kujon / rascal

(Very) WIP implementation of a Haskell 98-like Lisp in Racket

Rascal

Rascal is an attempt to implement a Haskell-like language with support for Racket’s macro system, built using the techniques described in the paper Type Systems as Macros. It is currently extremely work-in-progress.

Here are some of the features that Rascal supports right now:

• Parametric polymorphism
• Algebraic datatypes (ADTs)
• Hindley-Milner type inference
• Typeclasses
• Type-aware/type-directed macros

Here are some of the features that I am currently working on:

• Kindchecking
• Exhaustiveness checking
• Type inference for recursive definitions
• Type expanders (of which type aliases are a subset)

And finally, here is a (non-exhaustive) collection of features I would like to eventually support:

• Multi-parameter typeclasses
• Functional dependencies
• Syntax for infix operators
• GADTs
• Higher-rank polymorphism
• Row types
• Type families

Due to the way Rascal is implemented, many things that are language features in Haskell can be derived concepts in Rascal. In fact, Rascal’s ADTs are not primitives, they are actually implemented as a library via the data and case macros in rascal/private/adt. Other things, like newtype deriving and generics, should be possible to implement as derived concepts as well.

Here’s what some Rascal code might eventually look like:

#lang rascal

(data (Maybe a)
  nothing
  (just a))

(def x : Int
  (let ([y 3]
        [z 7])
    {y + z}))

(class (Show a)
  [show : {Int -> a -> String}])

(instance (forall [a] (Show a) => (Show (Maybe a)))
  (def+ show
    [nothing  -> "nothing"]
    [(just x) -> {"just " <> (show x)}]))

Trying Rascal

To reiterate: Rascal is extremely experimental right now. Things are not guaranteed to work correctly (or work at all), and things are likely to change dramatically. If you really want to install Rascal to play around with it, though, you can.

You will need to have Racket installed to use Rascal. Using raco, you can install Rascal as a package:

$ raco pkg install rascal

Now you can use Rascal by writing #lang rascal at the top of a file. For an example of what Rascal syntax currently looks like, check out the client.rkt file for some simple examples.