Create a generator function

generator() creates an generator factory. A generator is an iterator function that can pause its execution with yield() and resume from where it left off. Because they manage state for you, generators are the easiest way to create iterators. See vignette("generator").

The following rules apply:

Yielded values do not terminate the generator. If you call the generator again, the execution resumes right after the yielding point. All local variables are preserved.
Returned values terminate the generator. If called again after a return(), the generator keeps returning the exhausted() sentinel.

Generators are compatible with all features based on the iterator protocol such as loop() and collect().

Usage

generator(fn)

gen(expr)

Arguments

fn: A function template for generators. The function can yield() values. Within a generator, for loops have iterator support.
expr: A yielding expression.

Examples

# A generator statement creates a generator factory. The
# following generator yields three times and then returns `"d"`.
# Only the yielded values are visible to the callers.
generate_abc <- generator(function() {
  yield("a")
  yield("b")
  yield("c")
  "d"
})

# Equivalently:
generate_abc <- generator(function() {
  for (x in c("a", "b", "c")) {
    yield(x)
  }
})

# The factory creates generator instances. They are iterators
# that you can call successively to obtain new values:
abc <- generate_abc()
abc()
#> [1] "a"
abc()
#> [1] "b"

# Once a generator has returned it keeps returning `exhausted()`.
# This signals to its caller that new values can no longer be
# produced. The generator is exhausted:
abc()
#> [1] "c"
abc()
#> .__exhausted__.

# You can only exhaust a generator once but you can always create
# new ones from a factory:
abc <- generate_abc()
abc()
#> [1] "a"


# As generators implement the coro iteration protocol, you can use
# coro tools like `loop()`. It makes it possible to loop over
# iterators with `for` expressions:
loop(for (x in abc) print(x))
#> [1] "b"
#> [1] "c"

# To gather values of an iterator in a list, use `collect()`. Pass
# the `n` argument to collect that number of elements from a
# generator:
abc <- generate_abc()
collect(abc, 1)
#> [[1]]
#> [1] "a"
#> 

# Or drain all remaining elements:
collect(abc)
#> list()


# coro provides a short syntax `gen()` for creating one-off
# generator _instances_. It is handy to adapt existing iterators:
numbers <- 1:10
odds <- gen(for (x in numbers) if (x %% 2 != 0) yield(x))
squares <- gen(for (x in odds) yield(x^2))
greetings <- gen(for (x in squares) yield(paste("Hey", x)))

collect(greetings)
#> [[1]]
#> [1] "Hey 1"
#> 
#> [[2]]
#> [1] "Hey 9"
#> 
#> [[3]]
#> [1] "Hey 25"
#> 
#> [[4]]
#> [1] "Hey 49"
#> 
#> [[5]]
#> [1] "Hey 81"
#> 


# Arguments passed to generator instances are returned from the
# `yield()` statement on reentry:
new_tally <- generator(function() {
  count <- 0
  while (TRUE) {
    i <- yield(count)
    count <- count + i
  }
})
tally <- new_tally()
tally(1)
#> [1] 0
tally(2)
#> [1] 2
tally(10)
#> [1] 12

Usage

Arguments

See also

Examples