Manual

Basic usage

When a @resumable function is called, it continues where it left during the previous invocation:

@resumable function basic_example()
  @yield "Initial call"
  @yield "Second call"
  "Final call"
end

julia> basic_iterator = basic_example();

julia> basic_iterator()
"Initial call"

julia> basic_iterator()
"Second call"

julia> basic_iterator()
"Final call"

The @yield can also be used without a return argument:

@resumable function yield_example()
  @yield "Initial call"
  @yield
  "Final call"
end

julia> yield_iterator = yield_example();

julia> yield_iterator()
"Initial call"

julia> yield_iterator()

julia> yield_iterator()
"Final call"

The famous Fibonacci sequence can easily be generated:

@resumable function fibonacci()
  a = 0
  b = 1
  while true
    @yield a
    a, b = b, a + b
  end
end

julia> fib_iterator = fibonacci();

julia> fib_iterator()
0

julia> fib_iterator()
1

julia> fib_iterator()
1

julia> fib_iterator()
2

julia> fib_iterator()
3

julia> fib_iterator()
5

julia> fib_iterator()
8

The @resumable function can take arguments and the type of the return value can be specified:

@resumable function fibonacci(n) :: Int
  a = 0
  b = 1
  for i in 1:n
    @yield a
    a, b = b, a + b
  end
end

julia> fib_iterator = fibonacci(4);

julia> fib_iterator()
0

julia> fib_iterator()
1

julia> fib_iterator()
1

julia> fib_iterator()
2

julia> fib_iterator()

julia> fib_iterator()
ERROR: @resumable function has stopped!

When the @resumable function returns normally (i.e. at the end of the function rather than at a @yield point), an error will be thrown if called again.

Two-way communication

The caller can transmit a variable to the @resumable function by assigning a @yield statement to a variable:

@resumable function two_way()
  name = @yield "Who are you?"
  "Hello, " * name * "!"
end

julia> hello = two_way();

julia> hello()
"Who are you?"

julia> hello("Ben")
"Hello, Ben!"

When an Exception is passed to the @resumable function, it is thrown at the resume point:

@resumable function mouse()
  try
    @yield "Here I am!"
  catch exc
    return "You got me!"
  end
end

struct Cat <: Exception end

julia> catch_me = mouse();

julia> catch_me()
"Here I am!"

julia> catch_me(Cat())
"You got me!"

Iterator interface

The iterator interface is implemented for a @resumable function:

@resumable function fibonacci(n) :: Int
  a = 0
  b = 1
  for i in 1:n
    @yield a
    a, b = b, a + b
  end
end

julia> for val in fibonacci(10) println(val) end
0
1
1
2
3
5
8
13
21
34

Parametric `@resumable` functions

Type parameters can be specified with a normal Julia where clause:

@resumable function fibonacci(a::N, b::N=a+one(N)) :: N where {N<:Number}
  for i in 1:10
    @yield a
    a, b = b, a + b
   end
end

julia> for val in fibonacci(0.0) println(val) end
0.0
1.0
1.0
2.0
3.0
5.0
8.0
13.0
21.0
34.0

Caveats