View the source of an R package

Just enter the name of a function/method without parentheses:

R> base::rev.default function (x) if (length(x)) x[length(x):1L] else x<environment: namespace:base>

See also R-Help Desk - Accessing the Sources in R News Volume 6/4, October 2006.

How you find the source code depends on the type of function. See my answer to this related question.

As rcs pointed out, if you want to specify a package, you can use ::.

> lattice::xyplotfunction (x, data, ...) UseMethod("xyplot")<environment: namespace:lattice>

Not all functions from a package will be exported (i.e. made publically available); for these you need to use :::.

> lattice::xyplot.formulaError: 'xyplot.formula' is not an exported object from 'namespace:lattice'> lattice:::xyplot.formulafunction (x, data = NULL, allow.multiple = is.null(groups) ||     outer, outer = !is.null(groups), auto.key = FALSE, aspect = "fill",     panel = lattice.getOption("panel.xyplot"), prepanel = NULL,     scales = list(), strip = TRUE, groups = NULL, xlab, xlim,     ylab, ylim, drop.unused.levels = lattice.getOption("drop.unused.levels"),     ..., lattice.options = NULL, default.scales = list(), subscripts = !is.null(groups),     subset = TRUE) {    formula <- x    dots <- list(...)# etc.

To find out which methods you want to see, write methods(funcOfInterest)

Sometimes it does not suffice to print(funcOfInterest.class). Try print(getAnywhere(funcOfInterest.class)) then.