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coco's Issues

An arbitrary number of type variables

I've figured out a nice way to support an arbitrary number of type variables:

> :set -XDataKinds
> :set -XKindSignatures
> import Data.Proxy
> import Data.Typeable
> import GHC.TypeLits
> data TyVar (n :: Nat) = TyVar
> type A = TyVar 0
> let isTypeVar :: Typeable t => Proxy t -> Bool; isTypeVar p = fst (splitTyConApp (typeRep p)) == fst (splitTyConApp (typeRep (Proxy :: Proxy A)))
> isTypeVar (Proxy :: Proxy A)
True
> isTypeVar (Proxy :: Proxy Int)
False

The A, B, C, an D types can still be exported, but now if someone wants more they can have as many as they want.

Refinement symbol

There are two problems with the current syntax:

  1. -[- is not a valid Haskell operator name, because someone could do this:
instance Num a => Num [a] where
    fromIntegral n = [fromInteger n]
    a - b = [sum a - sum b]
  ...

> 3 -[- 6 ]
 9
  1. The real operator is actually read as "is refined by", not "refines". So it's backwards.

Merge type environments by unification, not equality

unifyAccum :: Bool -> (Maybe TypeEnv -> Maybe TypeEnv) -> [TypeRep] -> [TypeRep] -> Maybe TypeEnv
unifyAccum b f as bs = foldr go (Just []) (zip as bs) where
  go (tyA, tyB) (Just env) = do
    assignments <- f (unify' b tyA tyB)
    let env' = nub (env ++ assignments)
    guard $ all (\v -> length (filter ((==v) . fst) env') <= 1) tyvars
    pure env'
  go _ Nothing = Nothing

The nub / guard is too strict, it requires that type variables have equal assignments, rather than just unifying assignments. A better solution is a unifyTypeEnvs :: TypeEnv -> TypeEnv -> Maybe TypeEnv function which finds all clashes and attempts to unify them, continuing recursively if this introduces new constraints.

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