quo captures its argument literally, that is, without evaluating,
and constructs a quotation. A quotation has two parts: an
expression expr(q) with an environment env(q). (Like in
writing, an 'expression' may simply be a set of words, but a
'quotation' comes bundled with a citation, to reference a context
in which it was said.)
quo_(expr, env) is the normally evaluating version. It
constructs a quotation given an expression and environment.
as.quo(x) converts an object into a quotation. Closures,
formulas, and single-element dots can be converted this way.
quo(expr, env = arg_env_(quote(expr), environment()), force = FALSE)quo_(expr, env, force = FALSE)
env(q)
# S3 method for quotation
env(q)
env(q) <- value
# S3 method for quotation
env(q) <- value
expr(q)
# S3 method for quotation
expr(q)
expr(q) <- value
# S3 method for quotation
expr(q) <- value
is.quotation(x)
as.quo(x)
# S3 method for function
as.quo(x)
# S3 method for quotation
as.quo(x)
# S3 method for dots
as.quo(x)
# S3 method for formula
as.quo(x)
# S3 method for lazy
as.quo(x)
# S3 method for default
as.quo(x)
An expression. For quo this is taken literally and
not evaluated. For quo_ this is evaluated normally.
An environment.
Immediately evaluate the expression and create a forced quotation, i.e. one that stores an expression and value, but no environment.
A quotation object.
An updated value.
Any object.
quo_ and quo return an object of class "quotation".
as.quo returns a quotation.
A quo is parallel to a 'promise' which is the data structure R uses to hold lazily evaluated arguments. A quo is different from a promise because it is an immutable data object.
As a data object, a quo does not automatically evaluate like a promise, but can be evaluated explicitly with the methods value or force_. A quo is immutable, so it does not mutate into a "forced" state if you choose to evaluate it.
A function can capture its arguments as quotations using arg.
A dots object is a list of quotations.