Substitution Modes

Language substitution (subsMethod='langsubs')

library("wrapr")

let(
  c(X = 'y', F = 'sin'), 
  {
    d <- data.frame("X" = "X", X2 = "XX", d = X*X, .X = X_)
    X <- list(X = d$X, X2 = d$"X", v1 = `X`, v2 = ` X`, F(1:2))
    X$a
    "X"$a
    X = function(X, ...) { X + 1 }
  },
  eval = FALSE, subsMethod = 'langsubs')

## {
##     d <- data.frame(y = "X", X2 = "XX", d = y * y, .X = X_)
##     y <- list(y = d$y, X2 = d$y, v1 = y, v2 = ` X`, sin(1:2))
##     y$a
##     "X"$a
##     y = function(y, ...) {
##         y + 1
##     }
## }

Notice the substitution replaced all symbol-like uses of “X”, and only these (including correctly working with some that were quoted!).

String substitution (subsMethod='stringsubs')

let(
  c(X = 'y', F = 'sin'), 
  {
    d <- data.frame("X" = "X", X2 = "XX", d = X*X, .X = X_)
    X <- list(X = d$X, X2 = d$"X", v1 = `X`, v2 = ` X`, F(1:2))
    X$a
    "X"$a
    X = function(X, ...) { X + 1 }
  },
  eval = FALSE, subsMethod = 'stringsubs')

## expression({
##     d <- data.frame(y = "y", X2 = "XX", d = y * y, .y = X_)
##     y <- list(y = d$y, X2 = d$y, v1 = y, v2 = ` y`, sin(1:2))
##     y$a
##     "y"$a
##     y = function(y, ...) {
##         y + 1
##     }
## })

Notice string substitution has a few flaws: it went after variable names that appeared to start with a word-boundary (the cases where the variable name started with a dot or a space). Substitution also occurred in some string constants (which as we have seen could be considered a good thing).

These situations are all avoidable as both the code inside the let-block and the substitution targets are chosen by the programmer, so they can be chosen to be simple and mutually consistent. We suggest “ALL_CAPS” style substitution targets as they jump out as being macro targets. But, of course, it is better to have stricter control on substitution.

Think of the language substitution implementation as a lower-bound on a perfect implementation (cautious, with a few corner cases to get coverage) and string substitution as an upper bound on a perfect implementation (aggressive, with a few over-reaches).

Substitute substitution (subsMethod='subsubs')

let(c(X = 'y', F = 'sin'), 
    {
      d <- data.frame("X" = "X", X2 = "XX", d = X*X, .X = X_)
      X <- list(X = d$X, X2 = d$"X", v1 = `X`, v2 = ` X`, F(1:2))
      X$a
      "X"$a
      X = function(X, ...) { X + 1 }
    },
    eval = FALSE, subsMethod = 'subsubs')

## {
##     d <- data.frame(X = "X", X2 = "XX", d = y * y, .X = X_)
##     y <- list(X = d$y, X2 = d$X, v1 = y, v2 = ` X`, sin(1:2))
##     y$a
##     "X"$a
##     y = function(X, ...) {
##         y + 1
##     }
## }

Notice base::substitute() doesn’t re-write left-hand-sides of argument bindings. This is why I originally didn’t consider using this implementation. Re-writing left-hand-sides of assignments is critical in expressions such as dplyr::mutate( RESULTCOL = INPUTCOL + 1). Also base::substitute() doesn’t special case the d$"X" situation (but that really isn’t very important).