1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
|
## ---- echo = FALSE-------------------------------------------------------
knitr::opts_chunk$set(collapse = TRUE, comment = "#>")
rownames(mtcars) <- NULL
## ------------------------------------------------------------------------
library(lazyeval)
f <- function(x = a - b) {
lazy(x)
}
f()
f(a + b)
## ------------------------------------------------------------------------
a <- 10
b <- 1
lazy_eval(f())
lazy_eval(f(a + b))
## ------------------------------------------------------------------------
lazy_eval(f(), list(a = 1))
## ------------------------------------------------------------------------
lazy_eval(~ a + b)
h <- function(i) {
~ 10 + i
}
lazy_eval(h(1))
## ------------------------------------------------------------------------
subset2_ <- function(df, condition) {
r <- lazy_eval(condition, df)
r <- r & !is.na(r)
df[r, , drop = FALSE]
}
subset2_(mtcars, lazy(mpg > 31))
## ------------------------------------------------------------------------
subset2_(mtcars, ~mpg > 31)
subset2_(mtcars, quote(mpg > 31))
subset2_(mtcars, "mpg > 31")
## ------------------------------------------------------------------------
subset2 <- function(df, condition) {
subset2_(df, lazy(condition))
}
subset2(mtcars, mpg > 31)
## ------------------------------------------------------------------------
above_threshold <- function(df, var, threshold) {
cond <- interp(~ var > x, var = lazy(var), x = threshold)
subset2_(df, cond)
}
above_threshold(mtcars, mpg, 31)
## ------------------------------------------------------------------------
x <- 31
f1 <- function(...) {
x <- 30
subset(mtcars, ...)
}
# Uses 30 instead of 31
f1(mpg > x)
f2 <- function(...) {
x <- 30
subset2(mtcars, ...)
}
# Correctly uses 31
f2(mpg > x)
## ---- eval = FALSE-------------------------------------------------------
# x <- 31
# g1 <- function(comp) {
# x <- 30
# subset(mtcars, comp)
# }
# g1(mpg > x)
# #> Error: object 'mpg' not found
## ------------------------------------------------------------------------
g2 <- function(comp) {
x <- 30
subset2(mtcars, comp)
}
g2(mpg > x)
## ------------------------------------------------------------------------
library(lazyeval)
f1 <- function(x) lazy(x)
g1 <- function(y) f1(y)
g1(a + b)
## ------------------------------------------------------------------------
f2 <- function(x) lazy(x, .follow_symbols = FALSE)
g2 <- function(y) f2(y)
g2(a + b)
## ------------------------------------------------------------------------
a <- 10
b <- 1
lazy_eval(g1(a + b))
lazy_eval(g2(a + b))
|
