat : number (x) number (h) ; Vcat x -> number ; Vcat h -> >> number ; Any Vcat? -> Boolean >> >> ; a struct has two parts ( the x-coordinate of a kamneleon and >> the happiness of the kameleon) (define-struct Vcham [x >> happiness]) ; make Vcham : number (x) number (h) ; Vcham x -> >> number ; Vcham -> number >> >> ; Vanimal -> Boolean ; Function which cehcks if a struct is a >> Vcat (check-expect (check (make-Vanimal(make-Vcat 0 100))) true) >> (check-expect (check (make-Vanimal(make-Vcham 0 100))) false) >> (define (check s) ( equal? s Vchat) >> >> (check(make-Vanimal (make-Vcat 0 100))) >> >> >> But the answer is false not mather if the struct is a Vchat or a >> Vcham. Where am I missing something. You're missing that a(!) Vanimal is not a(!) Vcat or a(!) Vcham, although the Vanimal animal member might be. Further, (no article here!) `Vc(h)at' is not a(!) Vc(h)at (whichever way you typo-correct). Marijn -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk/tU90ACgkQp/VmCx0OL2yDHgCghJHd2x8KJaGalbeLjDzcYeNT /fAAn1nwFo/3ozKIxn88giuSYj+GlqTq =QSQE -----END PGP SIGNATURE----- ____________________ Racket Users list: http://lists.racket-lang.org/users
