How do I substitute symbols in a language object?

Question

Suppose I have the following language object:

lang <- quote( f(x=a) )

and I want to substitute in 1 for a. How

Answer 1

If you have previously defined a in some environment (.GlobalEnv) as:

a <- 1

You can generally run:

construct(deconstruct_and_eval(lang))
f(x = 1)

For the definitions of these custom functions, see Generalized function to substitute all variables in the quote()d expression, if they exist in an environment