Wow - some great responses on the thread - I have some more questions to chime in and hopefully it's not too late ;)  On 6/11/07, Chongkai Zhu <<a href="mailto:czhu@cs.utah.edu"> czhu@cs.utah.edu</a>> wrote: > Yes, at the C level, MzScheme knows the type tag. But, on the Scheme > level, everything is suppose to be transparent. If scheme's type is really supposed to be transparent, I would venture that it isn't - because if it is, then I shouldn't have to test for types to call the right procedures, or write separate procedures that essentially do the same thing for different types ( i.e. no built-in generic programming). <div>On 6/11/07, Shriram Krishnamurthi <<a href="mailto:sk@cs.brown.edu">sk@cs.brown.edu</a>> wrote:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> > What is the type of ADD1?  What is the type of CALL/CC?  How > about of (lambda (x) x)?  Or >   (let ([x 3]) >     (lambda (y) >       (when (= (random 2) 0) >           (set! x y)) >      x)) > ?</blockquote><div> Shouldn't mzscheme know the answer to these questions?  It seems possible within Haskell or Ocaml to infer the types - below is my attempt but I probably have the syntax wrong. <div style="margin-left: 40px;">add1 is int -> int (lambda (x) x) is any -> any I don't understand enough about call/cc so I won't attempt ;) (let ((x 3)) (lambda (y) (when (= (random 2) 0) (set! x y)) x)) can be void -> (any -> (union int any)) </div></div> And the contract system does allow such specifications manually - running (add1 "a") returns a run-time type mismatch error. <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> > PS: In general, when asking questions about "every object" in >     a language like Scheme, it's imperative to remember the >    higher-order values also.</blockquote><div> Does this mean that Scheme doesn't know about the higher order value itself?  This might be where my gap is - from my perspective as a user, I am used to having the language platform knows about the types of the values, and externally observing scheme's behavior - it does know. On 6/11/07, Jos Koot <<a href="mailto:jos.koot@telefonica.net">jos.koot@telefonica.net</a>> wrote: > It appears to me that there is even no agreement about what the type of a type > would have to be, let alone the question when two types are the same ;) Please > take notice that (as a fan of PLT) I am quite content with PLT's system of > predicates, those for structs and object-classes included. I am also a fan and my reason to ask these questions is to understand how scheme's way work better (or worse, but I hope it's the former :P) than say C# or Python's way.  My current feeling is that Scheme's type system requires explicit handling where I am used to handling implicitly.  Perhaps as shown in the thread <a href="http://www.cs.brown.edu/pipermail/plt-scheme/2007-May/018457.html">http://www.cs.brown.edu/pipermail/plt-scheme/2007-May/018457.html</a> this approach allows for type research and experimentations at the expense of normal programming - and if this is a conscious design decision, that's fine - I am just trying to understand ;) Cheers, yc </div></div>