19.3 Quoting

Capture expressions without evaluating them

	Developer	User
Expression (Quasiquotation)
One	expr()	enexpr()
Many	exprs()	enexprs()
Symbol (Quasiquotation)
One		ensym()
Many		ensyms()
R Base (Quotation)
One	quote()	alist()
Many	substitute()	as.list(substitute(...()))

• Non-base functions are from rlang
• Developer - From you, direct, fixed, interactive
• User - From the user, indirect, varying, programmatic

Also:

• bquote() provides a limited form of quasiquotation
• ~, the formula, is a quoting function (see Section 20.3.4)

expr() and exprs()

expr(x + y)
#> x + y
exprs(exp1 = x + y, exp2 = x * y)
#> $exp1
#> x + y
#> 
#> $exp2
#> x * y

enexpr()² and enexprs()

f <- function(x) enexpr(x)
f(a + b + c)
#> a + b + c

f2 <- function(x, y) enexprs(exp1 = x, exp2 = y)
f2(x = a + b, y = c + d)
#> $exp1
#> a + b
#> 
#> $exp2
#> c + d

ensym() and ensyms()

• Remember: Symbol represents the name of an object. Can only be length 1.
• These are stricter than enexpr/s()

f <- function(x) ensym(x)
f(a)
#> a

f2 <- function(x, y) ensyms(sym1 = x, sym2 = y)
f2(x = a, y = "b")
#> $sym1
#> a
#> 
#> $sym2
#> b

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2. enexpr() = enrich expr()↩︎