Function to add a command line to an existing function

I need to write a function to add a particular command/string of characters to an existing function.

Consider this function:

constraint = function(cov_matrix,max_variance){ 

function(x){
f = NULL 
g = NULL 
g = rbind(g,  t(x)%*%cov_matrix%*%x-max_variance) 
return(list(ceq = f, c= g)) 
} 
}

Ideally, I would like to write a function add_text() that would take as input ("some_text_or_command", constraint) that would generate the output.

constraint = function(cov_matrix, max_variance){ 

function(x){
f = NULL 
g = NULL 
g = rbind(g,  t(x)%*%cov_matrix%*%x-  max_variance) 
"some_text_or_command"
return(list(ceq = f, c= g)) 
} 

}

How to go about it?

Here's how to do it, except for one crucial step:

add_text <- function(addition, original) {
  body <- body(original)

  # the missing step:  figure out the path to the line before where you're putting
  # the addition, i.e. the line 
  #   g = rbind(g,  t(x)%*%cov_matrix%*%x-  max_variance) ` 
  # in your example.  The path in your example is `c(2,3,4)`, so
  # I'll use that.

  prev_line <- c(2,3,4)

  len <- length(prev_line)   # How deeply nested is that line?

  # Now the edits:

  if (len > 1)   # Is that line nested?  It usually is, but not always
    parent <- body[[prev_line[-len]]]
  else
    parent <- body

  insert_at <- prev_line[len]
  
  parent <- as.list(parent)
  parent <- append(parent, list(substitute(addition)), after = insert_at)
  parent <- as.call(parent)
  if (len > 1)
    body[[prev_line[-len]]] <- parent
  else 
    body <- parent

  body(original) <- body
  original
}

For example,

add_text(1 + 1, constraint)

will give the function

function (cov_matrix, max_variance) 
{
    function(x) {
        f = NULL
        g = NULL
        g = rbind(g, t(x) %*% cov_matrix %*% x - max_variance)
        1 + 1
        return(list(ceq = f, c = g))
    }
}