[racket] turning input-port into file-stream port

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-- code being discussed http://pasterack.org/pastes/8580
-- source for process*/ports https://github.com/plt/racket/blob/master/racket/collects/racket/system.rkt
-- answer by M. Flatt https://groups.google.com/forum/#!topic/racket-users/6GvjIbndNt8

hope this helps. 

---
Vlad Kozin <vladilen.kozin at gmail.com>



On Nov 13, 2013, at 11:19 AM, Vlad Kozin wrote:

> 
>> You have a synchronization problem.  The copy-port within foo starts
>> reading the input file stream and hits EOF before the copy-port within
>> the separate thread starts writing to the file.
> 
> 
> David Pierson, you're absolutely right. This is exactly the problem with my hacky solution, which your (sleep 1) addition sort of solves.
> 
>> Vlad, are you looking for pipes? 
> 
> Matthias, I'm trying re-create unix shell pipe behavior, but where you can splice Racket code wrapped in (begin ..) as part of the pipeline: every element in the pipe reads from (current-input-port). (subprocess ..) that I was using to run external binaries strictly expects file-stream-ports. In Racket however my 'pipe' form can be parametrized over any kind of input/output port: string, result of
> (make-pipe), etc. Hence the need to somehow wrap the input ports.
> 
> More insight came courtesy of #racket irc particularly Jens Axel Søgaard. As was pointed out to me process*/ports could be just what I was looking for and I should be using it in place of (subprocess ..). Cursory glance at the source code for process*/ports reveals that it calls the subprocess and does exactly the port manipulation magic I was looking for. I don't pretend to understand the code since I'm yet to learn concurrent programming in Racket, but this answer by Matthew Flatt seems to support my understanding.
> 
>> #lang racket
>> (define (make-file-stream-input-port in-port)
>>  (if (file-stream-port? in-port)
>>      in-port
>>      (match (process*/ports #f in-port 'stdout "/bin/cat")
>>        ((list stdout-in-port #f pid #f proc)
>>         stdout-in-port))))
> 
> 
> David Pierson, calling external "cat" to fake what essentially is copying one port onto another won't cut it (pun intended) :) I hope the guts of process*/ports will provide the solution. But the behavior of your code is exactly what I need. 
> 
> ---
> Vlad Kozin <vladilen.kozin at gmail.com>
> 
> 
> 
> On Nov 13, 2013, at 8:58 AM, Matthias Felleisen wrote:
> 
>> 
>> On Nov 12, 2013, at 11:55 PM, David T. Pierson <dtp at mindstory.com> wrote:
>> 
>>> Do you really need a file stream port and not just an input port?  
>> 
>> Vlad, are you looking for pipes? 
>> 
> 

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