[Rd] Capturing the expression representing the body of a function

[Gabor Grothendieck]

On Mon, May 2, 2011 at 4:11 PM, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:
> On 02/05/2011 3:21 PM, Hadley Wickham wrote:
>>
>> Hi all,
>>
>> What's the preferred way of capturing the expression representing the
>> contents of a function?
>>
>> * body(write.csv) gives me a braced expression
>> * body(write.csv)[-1] gives me a mangled call
>> * as.list(body(write.csv)[-1]) gives me a list of calls
>> * as.expression(as.list(body(write.csv)[-1])) is what I want but seems
>> like too much work
>>
>> Any suggestions?
>
> The body of a function isn't an expression, it's a language object.  A
> language object is represented internally as a pairlist, while an expression
> is represented as a generic vector, i.e. the thing that list() gives.
>
> Your 1st try gives you the language object.
>
> The other ones only work when the body consists of a call to `{`, as the
> body of most complex functions does, but not for simple ones like
>
> f <- function(x) 2*x
>
> So I would say your question should be:  "What's the best way to construct
> an expression vector s.t. evaluating its elements in order is like
> evaluating the body of a function?"
>
> And the answer to that is something like
>
> body2expr <- function(f) {
>  body <- body(f)
>  if (typeof(body) == "language" && identical(body[[1]], quote(`{`)))
> as.expression(as.list(body[-1]))
>  else as.expression(body)
> }
>


Also try as.expression(as.list(f)[[3]])

e.g.

> f <- function(x, y) { x + y }
> as.expression(as.list(f)[[3]])
expression({
    x + y
})
> g <- function(x, y) x + y
> as.expression(as.list(g)[[3]])
expression(x + y)




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