17.4 Evaluation runs code

• evaluate - run/execute an expression
• need both expression and environment
• eval() uses current environment if not set
• manual evaluation means you can tweak the environment!

xy <- expr(x + y)

eval(xy, env(x = 1, y = 10))
#> [1] 11
eval(xy, env(x = 2, y = 100))
#> [1] 102