partial_eval function

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 translation because expressions don't need to carry around their environment - all relevant information is incorporated into the expression.

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).

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

Examples

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)