Definition of = vs. <-

On Wed, Apr 1, 2009 at 10:55 AM, Duncan Murdoch <murdoch at stats.uwo.ca>wrote:

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

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 ="

Interesting way of handling the classic C glitch (some of us would say
"design flaw in C", but...)

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

The restriction is incorrect in many other cases as well, e.g. the following
are all assignments: function()a=3; if(...)a=3; while(...)a=3; a=b=3 (two
assignments), and even a*b=3 (parses as assignment, but `*<-` happens not to
be defined in the default environment).

In fact, the only cases I have found where = does *not* mean assignment is
in functional or array argument position (f(a=2) and f[a=2]), the following
contexts with function-like syntax: function(XXX)..., if(XXX)..., and
while(XXX)...; and for (i in XXX)....  Are there any others? Perhaps the
documentation could be updated?

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.

So are you saying that it is planned to make = and <- non-synonymous, unlike
a<-b and b->a, which parse the same and are therefore guaranteed to be
synonymous?