formals: Access to and Manipulation of the Formal Arguments
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Details Value See Also Examples

Get or set the formal arguments of a function.

1 2	formals(fun = sys.function(sys.parent())) formals(fun, envir = environment(fun)) <- value

`fun`	a `function`, or see ‘Details’.
`envir`	`environment` in which the function should be defined.
`value`	a `list` (or `pairlist`) of R expressions.

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

Only closures have formals, not primitive functions.

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.

formalArgs (from methods), a shortcut for names(formals(.)). args for a human-readable version, alist, body, function.

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