fold_demo.R

# solution via Hadley Wickham and others at: 
# http://www.mail-archive.com/r-help@stat.math.ethz.ch/msg85203.html

qq <- list(data.frame(c1=c('a', 'b', 'c')), data.frame(c1=c('a','d','e')), data.frame(c1=c('a','f','g')))

fold <- function(x, fun) {
        if (length(x) == 1) return(fun(x))
        
        accumulator <- fun(x[[1]], x[[2]])
        if (length(x) == 2) return(accumulator)

        for(i in 3:length(x)) {
                accumulator <- fun(accumulator, x[[i]])
        }
        accumulator
}

fold(lapply(qq, function(df) df$c1), intersect)