[Rd] Question about substitute() and function def

Duncan Murdoch murdoch at stats.uwo.ca
Fri Sep 15 01:24:32 CEST 2006

On 9/14/2006 3:49 PM, Robert Gentleman wrote:
> Duncan Murdoch wrote:
>> On 9/14/2006 3:01 PM, Seth Falcon wrote:
>>> Hi,
>>> Can someone help me understand why
>>>   substitute(function(a) a + 1, list(a=quote(foo)))
>>> gives
>>>   function(a) foo + 1
>>> and not
>>>   function(foo) foo + 1
>>> The man page leads me to believe this is related to lazy evaluation of
>>> function arguments, but I'm not getting the big picture.
>> I think it's the same reason that this happens:
>>  > substitute(c( a = 1, b = a), list(a = quote(foo)))
>> c(a = 1, b = foo)
>> The "a" in function(a) is the name of the arg, it's not the arg itself 
> yes, but the logic seems to be broken. In Seth's case there seems to be 
> no way to use substitute to globally change an argument and all 
> instances throughout a function, which seems like a task that would be 
> useful.

I think variables and argument names are fundamentally different things, 
so I don't see this as very surprising.  It should take two steps to 
make a change like that.  Use substitute() to change the variables, put 
the result in f, change the names of the formals using

names(formals(f)) <- "foo".

I'm not sure if there's a simple way to change the first "a" in the 
expression c(a=1, b=a), but there are probably convoluted ways to do it.

> even here, I would have expected all instances of a to change, not some
>> (which is missing).  Now a harder question to answer is why this happens:
>>  > substitute(function(a=a) 1, list(a=quote(foo)))
>> function(a = a) 1
>    a bug for sure



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