Access to and Manipulation of the Formal Arguments

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Description

Get or set the formal arguments of a function.

Usage

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

Arguments

fun

a function, or see ‘Details’.

envir

environment in which the function should be defined.

value

a list (or pairlist) 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 from the parent frame. If it is not specified, the function calling formals is used.

Only closures have formals, not primitive functions.

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.

See Also

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

Examples

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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

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