[plt-scheme] (typeof obj)

I wonder too, for all that is needed seems to be in the C code. Chapter 2 of 'Inside PLT MzScheme' gives info on how type info is recorded 'inside' MzScheme. The following is what I collected from looking into the doc and the C code: C macro SCHEME_TYPE returns type-info. File .../src/mzscheme/src/stypes.c enumerates scheme types used inside MzScheme. I guess it would not be very difficult to add C code that collects the scheme-type and uses it to collect a type-name from an array of strings. May be elsewhere such an array already exists, I would not know. Adding a primitive that returns the string may do what you are looking for. Jos Koot
  ----- Original Message ----- 
  From: YC 
  To: Jos Koot 
  Cc: PLT Scheme List ; Robert Nikander 
  Sent: Monday, June 11, 2007 7:49 AM
  Subject: Re: [plt-scheme] (typeof obj)


  Agreed that it would be implementation dependent, but if the type info has to be calculated every time on the fly it can be both inefficient and incorrect (if all values are just bytes, it could satisfy more than one disjointed types that lead to incorrect semantics), hence I venture that Scheme value has type attached - but sure would love to be enlightened on this point. 

  Regarding whether a list of number is a list or a list of numbers - I can see that comes up with either predicates or type info, so don't see a disadvantage one way or another, but predicate-based approach just makes type dispatching more manual. 

  Anyhow - just wondering about the reason(s) not to provide type info - sure love to know the historical design decisions ;)

  Thanks,
  yinso


  On 6/10/07, Jos Koot <jos.koot at telefonica.net> wrote:
    ----- Original Message ----- 
      From: YC 
      To: Jos Koot ; PLT Scheme List ; Robert Nikander 
      Sent: Monday, June 11, 2007 12:47 AM
      Subject: Re: [plt-scheme] (typeof obj)


      This is something I wonder as well... given that internally Scheme *does know* about the type of a value, why isn't there a function that expose the information?  Type-based dispatching is after all a legitimate development technique. 
    I am not sure whether or not Scheme *does know* THE type of a value. I am sure the PLT team has more knowledge of that. My way of looking at it is that there are predicates that know whether or not a value satisfies them. Furthermore there is the question how far you want to go in specifying the type. Is a list of numbers simply a list or is it 'a list of numbers'? The number of predicates you may think of is unlimited.
    Jos Koot
      Thanks,
      yinso 


      On 6/10/07, Jos Koot <jos.koot at telefonica.net> wrote: 

        ----- Original Message -----
        From: "Robert Nikander" <nikander at nc.rr.com>
        To: "PLT Scheme List" < plt-scheme at list.cs.brown.edu>
        Sent: Sunday, June 10, 2007 6:28 PM
        Subject: [plt-scheme] (typeof obj)


        > Is there a way to get a type-name symbol or type object for any  scheme
        > object?

        Of course there is a way, but you may not like it:

        (define (types-of x) (filter identity (map (lambda (predicate typename) (and
        (predicate x) type-name)) predicates type-names)))
        (define predicates (list integer? procedure?, etc) 
        (define type-names '(integer procedure, etc))

        Of course the procedure returns a list of types, for a 1 is an integer, a
        rational, a real and a complex number.
        You also can think of adapting procedure make-struct-type, such as to add their 
        predicates and names to the lists.
        Same for define-struct, let-struct ...
        Same for classes.

        If you want only one type returned in stead of a list, two conditions must be
        met:
        First: for every two types A and B either A and B disjunct or A a subset of B or 
        B a subset of A.
        Second: carefully compose the lists of predicates and their names.
        Jos Koot

        >
        > (type 1) => 'integer
        >
        > or
        >
        > (type 1) => #<type integer> 
        > (type (type x)) => #<type type>
        >
        >
        > Rob
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