C# style generators a.k.a. semi-coroutines for Julia.



C# has a convenient way to create iterators using the yield return statement. The package ResumableFunctions provides the same functionality for the Julia language by introducing the @resumable and the @yield macros. These macros can be used to replace the Task switching functions produce and consume which were deprecated in Julia v0.6. Channels are the preferred way for inter-task communication in julia v0.6+, but their performance is subpar for iterator applications. See the benchmarks section below.

Build Status & Coverage

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ResumableFunctions is a registered package and can be installed by running:

using Pkg


using ResumableFunctions

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

for fib in fibonacci(10)


The following block is the result of running julia --project=. benchmark/benchmarks.jl on a Macbook Pro with following processor: Intel Core i9 2,4 GHz 8-Core. Julia version 1.5.3 was used.

Fibonacci with Int values:

  27.184 ns (0 allocations: 0 bytes)
  27.503 ns (0 allocations: 0 bytes)
Channels csize=0: 
  2.438 ms (101 allocations: 3.08 KiB)
Channels csize=1: 
  2.546 ms (23 allocations: 1.88 KiB)
Channels csize=20: 
  138.681 μs (26 allocations: 2.36 KiB)
Channels csize=100: 
  35.071 μs (28 allocations: 3.95 KiB)
Task scheduling
  17.726 μs (86 allocations: 3.31 KiB)
  1.948 μs (82 allocations: 1.28 KiB)
Closure optimised: 
  25.910 ns (0 allocations: 0 bytes)
Closure statemachine: 
  28.048 ns (0 allocations: 0 bytes)
Iteration protocol: 
  41.143 ns (0 allocations: 0 bytes)

Fibonacci with BigInt values:

  5.747 μs (188 allocations: 4.39 KiB)
  5.984 μs (191 allocations: 4.50 KiB)
Channels csize=0: 
  2.508 ms (306 allocations: 7.75 KiB)
Channels csize=1: 
  2.629 ms (306 allocations: 7.77 KiB)
Channels csize=20: 
  150.274 μs (309 allocations: 8.25 KiB)
Channels csize=100: 
  44.592 μs (311 allocations: 9.84 KiB)
Task scheduling
  24.802 μs (198 allocations: 6.61 KiB)
  7.064 μs (192 allocations: 4.47 KiB)
Closure optimised: 
  5.809 μs (190 allocations: 4.44 KiB)
Closure statemachine: 
  5.826 μs (190 allocations: 4.44 KiB)
Iteration protocol: 
  5.822 μs (190 allocations: 4.44 KiB)

Licence & References

License status DOI



  • To discuss problems or feature requests, file an issue. For bugs, please include as much information as possible, including operating system, julia version, and version of MacroTools.
  • To contribute, make a pull request. Contributions should include tests for any new features/bug fixes.

Release notes

  • 2021: v0.6.0

    • introduction of @nosave to keep a variable out of the saved structure.
    • optimized for loop.
  • 2020: v0.5.2 is Julia v1.6 compatible.

  • 2019: v0.5.1

    • inference problem solved: force iterator next value to be of type Union of Tuple and Nothing.
  • 2019: v0.5.0 is Julia v1.2 compatible.

  • 2018: v0.4.2 prepare for Julia v1.1

    • better inference caused a problem;).
    • iterator with a specified rtype is fixed.
  • 2018: v0.4.0 is Julia v1.0 compatible.

  • 2018: v0.3.1 uses the new iteration protocol.

    • the new iteration protocol is used for a @resumable function based iterator.
    • the for loop transformation implements also the new iteration protocol.
  • 2018: v0.3 is Julia v0.7 compatible.

    • introduction of let block to allow variables not te be persisted between @resumable function calls (EXPERIMENTAL).
    • the eltype of a @resumable function based iterator is its return type if specified, otherwise Any.
  • 2018: v0.2 the iterator now behaves as a Python generator: only values that are explicitely yielded are generated; the return value is ignored and a warning is generated.

  • 2017: v0.1 initial release that is Julia v0.6 compatible:

    • Introduction of the @resumable and the @yield macros.
    • A @resumable function generates a type that implements the iterator interface.
    • Parametric @resumable functions are supported.


  • In a try block only top level @yield statements are allowed.
  • In a finally block a @yield statement is not allowed.
  • An anonymous function can not contain a @yield statement.

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