Marijn, The define-sequence-syntax form is for writing sequences such as in-list and in-vector. Adding a syntax binding in addition to the value binding allows constructs like for/list to inline the code produced by the macro, rather than calling out to an external procedure.
The for/fold/derived form is just a copy of for/fold; the only difference is that it takes the original syntax of a derived macro as an extra argument and reports all syntax errors in terms of that. To write for/dlist, I suggest first just writing a macro like you would write any other macro, and use the normal for/fold form. You can later improve your macro's error messages by adapting it to for/fold/derived. The documentation could probably use some examples of those. Carl Eastlund On Wed, Jun 1, 2011 at 8:40 AM, Marijn <[email protected]> wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > Hi, > > I'm working on a doubly-linked list (dlist) implementation and have been > able to succesfully implement the prop:sequence #:property, allowing the > use of my data structure as a sequence, allowing for example to convert > to an ordinary list with `for/list'. I'm trying to now do the converse, > that is implement my own `for/dlist' for constructing dlists from > sequences, but I am having some trouble with the docs. > > Specifically, in the reference there is "2.18.2 Deriving New Iteration > Forms"[1], which mentions `for/fold/derived' and > `define-sequence-syntax', but doesn't really explain what they are for > or how they should be used. The last time I thought I understood how to > proceed I tried to do: > > (define (for/dlist > (for/fold/derived for/dlist > > after fruitlessly trying to complete: > > (define-sequence-syntax for/dlist > ... > (for/fold/derived for/dlist > > but clearly I have no clue how this is supposed to work and I am > currently not even sure that these constructs are there for supporting > what I want to do. Some help would be highly appreciated. > > Some of the things that 2.18.2 does say, are worded rather poorly in my > opinion, so for my part I will present some rewordings that I hope are > more direct and clearer: > > Old paragraphs: > Defines id as syntax. An (id . rest) form is treated specially when used > to generate a sequence in a clause of for (or one of its variants). In > that case, the procedure result of clause-transform-expr is called to > transform the clause. > When id is used in any other expression position, the result of > expr-transform-expr is used. If it is a procedure of zero arguments, > then the result must be an identifier other-id, and any use of id is > converted to a use of other-id. Otherwise,expr-transform-expr must > produce a procedure (of one argument) that is used as a macro transformer. > > Proposed replacement paragraphs: > Defines id as syntax that is treated specially when used in/as > [whichever is correct] a clause of an iteration (for, for/list, ...). In > this special case, clause-transform-expr should evaluate to a syntax > transformer (a procedure that takes a single syntax object argument and > returns another) which is then called in the place of id except when it > would return #f. > When id is not used in a clause of an iteration, expr-transform-expr > should either evaluate to a syntax transformer which is called in the > place of id, > or a thunk which returns an identifier other-id, which is used to > convert each use of id to other-id. > > My understanding of the rest of that subsection is yet too low to > attempt a rewrite. > > Marijn > > [1]:http://docs.racket-lang.org/reference/for.html?q=for/hash#(part._.Deriving_.New_.Iteration_.Forms) > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v2.0.17 (GNU/Linux) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ > > iEYEARECAAYFAk3mM08ACgkQp/VmCx0OL2xeEgCgxxhcuxs5YwvKEGnekESb7y9k > 5t8An1743I1PHw1lxfpPc36kV4p+xl3k > =xsSP > -----END PGP SIGNATURE----- > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
