blockr.core (version 0.1.3)

bbquote: Quoting utilities

Description

Block expressions in blockr are evaluated in a 2-step manner: first in the context of arguments supplied by the user via UI elements and in a second step in the context of input data. The function base::bquote() does not allow for terms wrapped in .() or ..() to be missing and this makes it incompatible with this 2-step approach. A drop-in replacement, provided as bbquote() addresses this shortcoming.

Usage

bbquote(expr, where = parent.frame(), splice = FALSE)
.(x)
..(x)

Value

A language object in the same way as returned by base::bquote(). Functions .() and ..() throw errors when invoked an only exist to mask check notes "no visible global function definition" for their use.

Arguments

expr

A language object.

where

An environment.

splice

Logical; if TRUE splicing is enabled.

x

Object

Details

A block like new_head_block() is expected to return an expression of the form utils::head(data, n = 10L), which will then be evaluated in an environment where he have a name data bound to some dataset. In order to perform some manipulations of such block expressions it is required to somehow mark the terms that correspond to input data and for that we can use the syntax introduced by base::bquote(). What we would prefer to have as block expression therefore is not the above, but something like utils::head(.(data), n = 10L), as this affords us more possibilities for performing substitutions (and therefore generates cleaner code).

In order to interpolate certain arguments in a first step, we unfortunately cannot use base::bquote(), but we can use bbquote() instead to generate the desired expression.

bquote(utils::head(.(data), n = .(n)), list(n = 10L))
#> Error in eval(e[[2L]], where) : object 'data' not found
bbquote(utils::head(.(data), n = .(n)), list(n = 10L))
#> utils::head(.(data), n = 10L)

This also works with ..() and splicing.

Examples

bbquote(utils::head(.(data), n = .(n)), list(n = 10L))
bbquote(c(.(a), ..(bc)), list(a = "a"))
bbquote(c(.(a), ..(bc)), list(a = "a", bc = c("b", "c")), splice = TRUE)