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A prototype programming language for programming with runners


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Coop is a prototype programming language for programming with runners, also known as comodels.

Coop is part of ongoing research by Danel Ahman and Andrej Bauer. The theoretical aspects of our work are described in the paper Runners in action. You may also be interested in Danel's talk Interacting with external resources using runners (aka comodels) and his Haskell-Coop library.



To compile Coop you need:

  • OCaml and OPAM

  • The OPAM packages menhir, sedlex, and dune:

      opam install menhir
      opam install sedlex
      opam install dune
  • It is recommended that you also install the rlwrap or ledit command line wrapper.

Obtaining Coop

If you're reading this file then you have probably found the Coop GitHub repository, where Coop is available.


You can type:

  • make to make the coop.native executable.
  • make byte to make the bytecode coop.byte executable.
  • make clean to clean up.
  • make doc to generate HTML documentation (see the generated coop.docdir/index.html).

Language features

To find out more about Coop, please consult the Coop manual.

Theoretical background

Coop is an extension of λ-coop, a calculus devised by Danel Ahman and Andrej Bauer, cf. our paper Runners in action, to study how runners (also known as comodels) can be used to program with external resources. These are similar to handlers for algebraic effects, except that they carry state and use the continuation at most one in a tail-call position. There are two kinds of computations in λ-coop: the kernel computations implement access to resources as co-operations, while the user computations use resources by calling algebraic operations. The kernel mode has the ability to report errors to user mode by raising exceptions (recoverable errors), or aborting user code by sending it signals (unrecoverable errors).

The central construct of λ-coop is

using R @ I run
finally F

which runs user code M and handles its operations using the (kernel mode) co-operations of runner R. It is a kind of "virtual machine" which runs user code inside a kernel represented by R. The runner has access to state (hidden from the user code) which is initialized by I. The finalisation code F intercepts the return value, exceptions and signals, with the purpose of properly disposing of any resources used by the runner R. The calculus guarantee that F will be reached (unless an outer runner kills the present one).

The type system of λ-cup keeps track of effects and supports subtyping.


This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 834146.
This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17-1-0326.


A prototype programming language for programming with runners







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