r - How to pass an expression "from higher level" to mutate? -

i create higher level function wrap mutate. want give expression parameter function , being able use expression in mutate :

datas <- data.frame(x = sample(100)) fn <- function(datas, expr) {    mutate(datas, flag = eval(substitute(expr), datas)) }  fn(datas[1], x > 50) error in mutate_impl(.data, dots) : object 'x' not found  

but don't understand why fails since mutate(datas, flag = eval(substitute(x > 50), datas)) works.

what doing wrong ?

thanks

try this:

fn <- function(df, expr) eval(substitute(mutate(df, expr), list(expr=substitute(expr)))) 

or (preferably) this:

fn <- function(df, expr) mutate_(df, .dots= list(flag=substitute(expr)))