partial_eval

0th

Percentile

Partially evaluate an expression.

This function partially evaluates an expression, using information from the tbl to determine whether names refer to local expressions or remote variables. This simplifies SQL translation because expressions don't need to carry around their environment - all revelant information is incorporated into the expression.

Keywords
internal
Usage
partial_eval(call, tbl = NULL, env = parent.frame())
Arguments
call
an unevaluated expression, as produced by quote
tbl
a tbl object
env
environment in which to search for local values
Symbol substitution

partial_eval needs to guess if you're referring to a variable on the server (remote), or in the current environment (local). It's not possible to do this 100

  • If the tbl variables are known, and the symbol matches a tbl variable, then remote.
  • If the symbol is defined locally, local.
  • Otherwise, remote.

Aliases
  • partial_eval
Examples
if (require("Lahman")) {
bdf <- tbl_df(Batting)
partial_eval(quote(year > 1980), bdf)

ids <- c("ansonca01", "forceda01", "mathebo01")
partial_eval(quote(id %in% ids), bdf)

# You can use local to disambiguate between local and remote
# variables: otherwise remote is always preferred
year <- 1980
partial_eval(quote(year > year), bdf)
partial_eval(quote(year > local(year)), bdf)

# Functions are always assumed to be remote. Use local to force evaluation
# in R.
f <- function(x) x + 1
partial_eval(quote(year > f(1980)), bdf)
partial_eval(quote(year > local(f(1980))), bdf)

# For testing you can also use it with the tbl omitted
partial_eval(quote(1 + 2 * 3))
x <- 1
partial_eval(quote(x ^ y))
}
Documentation reproduced from package dplyr, version 0.3, License: MIT + file LICENSE

Community examples

Looks like there are no examples yet.