call: Function Calls

Description

Create or test for objects of mode "call" (or "(", see Details).

Usage

call(name, …)
is.call(x)
as.call(x)

Arguments

name

a non-empty character string naming the function to be called.

…

arguments to be part of the call.

an arbitrary R object.

Warning

call should not be used to attempt to evade restrictions on the use of .Internal and other non-API calls.

Details

call

returns an unevaluated function call, that is, an unevaluated expression which consists of the named function applied to the given arguments (name must be a quoted string which gives the name of a function to be called). Note that although the call is unevaluated, the arguments … are evaluated.

call is a primitive, so the first argument is taken as name and the remaining arguments as arguments for the constructed call: if the first argument is named the name must partially match name.

is.call

is used to determine whether x is a call (i.e., of mode "call" or "("). Note that

is.call(x) is strictly equivalent to typeof(x) == "language".
is.language() is also true for calls (but also for symbols and expressions where is.call() is false).

as.call(x):

Objects of mode "list" can be coerced to mode "call". The first element of the list becomes the function part of the call, so should be a function or the name of one (as a symbol; a quoted string will not do).

If you think of using as.call(<string>), consider using str2lang(*) which is an efficient version of parse(text=*). Note that call() and as.call(), when applicable, are much preferable to these parse() based approaches.

All three are primitive functions.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Examples

Run this code

# NOT RUN {
is.call(call) #-> FALSE: Functions are NOT calls

## set up a function call to round with argument 10.5
cl <- call("round", 10.5)
is.call(cl) # TRUE
cl
identical(quote(round(10.5)), # <- less functional, but the same
          cl) # TRUE
## such a call can also be evaluated.
eval(cl) # [1] 10

class(cl) # "call"
typeof(cl)# "language"
is.call(cl) && is.language(cl) # always TRUE for "call"s

A <- 10.5
call("round", A)        # round(10.5)
call("round", quote(A)) # round(A)
f <- "round"
call(f, quote(A))       # round(A)
## if we want to supply a function we need to use as.call or similar
f <- round
# }
# NOT RUN {
call(f, quote(A))  # error: first arg must be character
# }
# NOT RUN {
(g <- as.call(list(f, quote(A))))
eval(g)
## alternatively but less transparently
g <- list(f, quote(A))
mode(g) <- "call"
g
eval(g)
## see also the examples in the help for do.call
# }

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