| match.fun {base} | R Documentation |
Extract a Function Specified by Name
Description
When called inside functions that take a function as argument, extract the desired function object while avoiding undesired matching to objects of other types.
Usage
match.fun(FUN, descend = TRUE)
Arguments
FUN |
item to match as function: a function, symbol or character string. See ‘Details’. |
descend |
logical; control whether to search past non-function objects. |
Details
match.fun is not intended to be used at the top level since it
will perform matching in the parent of the caller.
If FUN is a function, it is returned. If it is a symbol (for
example, enclosed in backquotes) or a
character vector of length one, it will be looked up using get
in the environment of the parent of the caller. If it is of any other
mode, it is attempted first to get the argument to the caller as a
symbol (using substitute twice), and if that fails, an error is
declared.
If descend = TRUE, match.fun will look past non-function
objects with the given name; otherwise if FUN points to a
non-function object then an error is generated.
This is used in base functions such as apply,
lapply, outer, and sweep.
Value
A function matching FUN or an error is generated.
Bugs
The descend argument is a bit of misnomer and probably not
actually needed by anything. It may go away in the future.
It is impossible to fully foolproof this. If one attaches a
list or data frame containing a length-one character vector with the
same name as a function, it may be used (although namespaces
will help).
Author(s)
Peter Dalgaard and Robert Gentleman, based on an earlier version by Jonathan Rougier.
See Also
Examples
# Same as get("*"):
match.fun("*")
# Overwrite outer with a vector
outer <- 1:5
try(match.fun(outer, descend = FALSE)) #-> Error: not a function
match.fun(outer) # finds it anyway
is.function(match.fun("outer")) # as well