Function Verification for “Function Variables”

Description

This function is called inside functions that take a function as argument. It's purpose is to extract the desired function object while avoiding undesired matching to objects of other types. match.fun is not intended to be used at the top level since it will perform matching in the parent of the caller.

It is assumed that FUN is an argument passed to the caller and is a function or a character string containing the name of a function.

Returns an error if no matching function is found.

Usage

match.fun(FUN, descend=TRUE)

Arguments

Details

If FUN is a function, it is returned. If it is a symbol 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 now 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 character object with the same name of a system function, it will be used.

Author(s)

Peter Dalgaard and Robert Gentleman, based on an earlier version by Jonathan Rougier

See Also

match.arg, get

Examples

# Same as get("*"):
match.fun("*")
# Overwrite outer with a vector
outer <- 1:5
## Not run: 
match.fun(outer, descend = FALSE) #-> Error:  not a function

## End(Not run)
match.fun(outer) # finds it anyway
is.function(match.fun("outer")) # as well