That woulda been cool when I was going though that book.  When you finish you might want to try to get a link from Pierce's web site, and/or inclusion in his download directory for the book, as that's probably most likely place I would have seen it. Corey <div>On 1/5/06, David Van Horn <<a href="mailto:dvanhorn@cs.brandeis.edu">dvanhorn@cs.brandeis.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;"> Danny Yoo wrote: > I've been reading Pierce's excellent "Types and Programming Languages" > textbook, and thought it might be fun to try to use PLT Scheme instead of > OCaml. It is fun---I've done this sort of thing over and over and over again. >     <a href="http://hkn.eecs.berkeley.edu/~dyoo/tmp/tapl-plt.txt">http://hkn.eecs.berkeley.edu/~dyoo/tmp/tapl-plt.txt</a> Just a couple comments on the code: > (**********************************) > let rec isval t = match t with >     TmTrue -> true >   | TmFalse -> true >   | t when isnumericalval t -> true >   | _ -> false > (**********************************) > > > Here's our equivalent in PLT Scheme: > > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > (define (isval t) >   (match t >     ((struct True ()) #t) >     ((struct False ()) #t) >     (t (=> fail) (if (isnumericalval t) #t (fail))) >     (else #f))) > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > > It's slightly uglier than in OCaml --- the third clause uses the > enhanced "=>" form to make sure pattern matching can continue when > isnumericalval fails --- but still, it isn't bad.  (And of course we > could easily simplify the function so it doesn't need it, but it's > good to know about "=>", since eval1 uses the "when" form quite a > bit.) You might use the `?' pattern in plt-match.ss to correspond to `when' guards in OCaml.  Eg: (define (isval t)    (match t      ((struct True ()) #t)      ((struct False ()) #t)      ((? isnumericalval) #t)      (else #f))) Another thing to note is that in OCaml, isval is of type t -> bool, but your Scheme predicates are Any -> bool, which isn't bad in itself, but if you're applying isval to non-terms, chances are it's a bug.  I use contracts to catch those things. As for concrete syntax, I almost never write parsers when I'm experimenting with languages implemented in Scheme.  I just cook up an S-expression concrete syntax and write functions parse : S-expr -> t and unparse : t -> S-expr.  (Of course, learning the parser tools is a fine thing.)  There are examples of this in the pattern matcher documentation if I recall correctly. David _________________________________________________   For list-related administrative tasks:    <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </blockquote></div> -- ((lambda (y) (y y)) (lambda (y) (y y)))