R Code: Extract a fixed component of a list within a list and optionally apply a function

lf <- function(obj, comp, fun=identity, wrap=identity,
               return.vector=TRUE, verbose=FALSE,
               ...) {
    ## WARNING: "comp" have to be a real string instead of a variable
    ## Functional Apply
    ## "comp": a character string, indicated a list component.
    ## "obj" is a list. "comp" is a component of each element of the
    ## list "obj".  Apply a funcion (just an unquoted name) to each "comp" of the list
    ## "obj".  "..." is passed to "fun".  use "fun=identity" if no
    ## processing is needed.  "wrap" is applied to the final result.
    ## "return.vector" will force the result to be a vector.

    comp <- substitute(comp)
    e <- paste(deparse(substitute(wrap)), "(lapply(obj,",
               deparse(substitute(function(x) fun(x[[comp]], ...))), "))")
    ans <- eval(parse(text=e))
    if (verbose) cat("expression:", e, "\n")
    if (return.vector)
        ans <- sapply(ans, function(x) if (is.null(x)) NA else x)

    ans
}
if (F) {
    a <- list(x=c(1:3), y=c(4:6))
    lf(a, 2)                            # c(x=2, y=5)

    a <- list(x=list(median=3, mean=4), y=list(median=5, mean=6))
    lf(a, "mean")                       # c(x=4, y=6)
    lf(a, "mean", sqrt, verbose=T)               # c(x=2, y=2.4495)
    lf(a, "mean", sqrt, wrap=length, verbose=T)  # 2
}