This vignette briefly illustrates some examples of tracing parameters evaluated in non-standard ways. This first examples demonstrates that parameter values are captured at the initial point of function entry.
eval_x_late_NSE <- function (x, y) {
y <- 10 * y
eval (substitute (x))
}
inject_tracer (eval_x_late_NSE)
eval_x_late_NSE (y + 1, 2:3)
#> [1] 21 31
res <- load_traces ()
res$par_name
#> [1] "x" "y"
res$uneval
#> $x
#> [1] "y + 1"
#>
#> $y
#> [1] "2:3"
res$eval
#> $x
#> [1] 3 4
#>
#> $y
#> [1] 2 3
The parameter x
is evaluated at the point of function
entry as y + 1
which, with a value of y = 2:3
,
gives the expected evaluated result of x = 3:4
, while the
function ultimately returns the expected values of
(10 * 2:3) + 1
, or 21 31
, because the first
line of y <- 10 * y
is evaluated prior to substituting
the value passed for x
of y + 1
.
The second example specifies a default value of
x = y + 1
, with the actual call passing no value, and thus
having "NULL"
in the unevaluated version, while evaluated
versions remain identical.
clear_traces () # clear all preceding traces
eval_x_late_standard <- function (x = y + 1, y, z = y ~ x) {
y <- 10 * y
x
}
inject_tracer (eval_x_late_standard)
eval_x_late_standard (, 2:3)
#> [1] 3 4
res <- load_traces ()
res$par_name
#> [1] "x" "y" "z"
res$uneval
#> $x
#> [1] "NULL"
#>
#> $y
#> [1] "2:3"
#>
#> $z
#> [1] "NULL"
res$eval
#> $x
#> [1] 3 4
#>
#> $y
#> [1] 2 3
#>
#> $z
#> y ~ x
#> <environment: 0x55e34b2d9298>
The traces produced by typetracer
also include a column,
formal
, which contains the default values specified in the
definition of eval_x_late_standard()
:
res$formal
#> $x
#> y + 1
#>
#> $y
#>
#>
#> $z
#> y ~ x
Those three columns of formal
, uneval
, and
eval
thus contain all definitions for all parameters passed
to the function environment, in the three possible states of: