### Replacement functions :

v <- 1:5
names(v) <- LETTERS[1:5]
v <- 1:5
names(v) <- c("Hans", "Heiri")
v

`names<-`
get("names<-") # here equivalent to the use  `...`

"[<-"  <- function(){}
find("[<-") # in two places ..
rm("[<-")
`[<-.factor` ## the "factor" method of the `[<-` generic function
`[.factor`   ## This is the "factor" method of the `[` generic function

### An R function is made of three parts
### 1) formals()    : The argument list
### 2)    body()    : The function body
### 3) environment(): The environment in which the function has been defined

str ( formals(abline) )
?body
?formals ##  there is a replacement function
def.u  <- formals(abline)$untf
def.u
deff <- formals(formals)[["fun"]]
deff
class(deff)

## missing() allows to check if an argument has been specified
## --------- in the actual call:
tt  <- function(x, ab=TRUE) {
        cat("missing(x):", missing(x),
            "miss.(ab):", missing(ab),"\n")
        formals()
}
tt(1)
tt(1, TRUE)
tt() ## the formals arguments of tt()

sin     # formal argument  'x'
sin(pi) # actual argument 'pi' --- is matched to the formal:
## Something like 'x <- pi' happens implicitly.

## sin is  *Primitive* function -- these are made to be very fast in R
## by ``diving into C-level'' of R immediately.
## ==> They are the exeptions: all three parts are empty
formals(sin) ## NULL, ditto for the body
body   (sin)
environment(sin)