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Myriad is a code generator. It takes an arbitary file and the library provides different mechanisms to allow F# code to be produced in response to the file, whether that file be an F# source file or a simple text file.

Myriad can be used from either an MSBuild extension or from its CLI tool.

The idea behind Myriad is to un-complicate, as far as possible, the ability to do meta-programming in F#. By meta-programming in F# I mean generating idiomatic F# code using F# native types like discriminated unions and records, this is something that is not possible with F# Type Providers etc. which just output basic .NET classes.

Myriad is an evolution of the ideas I developed while working with F#'s type providers and other meta-programming functionality like quotations and AST manipulation. Myriad aims to make it easy to extend the compiler via Myriad plugins. Myriad provides an approach to compiler extension that isn't modifying or adjusting Type Providers or waiting a long time for other F# language improvements. You write a Myriad plugin that works on a fragment of AST input, and the plugin supplies AST output with the final form being source code that is built into your project. This lets the compiler optimise generated output in addition to allowing tooling to operate effectively.

If you want to help and contribute code than thats great check out the issues or leave a comment. If not and you want to help in another way then I have GitHub sponsors and also Ko-fi:

GitHub Sponsors

Usage via MSBuild

To use Myriad via its MSBuild support you add the Myriad.Core and Myriad.Sdk package references:

      <PackageReference Include="Myriad.Core" Version="0.5.0" />
      <PackageReference Include="Myriad.Sdk" Version="0.5.0" />

An input file is specified by using the usual Compile element:

<Compile Include="Library.fs"/>
<Compile Include="Generated.fs">

This is configuring Myriad so that a file called Generated.fs will be included in the build using Library.fs as input to the Myriad.

It is also possible to append the generated content to the input file.

<Compile Include="Library.fs">

Myriad works by using plugins to generate code. A plugin called fields is included with Myriad which takes inspiration from OCaml's ppx_fields_conv plugin of the same name.

The input file in this example Library.fs looks like this:

namespace Example
open Myriad.Plugins

[<Generator.Fields "fields">]
type Test1 = { one: int; two: string; three: float; four: float32 }
type Test2 = { one: Test1; two: string }

Attribute's are use so that the code generator knows which parts of the input AST are to be processed by the plugin. If you had several records and you only want the fields plugin to operate on Test1 then the attribute would be used like in the example to only apply Generator.Fields to Test1. Note, if you wanted a plugin that just needs the whole input AST then there is no need to provide an input. Myriad aims to be a library rather than a full framework that ties you to the mechanism used to input and generate code. The parameter passed to the attribute "fields" specifies the configuration section that is used for the plugin in the myriad.toml file. In this instance fields is used and the myriad.toml file is as follows:

namespace = "TestFields"

This specifies the namespace that is used for the plugin, which in this case is "TestFields".

The fields plugin in this example will generate the following code at pre-build time and compile the code into your assembly:

//        This code was generated by myriad.
//        Changes to this file will be lost when the code is regenerated.
namespace rec TestFields

module Test1 =
    open Example

    let one (x : Test1) =
    let two (x : Test1) = x.two
    let three (x : Test1) = x.three
    let four (x : Test1) = x.four

    let create (one : int) (two : string) (three : float) (four : float32) : Test1 =
        { one = one
          two = two
          three = three
          four = four }

    let map (mapone : int -> int) (maptwo : string -> string) (mapthree : float -> float) (mapfour : float32 -> float32) (record': Test1) =
      { record' with
          one = mapone record'.one
          two = maptwo record'.two
          three = mapthree record'.three
          four = mapfour record'.four }

The fields plugin generates a map for each field in the input record, a create function taking each field, and a map function that takes one function per field in the input record.

The map functions for each field are useful in situations where you just want to use a single field from a record in a lambda like a list of records:

let records = [{one = "a"; two = "aa"; three = 42.0; four = 172.0f}
               {one = "b"; two = "bb"; three = 42.0; four = 172.0f}]
 records |> List.sortBy

Lens plugin

Myriad can also generate lenses for records and single-case discriminated unions. Lens is a pair of a getter and a setter for one property of the type. Given the object Lens allows you to get the value of the property or to update it, creating a new object. The advantage of lenses is an ability to combine them to read or update nested fields of the object.

To create lenses for your type, first annotate the type for which you want lenses to be generated with Generator.Lenses attribute:

type Record =
    { one: int
      two: string }

Myriad will generate the following code:

module RecordLenses =
    let one = (fun (x: Test1) ->, (fun (x: Test1) (value: int) -> { x with one = value })
    let two = (fun (x: Test1) -> x.two), (fun (x: Test1) (value: string) -> { x with two = value })

Often lenses are defined as a single-case union around a pair of getter and setter. Myriad is also capable of adding the invocation of such DU's constructor.

