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. <br><br>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. <br><br>Anyhow - just wondering about the reason(s) not to provide type info - sure love to know the historical design decisions ;)<br><br>Thanks,<br>yinso<br><br><div><span class="gmail_quote">On 6/10/07, <b class="gmail_sendername"> Jos Koot</b> <<a href="mailto:jos.koot@telefonica.net">jos.koot@telefonica.net</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <div bgcolor="#ffffff"><span class="q"> <div>----- Original Message ----- </div> <blockquote style="border-left: 2px solid rgb(0, 0, 0); padding-right: 0px; padding-left: 5px; margin-left: 5px; margin-right: 0px;"> <div style="background: rgb(228, 228, 228) none repeat scroll 0% 50%; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial; font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"> <b>From:</b> <a title="yinso.chen@gmail.com" href="mailto:yinso.chen@gmail.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">YC</a> </div> <div style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>To:</b> <a title="jos.koot@telefonica.net" href="mailto:jos.koot@telefonica.net" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> Jos Koot</a> ; <a title="plt-scheme@list.cs.brown.edu" href="mailto:plt-scheme@list.cs.brown.edu" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">PLT Scheme List</a> ; <a title="nikander@nc.rr.com" href="mailto:nikander@nc.rr.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> Robert Nikander</a> </div> <div style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Sent:</b> Monday, June 11, 2007 12:47 AM</div> <div style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Subject:</b> Re: [plt-scheme] (typeof obj)</div> <div><br></div> <div>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. </div></blockquote></span> <div><font face="Arial"><font size="2"><font face="Arial" size="2">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.</font></font></font></div><span class="sg"> <div><font face="Arial" size="2">Jos Koot</font></div></span><div><span class="e" id="q_113192334e3c113a_4"> <blockquote style="border-left: 2px solid rgb(0, 0, 0); padding-right: 0px; padding-left: 5px; margin-left: 5px; margin-right: 0px;"> <div>Thanks,<br>yinso <br><br></div> <div><span class="gmail_quote">On 6/10/07, <b class="gmail_sendername">Jos Koot</b> <<a href="mailto:jos.koot@telefonica.net" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">jos.koot@telefonica.net</a>> wrote:</span> <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><br>----- Original Message -----<br>From: "Robert Nikander" <<a href="mailto:nikander@nc.rr.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">nikander@nc.rr.com</a>><br>To: "PLT Scheme List" <<a href="mailto:plt-scheme@list.cs.brown.edu" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> plt-scheme@list.cs.brown.edu</a>><br>Sent: Sunday, June 10, 2007 6:28 PM<br>Subject: [plt-scheme] (typeof obj)<br><br><br>> Is there a way to get a type-name symbol or type object for any  scheme<br>> object?<br><br>Of course there is a way, but you may not like it:<br><br>(define (types-of x) (filter identity (map (lambda (predicate typename) (and<br>(predicate x) type-name)) predicates type-names)))<br>(define predicates (list integer? procedure?, etc) <br>(define type-names '(integer procedure, etc))<br><br>Of course the procedure returns a list of types, for a 1 is an integer, a<br>rational, a real and a complex number.<br>You also can think of adapting procedure make-struct-type, such as to add their <br>predicates and names to the lists.<br>Same for define-struct, let-struct ...<br>Same for classes.<br><br>If you want only one type returned in stead of a list, two conditions must be<br>met:<br>First: for every two types A and B either A and B disjunct or A a subset of B or <br>B a subset of A.<br>Second: carefully compose the lists of predicates and their names.<br>Jos Koot<br><br>><br>> (type 1) => 'integer<br>><br>> or<br>><br>> (type 1) => #<type integer> <br>> (type (type x)) => #<type type><br>><br>><br>> Rob<br>> _________________________________________________<br>>  For list-related administrative tasks:<br>>  <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a><br>><br><br>_________________________________________________<br>  For list-related administrative tasks:<br>  <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a><br></blockquote></div><br></blockquote></span></div></div> </blockquote></div><br>