I just thought the auto-v change would be easier and would also make it available to alternative structure definition macros. But, with just the constructor naming it will be cleaner to override the default make- (or whatever you want to call it) and roll your own. <div class="gmail_quote">On Fri, Apr 2, 2010 at 5:35 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.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;"> Support for keyword and optional arguments would be good. I don't think I'd extend the auto-field description, but instead have something like (define-struct a (w [x (add1 w)] #:y y #:z [z (add1 y)])) be analogous to defining the constructor as a function (lambda (w [x (add1 w)] #:y y #:z [z (add1 y)]) ...) but with the same selector functions as (define-struct a (w x y z)) As another example, (define-struct a (w [x (add1 w)] #:y [y #:mutable] #:z [z (add1 y) #:mutable])) would have a constructor with the same signature as above, but with mutable `y' and `z' fields. Some challenges I see: * reporting enough static information for `match' to support the right syntax for patterns with `a'; * subtyping of a type whose constructor uses keyword arguments; * reflective access to constructors (via `struct-type-make-constructor', for example); and * printing values when the constructor has keyword arguments. I think all of these can be addressed, but it's a lot of work (so if it depends on me, I'm inclined to put it off a while longer). <div><div></div><div class="h5"> At Fri, 2 Apr 2010 16:11:10 -0600, Doug Williams wrote: > Would it be possible to combine the structure constructor name with a means > of providing default structure values on a per slot basis? I had always > thought it would be nice if the auto-v argument to make-struct-type took an > old-style lambda list (i.e., an improper list or, in the degenerate case, an > atom) to allow specification of slot-specific automatic (default) values - > with the cdr of an improper list (or the atom in the degenerate case) being > used for the remaining elements. Then, define-struct could support a > #:default field option for each slot. Just a thought. > > On Fri, Apr 2, 2010 at 3:30 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu">mflatt@cs.utah.edu</a>> wrote: > > > Version 4.2.5.5 in the SVN trunk includes experimental features to > > support the following proposed Racket features. You can try the > > proposals with `#lang racket' in MzScheme. > > > > Structure Constructor Names > > --------------------------- > > > > Proposal: The default constructor name bound by `define-struct' in > > Racket should be the same as the type name, instead of having a `make-' > > prefix. > > > > Example: > > > > > (define-struct a (x y)) > > > a > > #<procedure:a> > > > (a 1 2) > > #<a> > > > > To help support this potential feature, the `define-struct' form of > > `scheme/base' now accepts a `#:constructor-name' argument to give the > > constructor a name other than the one prefixed with `make-'. In > > particular, the constructor name can be the same as the type name: > > > > > (define-struct a (x y)) > > > make-a > > #<procedure:make-a> > > > (define-struct a (x y) #:constructor-name a) > > > a > > #<procedure:a> > > > (a 1 2) > > #<a> > > > > A natural (and generally backward-compatible) to change to `match' > > would be to treat structure-type names as pattern constructors, so that > > > > (match (a 1 2) > > [(a x y) x]) > > > > would produce 1. > > > > > > Semi-quasiquote Printing > > ------------------------ > > > > Proposal: Use quasiquote printing as Racket's default printing mode, > > but only for transparent values. > > > > Functional programmers long ago figured out that it's better to print a > > value in the same way as an expression that produces the value. > > Printing with `quasiquote', meanwhile, mostly preserves the Lisp > > tradition of printing values that represent expressions as the > > expressions that they represent. > > > > Some values, however, cannot be printed easily as expressions that > > produce the same value. For example, In DrScheme with quasiquote > > printing, > > > > (list 1 (let ([f (lambda (x) x)]) f)) > > > > prints as > > > > `(1 ,(lambda (a1) ...)) > > > > The