[Rd] Error "promise already under evaluation ..." with function(x, dim=dim(x))

[Henrik Bengtsson]

I've meant to ask the following for several years now.  I understand why:

> foo <- function(x, dim=dim) { dim }
> foo(1)
Error in foo(1) :
  promise already under evaluation: recursive default argument
reference or earlier problems?

gives an error, but why wouldn't/couldn't the following work?

> foo <- function(x, dim=dim(x)) { dim }
> foo(1)
Error in foo(1) :
  promise already under evaluation: recursive default argument
reference or earlier problems?

As a workaround I also tried:

> foo <- function(x, dim) { if (missing(dim)) dim <- dim(x); dim }
> foo(1)
Error in foo(1) : argument "dim" is missing, with no default

which surprised me too.


For the first case, is the rationale related to:

> foo <- function(x, a=dim(x), dim) { a }
> foo(1)
Error in foo(1) : argument "dim" is missing, with no default

and

> foo <- function(x, a=dim(x), dim=a) { a }
> foo(1)
Error in foo(1) :
  promise already under evaluation: recursive default argument
reference or earlier problems?

[since here argument 'dim' could take a function, e.g. foo(1,
dim=length)], and that R treats

foo <- function(x, dim=dim(x)) { dim }

in a similar way?  That is, is R not "clever" enough to detect this as
a special case, but instead goes ahead and tries to evaluate the
default expression (=dim(x)) of argument 'dim' in order to get its
default value?  If so, is there anything preventing R from support
this "special case", e.g. by evaluating the default expression without
argument/symbol 'dim' itself being in the picture to avoid "it finds
itself"?  (Sorry if I'm using the incorrect words here).


Yes, I understand that I can do:

> foo <- function(x, dim=base::dim(x)) { dim }
> foo(1)
NULL

> foo <- function(x, dim=NULL) { if (is.null(dim)) dim <- dim(x); dim }
> foo(1)
NULL

or

> foo <- function(x, dim.=dim(x)) { dim. }
> foo(1)
NULL

but I would prefer not to have to turn those rather ad hoc solutions in my code.


Thanks,

Henrik