What does class "call" mean? How do I make class "formula" into a "call"?

On Jun 25, 2011, at 15:24 , David Winsemius wrote:

On Jun 24, 2011, at 6:12 PM, StellathePug wrote:

Hello R Users!
I have a list called "tabs" that I would like to have the same  
structure as
my list "eqSystem." The two look like they have the same structure  
but they
are different because when I look at their attributes,  
class(eqSystem[[1]])
is "call" but class(tabs[[1]]) is "formula". I want to have  
class(tabs[[1]])
as a call too.

So what does "call" mean?

An as yet unevaluated function invocation with first as the named  
function followed by quoted arguments is a "call":

See the help(call) page:

f <- round
A <- 10.5
(g <- as.call(list(f, quote(A))))

.Primitive("round")(A)

eval(g)

[1] 10

call("mean", quote( c(1,2,3)))

mean(c(1, 2, 3))

eval( call("mean", quote( c(1,2,3))))

[1] 2

It seems very unlikely that a formula object could be coerced into  
a valid call simply by altering its class. To convince us otherwise  
you need to provide more information than you have supplied to the  
present. The results of str() on these objects might be a first step.

Actually, no. Any unevaluated expression in R is mode "call", unless  
atomic or symbol. It will also be class "call", unless expressedly  
overridden by an S3 class assignment. Notice that operators are  
really function calls.
I.e.

mode(quote(x+y))

[1] "call"

class(quote(x+y))

[1] "call"

But

class(quote(x))

[1] "name"

class(quote(3.14159))

[1] "numeric"

(This is why the R docs keep talking about "unevaluated expressions"  
instead of "call objects": They aren't always that.)

The "~" operator is also a function call. However, evaluating "~"  
returns an object which is the actual call assigned class  
"formula" (plus an environment attribute).

f <- y ~ x
class(f)

[1] "formula"

unclass(f)

y ~ x
attr(,".Environment")
<environment: R_GlobalEnv>

mode(f)

[1] "call"

class(unclass(f))

[1] "call"

I.e., an unevaluated formulae expression (as in quote(y~x)) is class  
"call", as is an unclassed formula object. So it is pretty easy to  
have objects of class "formula" very similar to objects of class  
"call".