haskell - Typeably casting GADTs -

let's i'm writing dsl , want have support both phantom type support , badly typed expressions. value types might be

{-# language gadts, datakinds #-}  data ty = num | bool deriving (typeable)  data val   vnum  :: int  -> val num   vbool :: bool -> val bool

and can work phantom erased version

{-# language existentialquantification #-}  data valunk = forall . valunk (v' a)

now, can operate on values of valunk caseing out both vnum , vbool , reestablish phantom types in way

getnum :: valunk -> maybe (val num) getnum (valunk n@(vnum _)) = n getnum _                   = nothing

but feels i'm reimplementing typeable machinery. unfortunately, ghc won't let me derive typeable val

src/types/core.hs:97:13:     can't make derived instance of `typeable (val a)':       val must have arguments of kind `*'     in data declaration val

is there way around restriction? i'd love write

getit :: typeable => valunk -> maybe (val a) getit (valunk v) = cast v

but right have resort machinery this

class typeably b x kast :: x -> maybe (x b) instance typeably num val    kast n@(vnum _) = n   kast _          = nothing

for of types.

first of all, need store witness quantified type in valunk in typeable:

data valunk = forall . typeable => valunk (val a)

once have this, can use gcast achieve you're asking for:

getit :: typeable => valunk -> maybe (val a) getit (valunk v) = gcast v

this tested with:

data val   vnum  :: int  -> val int   vbool :: bool -> val bool

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