> On Feb 25, 2015, at 5:35 PM, Benjamin Tyner wrote:
>
> Actually, it depends on the number of cores:
Under current semantics, yes. Each 'stream' of function calls is lazily
capturing the last value of `i` on that core.
Under Luke's proposed semantics (IIUC),
the result would be the same (2,4,6,8) for both parallel and serial execution.
This is what allows for 'drop-in' parallelism.
>> fun1 <- function(c){function(i){c*i}}
>> fun2 <- function(f) f(2)
>> sapply(mclapply(1:4, fun1, mc.cores=1L), fun2)
>[1] 8 8 8 8
>> sapply(mclapply(1:4, fun1, mc.cores=2L), fun2)
>[1] 6 8 6 8
>> sapply(mclapply(1:4, fun1, mc.cores=4L), fun2)
>[1] 2 4 6 8
>
>>> / On Feb 24, 2015, at 10:50 AM, >> > wrote:
>> />/
>> />/ The documentation is not specific enough on the indented semantics in
>> />/ this situation to consider this a bug. The original R-level
>> />/ implementation of lapply was
>> />/
>> />/lapply <- function(X, FUN, ...) {
>> />/FUN <- match.fun(FUN)
>> />/if (!is.list(X))
>> />/X <- as.list(X)
>> />/rval <- vector("list", length(X))
>> />/for(i in seq(along = X))
>> />/rval[i] <- list(FUN(X[[i]], ...))
>> />/names(rval) <- names(X) # keep `names' !
>> />/return(rval)
>> />/}
>> />/
>> />/ and the current internal implementation is consistent with this. With
>> />/ a loop like this lazy evaluation and binding assignment interact in
>> />/ this way; the force() function was introduced to help with this.
>> />/
>> />/ That said, the expression FUN(X[[i]], ...) could be replaced by
>> />/
>> />/local({
>> />/i <- i
>> />/list(FUN(X[[i]], ...)
>> />/})
>> />/
>> />/ which would produce the more desirable result
>> />/
>> />/> sapply(test, function(myfn) myfn(2))
>> />/[1] 2 4 6 8
>> />/
>> /
>> Would the same semantics be applied to parallel::mclapply and friends?
>>
>> sapply(lapply(1:4, function(c){function(i){c*i}}), function(f) f(2))
>>
>> # [1] 8 8 8 8
>>
>> sapply(mclapply(1:4, function(c){function(i){c*i}}), function(f) f(2))
>>
>> # [1] 6 8 6 8
>>
>> I understand why they differ, but making mclapply easier for 'drop-in'
>> parallelism might be a good thing.
>>
>> Michael
>
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