A type definition compiler supporting multiple output languages.

Stackage Nightly 2022-01-25:1.1.0
Latest on Hackage:1.1.0

See all snapshots gotyno-hs appears in

BSD-2-Clause licensed
Maintained by Rickard Andersson
This version can be pinned in stack with:gotyno-hs-1.1.0@sha256:dcb750e63b82a0a5afd95c6b68874198ce02306872da5dbbe8edd8696dc5498e,6383


A tool for generating validation/encoding/decoding code for types via a type definition language. See details of the language here.


Make sure you have the Haskell tool stack. This is used for building the entire project and once you’ve installed it, it will manage having the correct version of the compiler and all the Haskell libraries that are used as well.

Once we have stack installed we can simply run the following in this project directory:

stack install

Stack will now install gotyno-hs as a binary in your $HOME/.local/bin directory. If you’ve made sure that you have this directory in your $PATH you can execute gotyno-hs anywhere.

If we wanted to compile a file at examples/basic.gotyno into both TypeScript and F# into separate output directories as well as watch for file changes and recompile when they happen, we could execute the following at this point:

$ gotyno-hs --typescript ts-output --fsharp fsharp-output --watch examples/basic.gotyno

Supported languages

  • TypeScript
  • F#
  • Python

Planned support

  • Haskell
  • PureScript

Maybe supported in the future

  • Kotlin
  • Swift
  • Scala 3 (if it’s better now)
  • Elixir

Options you can pass to the compiler

The help text for the compiler illustrates most of what you can do.

$ gotyno-hs --help
gotyno - Compile type definitions into encoders/decoders for languages

Usage: gotyno-hs [--ts|--typescript =|-|PATH] [--fs|--fsharp =|-|PATH] 
                 [-w|--watch] [-v|--verbose] GOTYNOFILE
  Compile type definitions into encoders/decoders for languages

Available options:
  --ts,--typescript =|-|PATH
                           Set TypeScript output
  --fs,--fsharp =|-|PATH   Set FSharp output
  --py,--python =|-|PATH   Set Python output
  -w,--watch               Watch files and recompile automatically
  -v,--verbose             Output info about compilation
  -h,--help                Show this help text


The output destination for a language can be set to = for “same as input file” - for standard output or a path for all output for that language.

TypeScript example

basic.gotyno has an example of some types being defined and basic.ts is the automatically generated TypeScript output from this file. This directory contains other examples, used primarily for testing purposes.

Behind the scenes it’s using a validation library I wrote for validating unknown values (for the most part against given interface definitions).

F# example

basic.gotyno has an example of some types being defined and basic.fs is the automatically generated F# output from this file. This directory contains other examples, used primarily for testing purposes.

The F# version uses Thoth for JSON decoding, as well as an additional extension library to it for some custom decoding helpers that I wrote.

Import names (and by extension declaration source names) are automatically converted to use an initial uppercase letter for F#. Note, however, that PascalCase is not automatically converted to. If import names are written in snake_case the output will be Snake_case. For the most part camelCase will work well for Gotyno file names, since the output will be “CamelCase” for that particular example.

The Language

All supported type names are uppercase and type definitions currently are enforced as such as well.


  • ?TypeName signifies an optional type.
  • *TypeName signifies a pointer to that type. In languages where pointers are hidden from the user this may not be visible in types generated for it.
  • []TypeName signifies a sequence of several TypeName with a length known at run-time, whereas [N]TypeName signifies a sequence of several TypeName with a known and enforced length at compile-time. Some languages may or may not have the latter concept and will use only the former for code generation.
  • TypeName<OtherTypeName> signifies an application of a generic type, such that whatever type variable TypeName takes in its definition is filled in with OtherTypeName in this specific instance.
  • Conversely, struct/union TypeName <T>{ ... } is how one defines a type that takes a type parameter. The <T> part is seen here to take and provide a type T for the adjacent scope, hence its position in the syntax.
  • The type "SomeValue" signifies a literal string of that value and can be used very effectively in TypeScript.
  • The unsigned integer type is the same, but for integers. It’s debatable whether this is useful to have.


struct Recruiter {
    name: String

struct Person {
    name: String
    age: U8
    efficiency: F32
    on_vacation: Boolean
    hobbies: []String
    last_fifteen_comments: [15]String
    recruiter: ?Recruiter

struct Generic <T>{
    field: T
    other_field: ?T
    other_field_with_slice: []T


enum Colors {
    red = "FF0000"
    green = "00FF00"
    blue = "0000FF"



union InteractionEvent {
    Click: Coordinates
    KeyPress: KeyCode
    Focus: *Element

union Option <T>{
    Some: T

union Result<E, T>{
    Error: E
    Ok: T


Sometimes a union that carries no extra tags is required, though usually these will have to be identified less automatically, perhaps via custom tags in their respective payload:

struct SomeType {
    type: "SomeType"
    some_field: F32
    some_other_field: ?String

struct SomeOtherType {
    type: "SomeOtherType"
    active: Boolean
    some_other_field: ?String

untagged union Possible {

In TypeScript, for example, the correct type guard and validator for this untagged union will be generated, and the literal string fields can still be used for identifying which type one has.

Setting the tag key and embedding it in payload structures

The above can also be accomplished by setting the tag key to be embedded in options passed to the union keyword (we can also set which key is used):

struct SomeType {
    some_field: F32
    some_other_field: ?String

struct SomeOtherType {
    active: Boolean
    some_other_field: ?String

union(tag = type_tag, embedded) Possible {
    FirstConstructor: SomeType
    SecondConstructor: SomeOtherType

This effectively will create a structure where we get the field type_tag embedded in the payload structures (SomeType & SomeOtherType) with the values "FirstConstructor" and "SecondConstructor" respectively.

Note that in order to embed a type key we need the payload (if present) to be a structure type, otherwise we have no fields to merge the type tag field into.

Both checks for existence of the referenced payload types and checks that they are structures are done during compilation.


When a type that is outside of Gotyno files is needed, we can use a declaration to assert that it exists. It will function much like an import in the generated code and it’s up to the user to supply the required functionality for each language in the required file:

declare external.Option<T>

declare otherExternalModule.Plain

struct HasOption<T> {
    field: Option<T>
    field2: Plain

Note that the above code will generate an import for each unique source module (on the left of the ‘.’) and each usage is automatically recognized as a reference to a declaration. The code output will rely on this module being present in the same output directory as the generated code.





  • Made the parser much less strict, allowing extra whitespace in many places. This is experimental for now, but should allow people to have just a bit more of their own style.



  • Added checks for whether or not the user is applying enough/too many type parameters to the usage of a given definition/declaration. This was previously punted to the output language but if we are to output to languages with very poor static checks themselves, this safeguard is good to have.



  • Uploaded package to Stackage



  • Clearing definitions & declarations on module parsing success and failure
  • Using debouncing to handle duplicate file events when using --watch