[int_19h]

In R, you can also choose the context in which the argument expression is to be evaluated. If you just use the promise as if it were a value and rely on implicit evaluation, then it happens in the context of the caller, yes. But environments (i.e. sets of name-value bindings) in R are first-class objects, so they can be captured at any given point, and later used to explicitly evaluate promises after retrieving the latter's associated expression.

   foo <- function(x, env) {
     print(x);  # implicit eval
     
     x_expr <- substitute(x);  # gets the associated expression
     print(eval(x_expr, env));  # explicit eval in different environment
   }

   bar <- function(y) {
     environment()  # capture and return local environment of function
   }

   y <- 1
   env <- bar(2);
   foo(y * y, env);  # prints 1 then 4.

Side note: substitute() seems like a weird name for a function that returns the underlying expression of the promise. It's named that way because it's actually similar in intended use to quasiquotation - it lets you explicitly substitute variable names for something else in the expression before evaluating it. So e.g. substitute(x <- x + 1, x=2) returns the expression object for (1 <- 1 + 2). Not passing any named arguments is just a special case where no substitutions are made and the original expression is returned instead, although in practice that is probably the most common way to use it.