[racket] meta-languages and going back to the "normal" reader

On Jul 10, 2014, at 6:40 AM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> The readtable strategy works when <language> itself uses a
> readtable-based reader. The idea is that you install a mapping for `#λ`
> while leaving all the other mappings in place. If <language> uses a
> readtable-based reader, then it picks up your extension, otherwise it
> doesn't.
> 
> I think a `#lang afl at-exp racket` combination should work fine: `afl`
> installs a handler for `#λ`, `at-exp` installs a handler for `@`, and
> `racket` uses `read-syntax` to see both extensions.

Well for some reason it doesn’t:
#lang afl at-exp racket/base
(map #λ(+ % 1) '(1 2 3)) ; read: bad syntax `#λ’

But also for some reason this does:
#lang at-exp afl racket/base
(map #λ(+ % 1) '(1 2 3)) ; '(2 3 4)
(map #λ@+[% 1] ‘(1 2 3)) ; ‘(2 3 4)
By the way I only just got this to work yesterday by doing basically this but for afl:
https://github.com/AlexKnauth/rackjure/commit/5fa266e672d529dde227ef216aaef157fa5c618c

Also is there any way to get something like this to work?:
#lang afl at-exp racket/base
@#λ(+ % 1)[1] ; read: bad syntax `#λ'

> Adding `#fn` support is a little trickier if you want to fall back to
> `#f` or `#false` when the character after `#f` (as determined by a
> peek) is not `n`. For that case, the readtable addition for `#f` should
> remember the old readtable, and then when it needs to fall back, it
> calls `read/recursive` with the saved readtable as the third argument.
> That way, immediate parsing of `#f...` uses the saved readtable without
> `afl` extensions, while parsing of sub-expressions will return to the
> current readtable that includes the `afl` extensions.

Do you mean like this?:
(define lambda-readtable (current-readtable))
(parameterize ([current-readtable orig-readtable])
  (read-syntax/recursive src in #f lambda-readtable))

> Documentation for the functions from a "<language>/lang/reader.rkt" is
> in section 1.3.18 of the Reference, which defines `#lang` (as being
> "like `#reader`, which is described in the same section).

Ok I just found this in section 1.3.18:
The arity of the resulting procedure determines whether it accepts extra source-location information: a read procedure accepts either one argument (an input port) or five, and aread-syntax procedure accepts either two arguments (a name value and an input port) or six. In either case, the four optional arguments are the reader’s module path (as a syntax object in read-syntax mode) followed by the line (positive exact integer or #f), column (non-negative exact integer or #f), and position (positive exact integer or #f) of the start of the #reader form. 

But maybe there should be a link or something to section 1.3.18 from sections 17.2 and 17.3.1 of the Guide.  
That would make it a lot easier to find it.  

> 
> At Sat, 5 Jul 2014 13:33:27 -0400, "Alexander D. Knauth" wrote:
>> 
>> If I have a meta-language like this:
>> #lang my-meta-lang <language>
>> And my-meta-lang is similar to at-exp in that it can accept any arbitrary 
>> language with any arbitrary reader
>> (as long as it looks at the readtable), then how do I escape back to the reader 
>> specified by <language>
>> from inside a reader macro from my-meta-lang?
>> 
>> What I’m trying to do is something like #lang afl <language> where afl adds 
>> rackjure-like anonymous function literals
>> to <language>.  
>> 
>> So to parse this:
>> #lang afl racket
>> #λ(+ % 1)
>> It would use the racket reader but wrap it to use the afl-readtable, which 
>> includes dispatch-macros that would
>> read the (+ % 1) and parse the result into a lambda expression.  
>> 
>> But if <language> was something else, with a different reader, then how could I 
>> use that to read the (+ %1 1).
>> 
>> For example if it was something like this:
>> #lang afl at-exp racket
>> #λ@+[% 1]
>> 
>> There’s also another problem.  If it was this:
>> #lang afl <language>
>> #f
>> Or this:
>> #lang afl <language>
>> #false
>> Or some other thing starting with f that means something to <language>,
>> Then it would see the #f and hope that it would turn out to be #fn.  If it 
>> doesn’t, then it uses the racket reader
>> (instead of the one provided by <language>) to read the #f or the #false.  
>> 
>> So back to my original question: How do I escape back to the reader specified 
>> by <language>
>> from inside a reader macro?  
>> 
>> By the way I can’t find anything in the docs about what the arguments to the 
>> read and read-syntax functions
>> provided by <language>/lang/reader.rkt are supposed to be or mean.  
>> 
>> 
>> ____________________
>>  Racket Users list:
>>  http://lists.racket-lang.org/users

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