Stavros Macrakis wrote:
On Thu, Dec 18, 2008 at 1:37 PM, Wacek Kusnierczyk <
[email protected]> wrote:
Kenn Konstabel wrote:
...foo({x = 2})
...
This is legal but doesn't do what you probably expect -- although
documentation for `<-` says the value (returned by <-) is 'value' i.e.
whatever is on the right side ...
What do you expect this to do that is different from what it does, namely
assign 2 to x and call foo on the value 2, which is the same as the value of
x? As long as foo doesn't do tricks with substitute, all the following
should be identical:
foo(x<-2)
foo((x<-2))
foo((x=2))
foo({x=2})
foo({x<-2})
foo(a=(x=2))
{x=2; foo(invisible(x))}
{x<-2; foo(invisible(x))}
{x=2; foo(invisible(2))}
{x<-2; foo(invisible(2))}
foo(a=x<-2)
.....
Of course, if foo *does* use substitute, all bets are off, because it would
capture the argument as an unevaluated expression and could do whatever it
wanted with it.
Also watch out for lazy evaluation. It is required that a is evaluated
inside foo to have equivalence with the pre-assigning forms (and even
so, things may happen in different order).
Consider
> foo <- function(a){cat("hello, "); a}
> foo(cat("world\n"))
hello, world
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