glelm

Elm-to-Gleam transpiler.

WORK-IN-PROGRESS.

Currently supported:

• Naive transpilling of type declarations (e.g. Maybe is Maybe in generated Gleam code).
• Sum types
• Typed Records
• Function types
• Tuples
• Unit

Coming up soon:

• Type aliases type alias A = B
• Untyped records type alias R = { foo: Int }

Unsupported:

• Function declarations
• Ports

cat samples/Sample1.elm

module Sample1 exposing (Declaration(..), Node)


-- Elm declaration
module Sample1 exposing (Declaration(..), Node)

{-| Elm declaration
-}
type Declaration
    = FunctionDeclaration Function
    | AliasDeclaration TypeAlias
    | CustomTypeDeclaration Type
    | PortDeclaration Signature
    | InfixDeclaration Infix
    | Destructuring
        { pattern : Node Pattern
        , expression : Node Expression
        }


{-| Provides additional context
-}
type Node a
    = Node Range a

gleam run samples/Sample1.elm
[...]

pub type Declaration {
  FunctionDeclaration(Function)
  AliasDeclaration(TypeAlias)
  CustomTypeDeclaration(Type)
  PortDeclaration(Signature)
  InfixDeclaration(Infix)
  Destructuring(pattern: Node(Pattern), expression: Node(Expression))
}

pub opaque type Node(a) {
  Node(Range, a)
}
Success!

Development

gleam run   # Run the project
gleam test  # Run the tests

Running elm-syntax

There is a web app you can use to turn Elm source code into AST compatible with elm-syntax which is an inspiration for Elm AST in this project. The generated JSON isn’t prefect but it’s close enough to help figuring out some quirkier aspects.

cd tools/elm-syntax
elm reactor

Contributing: supporting more syntax

The flow for adding support for more Elm syntax is as follows:

1. Add a test case to test/transpile by adding two files, xyz.elm containing the input and xyz.expected.
2. If necessary, use tools/elm-syntax to inspect the corresponding elm-syntax AST.
3. If you need to add new tokens, add them in elm/lexer.gleam. Don’t forget about tests.
4. Implement the parser in elm/parser.gleam, adding tests where necessary.
5. Update the transpiler in transpile.gleam.
6. Run tests via gleam test. Iterate until they pass.