printer cannot actually print a function, so it has to invent a > > `lambda' expression that approximates the value. The problem is worse > > with objects, classes, and other opaque types. Expressions with graphs > > print as a `shared' expression. > > > > Other implementations of functional languages punt on opaque values. > > Here's an example in OCaml, which prints functions as just `<fun>': > > > > # Some 10;; > > - : int option = Some 10 > > # sqrt;; > > - : float -> float = <fun> > > # [sqrt;sqrt];; > > - : (float -> float) list = [<fun>; <fun>] > > > > This seems like the right compromise for Racket. For example, > > > > (list 1 (let ([f (lambda (x) x)]) f)) > > > > could print as > > > > `(1 #<procedure:f>) > > > > (Note that there's no need for an unquote when printing a value as a > > non-expression. Non-S-expression forms are "self-unquoting".) > > > > Transparent (or prtially transparent) structures can print with > > constructors, while opaque structures can print as non-S-expressions: > > > > > (define-struct a (x y)) > > > (list 1 (a 2 3)) > > `(1 #<a>) > > > (define-struct a (x y) #:transparent) > > > (list 1 (a 2 3)) > > `(1 ,(a 2 3)) > > > > Instances of prefab structure types, meanwhile, should stick to > > quasiquoting: > > > > > (define-struct b (x y) #:prefab) > > > (list 1 (b (a 2 3) 'x)) > > `(1 #s(b ,(a 2 3) x)) > > > > Graphs can still use the compact #n= notation: > > > > > (read (open-input-string "#0=(1 . #0#)")) > > `#0=(1 . #0#) > > > > Unlike DrScheme's quasiquote printing, semi-quasiquote printing is > > easily implemented by parameterizing our existing printer(s). > > > > A new `print-as-quasiquote' parameter directs `print' and > > `pretty-print' to use semi-quasiquote style. (The parameter does not > > affect `write'.) > > > > Welcome to MzScheme v4.2.5.5 [3m], Copyright (c) 2004-2010 PLT Scheme Inc. > > > 'x > > x > > > (print-as-quasiquote #t) > > > 'x > > 'x > > > (list 1 2 3) > > `(1 2 3) > > > sqrt > > #<procedure:sqrt> > > > (list 1 sqrt) > > `(1 #<procedure:sqrt>) > > > > The `port-print-handler' and `prop:write' protocols have been changed > > (in a mostly backward-compatible way) to make semi-quasiquote printing > > extensible. > > > > > > Language-Specific Run-Time Configuration > > ---------------------------------------- > > > > Proposal: The main language of a program should determine a run-time > > configuration, including the style for printing values. > > > > Assuming the changes above, we'd want > > > > #lang scheme > > (define-struct a (x y) #:transparent) > > (list (make-a 1 2)) > > > > to produce > > > > (#(struct:a 1 2)) > > > > while > > > > #lang racket > > (define-struct a (x y) #:transparent) > > (list (a 1 2)) > > > > should produce > > > > `(,(a 1 2)) > > > > Along the same lines, we'd want > > > > #lang scheme > > (define-struct a (x y) #:transparent) > > (+ 'x (list (make-a 1 2))) > > > > to produce the error message > > > > +: expects type <number> as 1st argument, given: x; other arguments > > were: (#(struct:a 1 2)) > > > > while > > > > #lang racket > > (define-struct a (x y) #:transparent) > > (+ 'x (list (a 1 2))) > > > > should produce the error message > > > > +: expects type <number> as 1st argument, given: 'x; other arguments > > were: `(,(a 1 2)) > > > > The different `define-struct's are easily support through different > > bindings imported by `scheme' and `racket'. Similarly, for printing > > top-level results in a module, you might imagine that `scheme' and > > `racket' use different printing functions. The different error formats, > > however, are not so easily controlled through bindings. > > > > Setting `print-as-quasiquote' to #t is enough to get the Racket-style > > error format, but having `#lang racket' inject `(print-as-quasiquote > > #t)' in the module top-level would not work well when modules from > > different languages are mixed together. For example, if a program > > imports both > > > > ;; s.ss: > > #lang scheme > > (define (s-bad v) (error 's-bad "~e" v)) > > (provide s-bad) > > > > and > > > > ;; r.rkt > > #lang racket > > (define (r-bad v) (error 'r-bad "~e" v)) > > (provide r-bad) > > > > the way an error message is printed by `s-bad' and `r-bad' shouldn't > > depend