R: passing expression to an inner function

Question

Further delving into the mysteries of R evaluation...This is closely related to my previous question ( How to write an R function that evaluates an expression within a data-

Answer 1

This is most easily avoided by passing strings into topfn instead of expressions.

topfn <- function(df, ex_txt) 
{
  fn(df, ex_txt) 
}

fn <- function(dfr, expr_txt) 
{        
   eval(parse(text = expr_txt), dfr) 
}

df <- data.frame(a = 1:5, b = 1:5 )
fn(df, "a")                              
fn(df, "2 * a + b")
topfn(df, "a")             
topfn(df, "2 * a + b")

EDIT:

You could let the user pass expressions in, but use strings underneath for your convenience.

Change topfn to

topfn <- function(df, ex) 
{
  ex_txt <- deparse(substitute(ex))
  fn(df, ex_txt) 
}
topfn(df, a)             
topfn(df, 2 * a + b)

ANOTHER EDIT:

This seems to work:

topfn <- function(df, ex) 
{
  eval(substitute(fn(df, ex)))
}

fn <- function(dfr, expr) 
{        
   eval(substitute(expr), dfr) 
}
fn(df, a)                              
fn(df, 2 * a + b)
topfn(df, a)             
topfn(df, 2 * a + b)