To achieve this, decorate your type with the Lens attribute, specifying the name of the DU constructor: [<Generator.Lenses("Lens")>], and Myriad will generate this code:

module RecordLenses =
    let one = Lens((fun (x: Test1) ->, (fun (x: Test1) (value: int) -> { x with one = value }))
    let two = Lens((fun (x: Test1) -> x.two), (fun (x: Test1) (value: string) -> { x with two = value }))

You can provide the name of DU constructor in several ways:

  • As a string: [<Generator.Lenses("lens", "Lens")>];
  • Or as a type: [<Generator.Lenses("lens", typedefof<Lens<_, _>>)>] or [<Generator.Lenses(typeof<Lens<_, _>>)>].

If the Lens type is in different namespace/module than the type decorated with the attribute, provide the full name of the Lens constructor: [<Generator.Lenses("Namespace.And.Module.Of.Lens")>].

The full fsproj is detail below:

<Project Sdk="Microsoft.NET.Sdk">
        <Compile Include="Library.fs" />
        <Compile Include="Generated.fs">
      <PackageReference Include="Myriad.Core" Version="0.5.0" />
      <PackageReference Include="Myriad.Sdk" Version="0.5.0" />


Plugins for Myriad are supplied by including the nuget package in your project. The nuget infrastructure supplies the necessary MSBuild props and targets so that the plugin is used by Myriad automatically. Following the source for the fields plugin can be used as reference until more details about authoring plugins is created.

Using external Plugins

To consume external plugins that aren't included in the Myriad.Plugins package, you must register them with Myriad. If you are using the CLI tool then the way to do this is by passing in the --plugin <path to dll> command-line argument. If you are using MSBuild then this can be done by adding to the MyriadSdkGenerator property to your project file:

    <MyriadSdkGenerator Include="<path to plugin dll>" />

For example, if you had a project layout like this:

 - Generator.fs
 - Generator.fsproj
 - Tests.fs
 - GeneratorTests.fsproj

You would add the following to Generator.fsproj:

    <Content Include="build\Generator.props">

Then add a new folder build with the Generator.props file within:

        <MyriadSdkGenerator Include="$(MSBuildThisFileDirectory)/../lib/netstandard2.1/Generator.dll" />

Often an additional props file (In this sample the file would be Generator.InTest.props) is used to make testing easier. The matching element for the tests .fsproj would be something like this:

        <MyriadSdkGenerator Include="$(MSBuildThisFileDirectory)/../bin/$(Configuration)/netstandard2.1/Generator.dll" />

Notice the Include path is pointing locally rather than within the packaged nuget folder structure.

In your testing fsproj you would add the following to allow the plugin to be used locally rather that having to consume a nuget package:

<!-- include plugin -->
<Import Project="<Path to Generator plugin location>\build\Myriad.Plugins.InTest.props" />


To debug Myriad, you can use the following two command line options:

  • --verbose — write diagnostic logs out to standard out
  • --wait-for-debugger — causes Myriad to wait for a debugger to attach to the Myriad process

These can be triggered from MSBuild by the <MyriadSdkVerboseOutput>true</MyriadSdkVerboseOutput> and <MyriadSdkWaitForDebugger>true</MyriadSdkWaitForDebugger> properties, respectively.


The nuget package for Myriad can be found here: Nuget package.

Dotnet template

A dotnet template for a Myriad plugin/generator is available here:

#install dotnet template
dotnet new -i Myriad.Templates

#create myriad generator from the template
dotnet new myriadgenerator -n myMyriadPlugin

How to build and test

  1. Make sure you have .Net Core SDK installed - check required version in global.json
  2. Run dotnet tool restore
  3. Run dotnet build -c Release -t:Build

How to release new version

  1. Update by adding new entry (## [0.X.X]) and commit it.
  2. Create version tag (git tag v0.X.X)
  3. Update the VersionPrefix in build.proj to match the tag above.
  4. Run dotnet build -t:Pack to create the nuget package and test/examine it locally.
  5. Push the tag to the repo git push origin v0.X.X - this will start CI process that will create GitHub release and put generated NuGet packages in it
  6. Upload generated packages into

Also see

External plugins

Here is a list of external plugins that have been built

SqlHyra JsonWrapper TypeSafeInternals

Terms of use

By using this project or its source code, for any purpose and in any shape or form, you grant your implicit agreement to all the following statements:

  • You condemn Russia and its military aggression against Ukraine
  • You recognize that Russia is an occupant that unlawfully invaded a sovereign state
  • You support Ukraine's territorial integrity, including its claims over temporarily occupied territories of Crimea and Donbas
  • You reject false narratives perpetuated by Russian state propaganda

To learn more about the war and how you can help, click here. Glory to Ukraine! 🇺🇦