[plt-dev] some Racket proposals & implementation

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).

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 <mflatt at cs.utah.edu> 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).
> >
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> >