<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META content="text/html; charset=utf-8" http-equiv=Content-Type> <META name=GENERATOR content="MSHTML 8.00.7600.16588"></HEAD> <BODY> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>Is it a good idea to adapt PLT-Scheme implementation of srfi41 to Racket?</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>In principle, this is trivial.</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>However, when I adapted srfi41 for PLT-Scheme, Eli Barzilay advised me not to double the code for promises.</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>I did not follow his advice.</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>I now intend to follow his advice.</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>This has consequences that at first sight may seem undesirable. For example it would no longer be possible to distinguish a stream from a plain promise. It is not possible to provide a reliable predicate for streams. This also holds for the standard implementation. For example:</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>#lang racket<BR>(require srfi/41)</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>(define improper-stream ((stream-lambda () 1)))<BR>(stream? improper-stream) ; -> #t<BR>(stream-null? improper-stream) ; -> error<BR>(stream-pair? improper-stream) ; -> error</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>Hence the claim that a stream is either stream-null or a stream-pair is not fulfilled.</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>The document of srfi41 also claims that stream-null is unique. It is not:</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>(define another-stream-null (stream-filter number? (stream 'a 'b 's)))<BR>(equal? stream-null another-stream-null) ;-> #f, yet:<BR>(stream-null? another-stream-null) ; -> #t</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>In the docs for a new implementation I would like to make a distinction between abstract streams (with abstract predicates but without true (complete) predicates) and concrete streams. A concrete stream would be:</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>concrete-stream ::= null</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>concrete-stream ::= promise (any promise, and forcing may yield anything)</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>concrete-stream ::= pair whose cdr is a concrete stream (and the car probably, but not necessarily a promise)</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>When using stream functions on concrete streams that do not satisfy the requirements of abstract streams, errors will be reported (just like in th standard implementation) I</SPAN></FONT><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>t is possible to make a predicate for concrete-streams, but it seems rather useless to me.</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010></SPAN></FONT> </DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>In srfi41 attempts have been made to conceal the internally defined syntax stream-lazy. Yet stream-lazy can easily be exposed, as in:</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010>(define-syntax my-stream-lazy</SPAN></FONT></DIV> <DIV><FONT size=2 face="Courier New"><SPAN class=181565615-19072010> (syntax-rules ()</SPAN></FONT></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">  ((_ expr) ((stream-lambda () expr)))))</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New"></FONT></SPAN> </DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">While it is possible to maintain almost all provided syntaxes and procedures of PLT's srfi41, some changes would be noticeable:</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New"></FONT></SPAN> </DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">1:</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">(stream-lambda formals definition ... body ...1)</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">can also be written as</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">(lambda formals (lazy (let () definition ... body ...)))</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">To me this seems an improvement.</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New"></FONT></SPAN> </DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">2:</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">stream-null would just be a synonym for null. There is no reason to wrap it in a promise.</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New"></FONT></SPAN> </DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">3:</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">A stream-pair could be a promise that yieds a pair, but it could also be a pair whose cdr is a promise. There is no reason to wrap a pair of promises in another promise.</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New"></FONT></SPAN> </DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">Your comments/critics/advises are welcome.</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New">Thanks, Jos</FONT></SPAN></DIV> <DIV><SPAN class=181565615-19072010><FONT size=2 face="Courier New"></FONT></SPAN> </DIV> <P align=left><FONT size=2><FONT face="Courier New"><SPAN lang=en-gb>Hartelijke groet,</SPAN> <BR><SPAN lang=en-gb>Jos Koot</SPAN> <BR><SPAN lang=en-gb>Carrer d'Argentona 17 baixos</SPAN> <BR><SPAN lang=en-gb>ES 08302 MATARO</SPAN> <BR><SPAN lang=en-gb>España/Spanje /Spain</SPAN> </FONT></FONT></P> <P><FONT size=2><FONT face="Courier New"><SPAN lang=en-gb>+34937412360</SPAN> <BR><SPAN lang=en-gb>+34679944599</SPAN> </FONT></FONT></P> <P><FONT size=2><FONT face="Courier New"><SPAN lang=en-gb>((((</SPAN><SPAN lang=el>λ(x)(((((((x x)x)x)x)x)x)x))</SPAN> <BR><SPAN lang=el>   (λ(x)(λ(y)(x(x y)))))</SPAN> <BR><SPAN lang=el>  (λ(x)(cons'greeting x)))</SPAN> <BR><SPAN lang=el>'())</SPAN><SPAN lang=en-gb> ; That's a lot of greetings.</SPAN> </FONT></FONT></P> <DIV><FONT size=2 face=Arial></FONT> </DIV></BODY></HTML>