R function for getting the default value of a function argument

default.md

default <- function(sym) {
    Sym <- substitute(sym)
    if (!(is.symbol(Sym)))
        stop('\nExpected a symbol as an argument, got a ', class(sym))
    ArgList <- eval.parent(quote(formals()))
    ArgName <- as.character(Sym)
    if (!(ArgName %in% names(ArgList)))
        stop('\nThere is no argument named `', ArgName,'`')
    if (is.symbol(ArgList[[ArgName]]))
        stop('\nThere is no default value for argument `', ArgName,'`')
    eval.parent(ArgList[[ArgName]])
}

Usage examples:

aa <- function(alpha = c('aa','bb')) {
    stopifnot(alpha %in% default(alpha))
    alpha
}

aa('aa')

## [1] "aa"

aa('ww')

## Error: alpha %in% default(alpha) is not TRUE

bb <- function(x=1,y=10:12)
    list(X=default(x),
         Y=default(y))

bb()

## $X
## [1] 1
## 
## $Y
## [1] 10 11 12

bb(7,8)

## $X
## [1] 1
## 
## $Y
## [1] 10 11 12