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Partially evaluate an expression.

Source: R/tidyeval.R
partial_eval.Rd

This function partially evaluates a quosure yielding an expression. It uses information from the current tbl and the local environment to yield a standalone expression. This simplifies SQL translation because we can just pass around expressions rather than expressions + environments (i.e. quosures).

Usage

partial_eval(call, data, env = caller_env(), error_call = caller_env())

Arguments

call

an unevaluated expression, as produced by quote()

data

A lazy data frame backed by a database query.

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% perfectly. partial_eval() uses the following heuristic:

  • 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, by using the .data and .env pronouns of tidy evaluation, or by using dbplyr's own .sql pronoun.

Examples

lf <- lazy_frame(year = 1980, id = 1)
partial_eval(quote(year > 1980), data = lf)
#> year > 1980

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

# cf.
partial_eval(quote(id == .data$id), lf)
#> id == id

# 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), lf)
#> year > year
partial_eval(quote(year > local(year)), lf)
#> year > 1980
partial_eval(quote(year > .env$year), lf)
#> year > 1980

# 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)), lf)
#> year > f(1980)
partial_eval(quote(year > local(f(1980))), lf)
#> year > 1981

# You can use `.sql` to make it clear that the function comes from SQL,
# and inside a package, reduce the number of globalVariables() directives
# needed
partial_eval(quote(.sql$EXTRACT_YEAR(year)), lf)
#> EXTRACT_YEAR(year)