[racket-dev] expand, local-expand, and syntax-procedure-converted-arguments-property

Carl Eastlund


On Wed, Jul 10, 2013 at 11:28 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> At Wed, 10 Jul 2013 23:15:46 -0400, Ryan Culpepper wrote:
> > On 07/10/2013 09:04 PM, Asumu Takikawa wrote:
> > > Hi all,
> > >
> > > I'm currently trying to fix the Typed Racket unit tests. I think I've
> > > narrowed down the issue to a certain syntax property for keyword
> > > functions.
> > >
> > > The issue is illustrated by the following example:
> > >
> > >    #lang racket
> > >
> > >    (require racket/file
> > >             (for-syntax racket/file
> > >                         racket/keyword-transform))
> > >
> > >    ;; the property is #f
> > >    (begin-for-syntax
> > >     (displayln
> > >       (syntax-case (expand-syntax #'(copy-directory/files 1 2)) ()
> > >         [(let-values (((temp1) _)
> > >                       ((temp2) _))
> > >            (if _
> > >                (#%plain-app1 copy-directory/files15 e1 ...)
> > >                (#%plain-app2 copy-directory/files17 e2 ...)))
> > >          (syntax-procedure-converted-arguments-property
> > #'copy-directory/files15)])))
> > >
> > >    ;; property is syntax
> > >    (begin-for-syntax
> > >     (displayln
> > >       (syntax-case (local-expand #'(copy-directory/files 1 2)
> 'expression
> > null) ()
> > >         [(let-values (((temp1) _)
> > >                       ((temp2) _))
> > >            (if _
> > >                (#%plain-app1 copy-directory/files15 e1 ...)
> > >                (#%plain-app2 copy-directory/files17 e2 ...)))
> > >          (syntax-procedure-converted-arguments-property
> > #'copy-directory/files15)])))
> > >
> > > There are two syntax-time computations here. Both are expanding an
> > > application of a keyword function (one with local-expand, one with
> > > expand) and looking at the resulting syntax.
> > >
> > > The key point here is that I want to find the property looked up by
> > > `syntax-procedure-converted-arguments-property` on an output identifier
> > > because Typed Racket needs it to type-check the expansion.
> > >
> > > Unfortunately, as the comments indicate, only the second piece of code
> > > can find the property. The reason appears to be that the property key
> is
> > > actually a private `gensym`ed symbol and the two pieces of code appear
> > > to get separate instantiations of the kw.rkt module (perhaps at
> different
> > > phases).
> > >
> > > To check that, if I modify kw.rkt to use a plain symbol, both of the
> > > snippets above return the same property value.
> > >
> > > Anyone have any idea how I can keep using `expand` but still be able to
> > > look up the property?
> >
> > To get information about a phase-0 '#%app' expansion, you need to call
> > the phase-1 version of 'syntax-procedure-converted-arguments-property'.
> > That's going to require a bit of phase-crossing trickery, because the
> > identifier you want to query is a phase-0 (dynamic) value, and you want
> > the result as a phase-0 value, but the phase-1 function naturally
> > consumes and produces phase-1 values.
> >
> > One solution is to use 'quote-syntax', 'eval', and 'phase1-eval' all
> > together. Use 'eval' with 'quote-syntax' to convert the phase-0
> > identifier to a phase-1 identifier. Use 'phase1-eval' to run the
> > computation at phase 1 and capture the phase-1 result as a phase-0 value
> > (also using 'quote-syntax').
>
> I haven't thought about this much, but would it make more sense to move
> the property key to a cross-phase persistent module (with a `protected'
> provide) in this case?
>

That came up on IRC.  I believe Asumu said the key is an uninterned
identifier, meaning it's a stateful value.  Right now, cross-phase
persistent modules can't store anything stateful other than generative
struct types.

--Carl
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