Why are = and <- not equivalent in within()?

Question

> within( list(a="a",b="b"), c="c" )
Error in eval(expr, envir, enclos) : argument is missing, with no default
> within( list(a="a",b="b"), c<-"c" )
$a
[1] "a"

$b
[1] "b"

$c
[1] "c"

I'm not sure exactly why these two shouldn't be equivalent. It seems like the the = version getting interpreted as an argument named c to within because of the .... Is there any way to disable this behavior? I tried,

within( list(a="a",b="b"), `c`="c" )

but that fails too.

Answer 1

You are correct that c="c" (or any clause of that form) is getting interpreted as a supplied argument. And no, there's no way to disable that -- it's presumably handled down at the level of the R parser.

This difference between = and <- is documented ?"<-"

The operators ‘<-’ and ‘=’ assign into the environment in which they are evaluated. The operator ‘<-’ can be used anywhere, whereas the operator ‘=’ is only allowed at the top level (e.g., in the complete expression typed at the command prompt) or as one of the subexpressions in a braced list of expressions.

The prime example of a "braced list of expressions" is a function body, which you can verify by typing, e.g. is(body(plot.default)), length(body(plot.default)).