Access to and Manipulation of the Formal Arguments (original) (raw)
| formals {base} | R Documentation |
|---|
Description
Get or set the formal arguments of a [function](../../base/help/function.html).
Usage
formals(fun = sys.function(sys.parent()), envir = parent.frame())
formals(fun, envir = environment(fun)) <- value
Arguments
| fun | a function, or see ‘Details’. |
|---|---|
| envir | environment in which the function should be defined (or found via get() in the first case and whenfun a character string). |
| value | a list (or pairlist, hence possiblyNULL) of R expressions. |
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](../../base/help/pairlist.html), 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](../../base/help/attributes.html).
See Also
[formalArgs](../../methods/html/methodUtilities.html) (from methods), a shortcut for names(formals(.)).[args](../../base/help/args.html) for a human-readable version, and as intermediary to get formals of a primitive function.[alist](../../base/help/alist.html) to construct a typical formals value, see the examples.
The three parts of a (non-primitive) [function](../../base/help/function.html) are itsformals, [body](../../base/help/body.html), and [environment](../../base/help/environment.html).
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
[Package _base_ version 4.6.0 Index]