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 can I do this?

I would expect substitute to do what I want, but

substitute(lang, list(a=1))

just returns lang, while

substitute(f(x=a), list(a=1))

does in fact do what I expect.

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`.

Answer 2

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