gestalt (version 0.2.0)

constant: Values as Functions

Description

A constant is a fixed value that incorporates its very computation. This is none other than a function that computes a fixed value when called without arguments. constant() declares such a function as a bona fide constant by transforming it to a function that caches the value of its void call (i.e., constant() memoizes void functions).

Combine %>>>% with constant() for a lazy, structured alternative to the magrittr `%>%` operator (see ‘Examples’).

Usage

constant(f)
variable(f)

Value

constant() yields a function without formal arguments that returns the (cached, visibility-preserving) value of the void call f().

variable() is the inverse transformation of constant(): it recovers the underlying (uncached) function of a constant function.

Arguments

f

Function, or symbol or name (string) thereof, that can be called without arguments. (NB: constant() itself does not check whether f() is indeed a valid call.)

See Also

%>>>%

Examples

# Function with a constant return value
val <- {message("Computing from scratch"); mtcars} %>>>%
  split(.$cyl) %>>>%
  lapply(function(data) lm(mpg ~ wt, data)) %>>>%
  lapply(summary) %>>>%
  sapply(`[[`, "r.squared")

# With every invocation, `val()` is computed anew:
val()
val()

# Declaring `val` as a constant ensures that its value is computed only once.
# On subsequent calls, the computed value is simply fetched:
const <- constant(val)
const()
const()

# As values, `val()` and `const()` are identical. But `const()`, moreover,
# has structure, namely the function `const`:
const

# For instance, you can inspect the intermediate summaries:
head(const, -1)()

# Which can itself be a constant:
summ <- constant(head(const, -1))
summ()
summ()

if (FALSE) {
# Think of `%>>>%` combined with `constant()` as a lazy, structured
# alternative to the magrittr `%>%` operator.
library(magrittr)

val2 <- mtcars %>%
  split(.$cyl) %>%
  lapply(function(data) lm(mpg ~ wt, data)) %>%
  lapply(summary) %>%
  sapply(`[[`, "r.squared")

# `val2` and `const()` are identical values. But whereas `val2` is computed
# immediately and carries no structure, `const` embodies the process that
# produces its value, and allows you to defer its realization to the
# invocation `const()`.
stopifnot(identical(val2, const()))}

# Use `variable()` to recover the original (\dQuote{variable}) function
val_var <- variable(const)
stopifnot(identical(val_var, val))
val_var()
val_var()