Defaulting.md

(Quick notes written up in response to a discussion on defaulting in GHC.)

Though I argued that implicitly imported defaults should be opt-in, I have a use case for them in this library. It makes heavy use of "custom literals", defined thus:

class IsPitch a where
  c :: a
  d :: a
  -- etc

This is basically "IsString on steroids". Compare:

2   :: Num a => a
"x" :: IsString a => a
c   :: IsPitch a => a

There are various instances for IsPitch, with different trade-offs in terms of expressivity. Notably we also have container instances such as

instance IsPitch a => IsPitch (Chord a) where
   ... 
instance IsPitch a => IsPitch (Score a) where
   ... 

This allow us to build polymorphic music expressions like the below:

-- Two notes at the same time (a chord)
c <> d :: (Monoid a, IsPitch a) => a

-- Two notes in sequence (a melody)
c |> d :: (Monoid a, HasPosition a, IsPitch a) => a

--  A melody with dynamics
foldr1 (|>) [c,d,level pp e] :: (Monoid a, HasPosition a, IsPitch a, HasDynamics a) => a

Ambiguity errors occur when we combine these expresisons with monomorphizing functions. For example we have a function called defaultMain :: (IsPitch a, ...) => Score a -> IO (), designed to be used with main like this:

main = defaultMain (c |> d)

main = defaultMain (c :: Pitch)

main = defaultMain (c :: Score Pitch)

This is in a way an encoding allowing more than one type for main, which is supported by e.g. Elm.