type-sets

Dedication

Obstacles don't have to stop you. If you run into a wall, don't turn around and give up. Figure out how to climb it, go through it, or work around it.

-- Michael Jordan, on complexity analysis in Haskell

Overview

How much do you hate programming at the type level, but only being able to use lists? A million? Two million? Some mathematicians suspect that there may be even larger degrees of hate.

Enter type-sets. They're sets... at the type-level! Check this:

{-# OPTIONS_GHC -fplugin=Type.Compare.Plugin #-}

import Type.Set

type MySet = Insert Bool (Insert String (Insert (Maybe Int) 'Empty))

test1 :: Proxy (Member Bool MySet) -> Proxy 'True
test1 = id  -- Bool is a member :)

test2 :: Proxy (Member Char MySet) -> Proxy 'False
test2 = id  -- Char is not a member :(

See the Type.Set module for more operations.

But wait, there's more! There's also a proof-of-concept Type.Set.Variant for describing open-sums indexed by type-sets. PRs to make it better would be greatly appreciated!

One love.

Acknowledgments

Huge shout-outs to Boris Rozinov for all of his help on this library, including the original idea, help with the implementation, infinite amounts of coffee, and a couch to sleep on while it was being made.