Opt for Num and Fractional type, and more

[AliceRixte]

Hi there !

Thank you for this library, this is almost exactly what I needed.

However, it would be great if one could use Opt with Int (or any other Num type, or Rationaltype, and so on so forth).

This is possible by changing Opt to :

data Opt (def :: k) a where
  Def :: forall {k} def a. SingDef def a => Opt (def :: k) a
  Some :: forall {k} def a. a -> Opt (def :: k) a

and then define what the conversion function is for each kind via the class

class SingKind k => SingDef (def :: k) a where
  demoteDef :: a

What do you think ?

I made a PR #2 that showcases what I mean.

Cheers !

Alice

[AliceRixte]

Maybe a better path would be to do

class SingKind k => FromSing' (s :: k) a where
  fromSing' :: Sing (s :: k) -> a

instance Num a => FromSing' (s :: Nat) a where
  fromSing' s = fromIntegral $ fromSing s

instance {-# OVERLAPPING #-} FromSing' (s :: Symbol) Text where
  fromSing' s = fromSing s

instance (KnownSymbol s, IsString a) => FromSing' (s :: Symbol) a where
  fromSing' s = fromString $ symbolVal s

....

and then use fromSing' for definite

[AliceRixte]

Or even

class SingKind k => FromDemote k a where
  fromDemote :: Demote k -> a

[AliceRixte]

There is a simpler solution : simply use the Convertible class from the convertible package. See #2