R: Function Verification for “Function Variables”

match.fun {base} | R Documentation

## Function Verification for “Function Variables”

### 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 `attach`es a list or data frame containing a length-one character vector with the same name as a function, it may be used (although name spaces will help).

### 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

[Package base version 2.8.1 Index]