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help with sockets

2 messages · Erich Neuwirth, Brian Ripley

Original
Erich Neuwirth
# 
i would like to use sockets to accept commands over the net.
so a socket should be a replacement for stdin.
whenever i have a complete line, i want to hand it over to the parser.

i cannot do it simply with eval because eval expects a complete
expression.

additionally, eval and evalq
seem not to accept strings.

is there an easy way to do this?

let me rephrase

my program should run a loop reading a socket,
and whenever an and of line arrives,
the string should be handled like if it had been typed on the command
line.

can this be done easily?



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Brian Ripley
#
On Fri, 29 Dec 2000, Erich Neuwirth wrote:

            

      
      
      
    

        
Not yet, I think. This is the aim of connections (new in 1.2.0), but
socket connections are not yet fully implemented (and nothing is in the
R sources yet).  I bet you want to do this on Windows too!

First off, you need to parse, not evaluate the string: eval is called on a
parse tree.  So what you will be able to do with a socket connection
is to call eval(parse(file=conname, n = 1)) and that will read one
expression from the connection conname and then evaluate it.  So you
could run that in a loop.

I think you can fake this now.  When each line arrives, add it to a 
character vector `buffer'.  Then try parse(n = 1, text=buffer),
and if that succeeds evaluate the result and reinitialize `buffer'
to be empty.

A few details: giving parse() an incomplete expression will get a
parse error, so the call to parse has to be done inside try().
I don't know how to distinguish between an incomplete expression and a
parse error, so this strategy will only work if you always send correct
expressions.  (I met exactly the same problem using DDE connections to
S-PLUS for Windows.)  Perhaps we can make parse give different error
messages?


An alternative is to use OS facilities to do this. That is, to write a 
small program `getfromsocket' that accepts input from the socket and
writes line by line to stdout. Then just use

getfromsocket args | R (or Rterm on Windows).