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, vars = character(), env = caller_env())
Arguments
call

an unevaluated expression, as produced by quote()

vars

character vector of variable names.

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.

You can override the guesses using local() and remote() to force computation, or by using the .data and .env pronouns of tidy evaluation.

Aliases
  • partial_eval
Examples
# NOT RUN {
vars <- c("year", "id")
partial_eval(quote(year > 1980), vars = vars)

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

# cf.
partial_eval(quote(id == .data$ids), vars = vars)

# You can use local() or .env to disambiguate between local and remote
# variables: otherwise remote is always preferred
year <- 1980
partial_eval(quote(year > year), vars = vars)
partial_eval(quote(year > local(year)), vars = vars)
partial_eval(quote(year > .env$year), vars = vars)

# 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)), vars = vars)
partial_eval(quote(year > local(f(1980))), vars = vars)

# 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 dbplyr, version 1.4.0, License: MIT + file LICENSE

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