An experimental Agda backend that generates Scala 2 or Scala 3 source files.

Currently supported:

• ✅ Sum types / ADTs (also polymorphic ADTs like Maybe[A], List[A])
• ✅ Product types (records) (with polymorphic parameters)
• ✅ Simple and polymorphic functions (e.g. id[A])
• ✅ Literals: Int/Nat, Bool, String (subset)
• 🚧 Pattern matching (match / cases) – planned
• 🚧 Mapping Agda stdlib names to Scala stdlib (Nat, List, pairs) – planned / partial

Basic usage:

cabal run -- agda2scala ./examples/adts.agda

compile following Agda code:

module examples.adts where

-- sum types
data Rgb : Set where
  Red : Rgb
  Green : Rgb
  Blue : Rgb
{-# COMPILE AGDA2SCALA Rgb #-}

-- polymorphic ADT

data List (X : Set) : Set where
  []   : List X
  _::_ : X -> List X -> List X
{-# COMPILE AGDA2SCALA List #-}

-- literals String

hello : String
hello = "Hello World!"
{-# COMPILE AGDA2SCALA hello #-}

into Scala code:

package examples

object adts:
  enum Rgb:
    case Red
    case Green
    case Blue

  enum List[X]:
    case Nil extends List[Nothing]
    case Cons[X](x0: X, x1: List[X]) extends List[X]

  def hello(): String = "Hello World!"

More Agda examples

Identifier mapping

Agda constructor names are not always valid Scala identifiers (e.g. []). We apply a small mapping table for common constructors and then sanitize the result:

• [] → Nil
• _∷_ / _::_ / _cons_ → Cons
• _,_ → Pair

See Agda.Compiler.Scala.NamePolicy.

Working with source code

• continuous compilation loop using entr

find -name '*.hs' | entr cabal test all

or using ghcid

ghcid

• Build

cabal build all

• Run tests

cabal test all
cabal test agda2scala-test
cabal test agda2scala-hedgehog

• Simple way to run Scala backend

cabal run -- agda2scala --help
cabal run -- agda2scala ./examples/adts.agda

• Generate Scala2 output

cabal run -- agda2scala --compile --no-main --out-dir=scala2/src/main/scala ./examples/adts.agda

• Generate Scala3 (dotty) output

cabal run -- agda2scala --compile --no-main --scala-dialect=Scala3 --out-dir=scala3/src/main/scala ./examples/adts.agda

cabal run -- agda2scala --help
cabal run -- agda2scala ./examples/adts.agda
cabal run -- agda2scala --compile --no-main --out-dir=scala2/src/main/scala ./examples/adts.agda

• format code

fourmolu -i $(find src app test -name '*.hs')
cabal-fmt -i agda2scala.cabal
hlint src app test

end-to-end tests

There are Scala 3 and Scala 2 projects for code generated from Agda examples

They have unit tests, that use code generated from examples.

                agda2scala                
                (generate)                     sbt test
Agda examples ==============> src/main/scala <============ src/test/scala

checks:
* compile Agda examples
* run Scala unit tests that calls Scala code

Those tests are run on CI - Github Actions

Generate Scala 2 code from Agda examples and running tests:

cabal run -- agda2scala --compile --no-main --out-dir=scala2/src/main/scala ./examples/adts.agda
cd scala2
sbt ~test

generate Scala 3 code:

cabal run -- agda2scala --compile --no-main --scala-dialect=Scala3 --out-dir=scala3/src/main/scala ./examples/adts.agda
cd scala3
sbt ~test

Resources

• Documentation for Agda as Haskell library on Hackage including
  • docs for Agda.Compiler.Backend
  • build-in JS backend
  • build-in Haskell backend
• external project with Agda backends
  • omelkonian/agda-minimal-backend
  • jespercockx/agda2scheme
  • omelkonian/agda2train
  • agda/agda2hs, (publication)
  • HectorPeeters/agda2rust, (publication)
  • omelkonian/agda2rust