[Rd] substitute and expression

[Peter Dalgaard]

Deepayan Sarkar wrote:
> Hi,
>
> I'm trying to understand whether the use of substitute() is
> appropriate/documented for plotmath annotation. The following two
> calls give the same results:
>
>   
>> plot(1:10, main = expression(alpha == 1))
>> do.call(plot, list(1:10, main = expression(alpha == 1)))
>>     
>
> But not these two:
>
>   
>> plot(1:10, main = substitute(alpha == a, list(a = 2)))
>> do.call(plot, list(1:10, main = substitute(alpha == a, list(a = 2))))
>>     
> Error in as.graphicsAnnot(main) : object "alpha" not found
>
> (as a consequence, xyplot(..., main = substitute(alpha)) doesn't
> currently work.)
>
> On the other hand, this works:
>
>   
>> foo <- function(x) plot(1, main = x)
>> foo(substitute(alpha))
>>     
>
> I'm not sure how to interpret ?plotmath; it says
>
>      If the 'text' argument to one of the text-drawing functions
>      ('text', 'mtext', 'axis', 'legend') in R is an expression, the
>      argument is interpreted as a mathematical expression...
>
> and uses substitute() in its examples, but
>
>   
>> is.expression(substitute(alpha == a, list(a = 1)))
>>     
> [1] FALSE
>   
I think you need to take plotmath out of the equation and study the 
difference between objects of mode "call" and those of mode 
"expression". Consider this:

 > f <- function(...)match.call()
 > do.call(f, list(1:10, main = substitute(alpha == a, list(a = 2))))
function(...)match.call()
(1:10, main = alpha == 2)
 > do.call(list, list(1:10, main = substitute(alpha == a, list(a = 2))))
Error in do.call(list, list(1:10, main = substitute(alpha == a, list(a = 
2)))) :
        object "alpha" not found

The issue is that function ends up with an argument  alpha == 2 which it 
proceeds to evaluate (lazily), where a direct call sees 
substitute(.....). It is a general problem with the do.call mechanism 
that it effectively pre-evaluates the argument list, which can confuse 
functions that rely on accessing the original argument expression. Try, 
e.g., do.call(plot, list(airquality$Wind, airquality$Ozone)) and watch 
the axis labels.

Does it work if you use something like

 main = substitute(quote(alpha == a), list(a = 2))?