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`.

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

Answer 3

You can use substituteDirect rather than substitute:

substituteDirect(lang, list(a=1))

Eventually you may be able to use substitute. According to ?substituteDirect:

The goal is to replace this with an eval= argument to substitute.