Function modification on the fly using trace, untrace and substitute

## Function modification on the fly using trace, untrace and substitute

# You want to create a function which is a test version of another function

#lets start by creating a function
# The original function
sum_func <- function(x)
{
  y <- 2
  return(x+y)
}

# This gives you 5
sum_func(3)

# Use this to view the body of the function. You need this to find out where you need to place the modification
as.list(body(sum_func))

#Save the function into a new function
new_sum_func <- sum_func

#Change the new function body

body(new_sum_func)[[2]] <- substitute(y <- 3)

class(new_sum_func)

as.list(body(new_sum_func)) #You can view the modified function

#The modified function 
new_sum_func(3) #This output will be 6


#Suppose instead you want to modify the original function just for testing
# but then you want to revert back to the original 

# Use the trace function to insert a piece of code (not overwrite)
trace(sum_func, quote(y <- 4), at = 3) 

#The modified test function
as.list(body(sum_func))

sum_func(3) #This gives you 7

#Reset the original code to unchanged version by removing the test changes
untrace(sum_func)

# You get back the original function
sum_func(3) 'this gives you back 5