Access to and Manipulation of the Formal Arguments

Description

Get or set the formal arguments of a function.

Usage

formals(fun = sys.function(sys.parent()), envir = parent.frame())
formals(fun, envir = environment(fun)) <- value

Arguments

Details

For the first form, fun can also be a character string naming the function to be manipulated, which is searched for in envir, by default from the parent frame. If it is not specified, the function calling formals is used.

Only closures, i.e., non-primitive functions, have formals, not primitive functions.
Note that formals(args(f)) gives a formal argument list for all functions f, primitive or not.

Value

formals returns the formal argument list of the function specified, as a pairlist, or NULL for a non-function or primitive.

The replacement form sets the formals of a function to the list/pairlist on the right hand side, and (potentially) resets the environment of the function, dropping attributes.

See Also

formalArgs (from methods), a shortcut for names(formals(.)). args for a human-readable version, and as intermediary to get formals of a primitive function.
alist to construct a typical formals value, see the examples.

The three parts of a (non-primitive) function are its formals, body, and environment.

Examples

require(stats)
formals(lm)

## If you just want the names of the arguments, use formalArgs instead.
names(formals(lm))
methods:: formalArgs(lm)     # same

## formals returns a pairlist. Arguments with no default have type symbol (aka name).
str(formals(lm))

## formals returns NULL for primitive functions.  Use it in combination with
## args for this case.
is.primitive(`+`)
formals(`+`)
formals(args(`+`))

## You can overwrite the formal arguments of a function (though this is
## advanced, dangerous coding).
f <- function(x) a + b
formals(f) <- alist(a = , b = 3)
f    # function(a, b = 3) a + b
f(2) # result = 5