18.1 Introduction

To compute on the language, we first need to understand its structure.

• This requires a few things:
  • New vocabulary.
  • New tools to inspect and modify expressions.
  • Approach the use of the language with new ways of thinking.
• One of the first new ways of thinking is the distinction between an operation and its result.

y <- x * 10
#> Error in x * 10: non-numeric argument to binary operator

• We can capture the intent of the code without executing it using the rlang package.

z <- rlang::expr(y <- x * 10)

z
#> y <- x * 10

• We can then evaluate the expression using the base::eval function.

x <- 4

base::eval(expr(y <- x * 10))

y
#> [1] 40

18.1.1 Evaluating multiple expressions

• We can write multiple expressions at once and it acts similar to source().

• expression() returns a vector and can be passed to eval().

eval(expression(x <- 4, x * 10))
#> [1] 40

exprs() does not and has to be used in a loop

for (i in exprs(x <- 4, x * 10)) {
print(i)
print(eval(i))
}
#> x <- 4
#> [1] 4
#> x * 10
#> [1] 40