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FEZ - an F# to core erlang experiment

What is it?

Fez is an early doors experiment in compiling fsharp to BEAM compatible core erlang.

The primary aim is to implement enough of the language to evaluate what how well an ML type of language could become a practical language for writing code to be run on the beam.

Should this experiment succeed then it may lead to a fuller and usable fsharp backend.

See for some discussions on potential implementation approaches.

Getting started


./   [on Linux/Mac]
./build.cmd  [on Windows]

after this you can:

./fezc <file.fs> <file2.fs> -o out     [on Linux/Mac]
./fezc.cmd <file.fs> <file2.fs> -o out [on Windows]

to try to compile fsharp modules to the beam.

You can use the --nobeam parameter if you only want to compile as core erlang files.

To run the tests:


Self contained releases

See releases page.

Download and extract the zip for your operating system. Then run: fez help to see the available commands and options.

To compile:

fez compile <files>

The module can then be run used in an erlang shell. The release ships with a set of .beam files that need to be included. E.g:

erl -pa <path_to_fez>/ebin

What works?

FSharp is a multi-paradigmatic language and fez is targeting a solely functional language (core erlang). Given this impedence mismatch it is only reasonable to expect fez to adequately cover the functional language constructs of FSharp. That said there are aspects of the imperative and object oriented paradigms that can be covered, to a certain extent.

The most up-to-date resource for what language construct are covered is to look at basics.fs but it is fairly unstructured.

The lists below list feature that are implemented to one of the following degrees:

  • complete

    The feature is reasonably complete and unsupported APIs and constructs either generate appropriate compile-time or run-time errors. Any strange behaviour here needs to be reported!

  • partial

    The feature is tried and proven viable but not yet fully implemented.

  • prototype

    The feature is present to some limited degree. Likely to have unexpected or unusual constraints. Feature may not be kept.

  • unstarted

    The feature has not been started or investigated for feasibilty.


  • Modules [complete]

    An fsharp module is compiled as an erlang module. Nested modules are compiled as indiviual erlang modules with fully-qualified names.

  • Functions [complete]

    Private functions are not included in the erlang export statement. Function application, currying, inner functions and lambdas are all supported.

  • Operators [partial]

    Most of the common operators (|>, >>, +, -, /, snd, fst) have been implemented but not to completion. Please report omissions.

  • Pattern matching / control flow. [complete]

    Pattern matching, including guards is supported as are if then else expressions.

  • list/seq comprehensions [partial]

    List and Seq comprehensions are supported but haven't been verified for completeness.

  • List.* [complete]

    List API is complete apart from any functions including Array.

  • Seq.* [partial]

  • Set.* [partial],

  • Option.* [partial]

  • Result.* [complete]

  • Map.* [partial],

  • String.* [complete]

  • Array* [unstarted]

  • Tuples [complete]

  • Records [complete]

  • Discriminated Unions [complete]

  • Active Patterns [partial]

    Limited testing. Please report issues.

  • try/finally/with [partial]

  • Primitive types [partial]

    • Currently all integers are translated to erlang integers and all float types to erlang floats. Erlang integers are "big ints" hence things like overflows and bitshifting probably wont work as expected.
  • sprintf / printfn [partial]

  • query expressions [prototype]

    Only works with seq.

  • async [partial]

    Async.Start and Async.StartChild will execute computation in a separate process. The rest is mostly erased as all IO is already non-blocking in erlang.

  • Computation Expressions [partial]

    Limited testing.

Object programming

  • Member methods on records and discriminated unions [complete]

    NB: type based method overload do not work.

  • Static members on records and discriminated unions [complete]

  • Extension methods [complete]

  • Interface implementations on records and unions [complete]

  • use expressions [complete]

  • constructors [prototype]

  • inheritance [prototype]


  • Ref cells [prototype]

    No automatic GC of the backing process dictionary key.

  • Fast integer for loop. [partial]

  • While loops [unstarted]

Erlang interop

  • FFI [partial]

    The ModCall attribute can be used to generate a function stub that delegates to the configured erlang function.

  • ErlangTerms [partial]

    The ErlangTerm attribute can be used to decorate a discriminated union such that a more erlang compatible encoding will be generated instead of the standard DU representation.

      * Cases without fields will be lowercased and generated as atoms
      * Cases with fields will be generated as tuples.

How can I help?

Try it! Right now I need people to try stuff and find constructs that break or don't behave as expected. Especially for functional code. Once you find something (and I'm sure you will) please raise an issue including the smallest possible repro code.

Code is also welcome of course. Especially for the standard library erlang modules.


F# to core erlang compiler experiment








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