Definition of = vs. <-

On 4/1/2009 10:38 AM, Stavros Macrakis wrote:

NOTA BENE: This email is about `=`, the assignment operator (e.g. {a=1}
which is equivalent to { `=`(a,1) } ), not `=` the named-argument syntax
(e.g. f(a=1), which is equivalent to
eval(structure(quote(f(1)),names=c('','a'))).
As far as I can tell from the documentation, assignment with = is
precisely
equivalent to assignment with <-.  Yet they call different primitives:

The parser does treat them differently:

 > if (x <- 2) cat("assigned\n")

assigned

 > if (x = 2) cat("assigned\n")

Error: unexpected '=' in "if (x ="

The ?"=" man page explains this:

" 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. "

though the restriction on '=' seems to be described incorrectly:

 > if ((x = 2)) cat("assigned\n")

assigned

in which the assignment is in parentheses, not a braced list.

As to the difference between the operations of the two primitives:
see do_set in src/main/eval.c.  The facility is there to distinguish
between them, but it is not used.

x = y <- 1
rm(x,y)
x <- y = 1

Duncan Murdoch

`=`

.Primitive("=")

`<-`

.Primitive("<-")
(Perhaps these are different names for the same internal function?)
Also, the difference is preserved by the parser:

quote({a=b})

{
    a = b
}

quote({a<-b})

{
    a <- b
}
even though in other cases the parser canonicalizes variant syntax,
e.g. ->
to <-:

quote({a->b})

{
    b <- a
}

`->`

Error: object "->" not found
Is there in fact some semantic difference between = and <- ?
If not, why do they use a different operator internally, each
calling a
different primitive?
Or is this all just accidental inconsistency resulting from the '=',
<-',
and '->' features being added at different times by different people working
off different stylistic conventions?
            -s
	[[alternative HTML version deleted]]