rlang (version 0.2.2)

is_function: Is object a function?

Description

The R language defines two different types of functions: primitive functions, which are low-level, and closures, which are the regular kind of functions.

Usage

is_function(x)
is_closure(x)
is_primitive(x)
is_primitive_eager(x)
is_primitive_lazy(x)

Arguments

x

Object to be tested.

Details

Closures are functions written in R, named after the way their arguments are scoped within nested environments (see https://en.wikipedia.org/wiki/Closure_(computer_programming)). The root environment of the closure is called the closure environment. When closures are evaluated, a new environment called the evaluation frame is created with the closure environment as parent. This is where the body of the closure is evaluated. These closure frames appear on the evaluation stack (see ctxt_stack()), as opposed to primitive functions which do not necessarily have their own evaluation frame and never appear on the stack.

Primitive functions are more efficient than closures for two reasons. First, they are written entirely in fast low-level code. Secondly, the mechanism by which they are passed arguments is more efficient because they often do not need the full procedure of argument matching (dealing with positional versus named arguments, partial matching, etc). One practical consequence of the special way in which primitives are passed arguments this is that they technically do not have formal arguments, and formals() will return NULL if called on a primitive function. See fn_fmls() for a function that returns a representation of formal arguments for primitive functions. Finally, primitive functions can either take arguments lazily, like R closures do, or evaluate them eagerly before being passed on to the C code. The former kind of primitives are called "special" in R terminology, while the latter is referred to as "builtin". is_primitive_eager() and is_primitive_lazy() allow you to check whether a primitive function evaluates arguments eagerly or lazily.

You will also encounter the distinction between primitive and internal functions in technical documentation. Like primitive functions, internal functions are defined at a low level and written in C. However, internal functions have no representation in the R language. Instead, they are called via a call to base::.Internal() within a regular closure. This ensures that they appear as normal R function objects: they obey all the usual rules of argument passing, and they appear on the evaluation stack as any other closures. As a result, fn_fmls() does not need to look in the .ArgsEnv environment to obtain a representation of their arguments, and there is no way of querying from R whether they are lazy ('special' in R terminology) or eager ('builtin').

You can call primitive functions with .Primitive() and internal functions with .Internal(). However, calling internal functions in a package is forbidden by CRAN's policy because they are considered part of the private API. They often assume that they have been called with correctly formed arguments, and may cause R to crash if you call them with unexpected objects.

Examples

# NOT RUN {
# Primitive functions are not closures:
is_closure(base::c)
is_primitive(base::c)

# On the other hand, internal functions are wrapped in a closure
# and appear as such from the R side:
is_closure(base::eval)

# Both closures and primitives are functions:
is_function(base::c)
is_function(base::eval)

# Primitive functions never appear in evaluation stacks:
is_primitive(base::`[[`)
is_primitive(base::list)
list(ctxt_stack())[[1]]

# While closures do:
identity(identity(ctxt_stack()))

# Many primitive functions evaluate arguments eagerly:
is_primitive_eager(base::c)
is_primitive_eager(base::list)
is_primitive_eager(base::`+`)

# However, primitives that operate on expressions, like quote() or
# substitute(), are lazy:
is_primitive_lazy(base::quote)
is_primitive_lazy(base::substitute)
# }