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

Use do.call:

do.call(substitute, list(lang, list(a=1)))

By using do.call, we force evaluation of the name `lang` to its actual underlying value, f(x=a). Then substitution is performed on f(x=a), rather than the name `lang`.