on the order that the modules are instantiated. > > > > To accommodate run-time configuration of the environment, such as > > setting the way that values are printed, `mzscheme' now treats the main > > module of a program specially. It extracts information about the > > module's language --- specifically, whether the language declares a > > run-time configuration action. If so, `mzscheme' runs the > > language-configuration action before it instantiates the module. > > > > As a result, when you put either version of the code above in "ex.ss", > > then `mzscheme ex.ss' produces the right error message. > > > > Here's how it works in more detail for the case of `#lang racket': > > > > * The `racket' module reader has implemented in `racket/lang/reader' > > associates a 'module-language property with `module' form that it > > produces from "ex.ss". The 'module-language property essentially > > points back to `racket/lang/reader'. > > > > * The macro expander and bytecode compiler preserves the > > 'module-language information so that it's available through > > `module-compiled-language-info' (from the unevaluated bytecode) > > and/or `module->language-info' (from the evaluated module > > declaration). > > > > * When the `mzscheme' executable is given a module to run, it uses > > `module->language-info' to get the module's language information > > before `require'ing the module. The `module->language-info' loads > > "ex.ss" (from source or bytecode) and extracts language info from > > the declared module. > > > > The language info on the declaration of the module from "ex.ss" > > points back to the `get-info' export of `racket/lang/reader'. The > > `mzscheme' executable calls that function with the > > 'configure-runtime key. > > > > * The `get-info' function of `racket/lang/reader' recognizes the > > 'configure-runtime key and reports back the `configure' function > > provided by `racket/private/runtime'. > > > > [Why doesn't `get-info' just call `configure' directly? See below > > on creating executables.] > > > > * The `mzscheme' executable calls the `configure' function of > > `racket/private/runtime' calls it. The `configure' function simply > > sets the `print-as-quasiquote' parameter to #t. > > > > * Having finished running the language's configuration action, the > > `mzscheme' executable `require's the "ex.ss" module to instantiate > > it. (Although `module->language-info' has already loaded the module, > > `module->language-info' doesn't instantiate the module.) > > > > Instantiating the module runs the expressions in its body, > > triggering the `+' error. The error message uses the right style for > > printing values because the `print-as-quasiquote' parameter was set > > to #t by `configure'. > > > > If you run `mzc --exe ex ex.ss', the generated executable prints the > > right error message, too. That's because `mzc' extracts the main > > module's language information in the same way as `mzscheme'. Based on > > the result for 'confgure-runtime for the module's language, `mzc' > > embeds the `racket/private/runtime' module in the generated executable > > (and that's why `get-info' doesn't call `configure' itself). The > > generated executable includes a start-up action that calls `configure' > > before running the main module. > > > > > > DrScheme should similarly extract language information and call > > `configure' before running the module. It may be that a single > > side-affecting `configure' function isn't the right interface for > > DrScheme, and so experiments with DrScheme may lead to a different > > protocol for `mzscheme' and `mzc'. > > > > > > When `mzscheme' is run in interactive module, the initialization > > module's language is used to initialize the run-time configuration. The > > `racket', `racket/base' and `racket/init' modules are implemented in > > Racket, so > > > > mzscheme -I racket/init > > > > gives you a REPL like `racket' could give you (when it exists). > > > > _________________________________________________ > > For list-related administrative tasks: > > <a href="http://list.cs.brown.edu/mailman/listinfo/plt-dev" target="_blank">http://list.cs.brown.edu/mailman/listinfo/plt-dev</a> > > </div></div></blockquote></div>