doMC - compiler - concatenate an expression vector into a single expression?

I think that the result of the concatenation should be a call object,
rather than an expression object.  How about something along the
lines of:

'%dopar2%'<- function(obj, ex) {
   ex<- as.call(list(as.name('{'),
                           quote({ a<- i; message("Custom ", a) }),
                           substitute(ex)))
   do.call('%dopar%', list(obj, ex), envir=parent.frame())
}

- Steve


On Tue, Jun 28, 2011 at 8:26 AM, Renaud Gaujoux
<renaud at mancala.cbio.uct.ac.za>  wrote:

Hi,

this post is about foreach operators, the compiler package and the last
update of doMC that includes support for the compiler functionality.

I am using a home-made %dopar%-like operator that adds some custom
expression to be executed before the foreach loop expression itself (see
sample code below).
It used to work perfectly with doMC 1.2.1, but with the introduction of
the compiler functionality, things do not work properly.
The change in the doMC package consists in evaluating a compiled
expression instead of the original R expression:

# from doMC:::doMC ...
c.expr<- comp(expr, env = envir, options = list(suppressUndefined = TRUE))

and for R>= 2.13.0 comp is defined as compiler::compile:
function (e, env = .GlobalEnv, options = NULL)
{
    cenv<- makeCenv(env)
    cntxt<- make.toplevelContext(cenv, options)
    cntxt$env<- addCenvVars(cenv, findLocals(e, cntxt))
    genCode(e, cntxt)
}
<environment: namespace:compiler>

My guess is that the function findLocals or genCode can not handle a
2-length expression vector.

Maybe somebody who knows the internals of these functions could explain
better this behaviour?
How can I concatenate two expressions into a single one?

Thank you,
Renaud


##################################################
# Sample code
##################################################

`%dopar2%`<- function(obj, ex){

    # append custom code to the expression
    ex<- c(expression({ a<- i; message("Custom ", a);}), substitute(ex))

    # call the standard %dopar% operator
    do.call(`%dopar%`, list(obj, ex), envir=parent.frame() )
}
res<- foreach(i=1:3) %dopar2% { print(i); i*2; }
res


#########################
# Output with doSEQ or doMC 1.2.1
#########################
Custom 1
[1] 1
Custom 2
[1] 2
Custom 3
[1] 3

  res

[[1]]
[1] 2

[[2]]
[1] 4

[[3]]
[1] 6


#########################
# Output with doMC 1.2.2
#########################

[[1]]
expression({
    a<- i
    message("Custom ", a)
}, {
    print(i)
    i * 2
})

[[2]]
expression({
    a<- i
    message("Custom ", a)
}, {
    print(i)
    i * 2
})

[[3]]
expression({
    a<- i
    message("Custom ", a)
}, {
    print(i)
    i * 2
})





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