[Haskell-cafe] Could Haddock export documentation with type family applications normalised?
ollie at ocharles.org.uk
Tue May 3 10:20:50 UTC 2016
I'm working on a library that uses quite a lot of type "magic" (as some
would call it), but really it's all just implementation details. The type
families are necessary for me to write the code, but an outside
understanding of these type families shouldn't be necessary to understand
the library. Unfortunately, Haddock does not align with that goal. To give
you a feeling for things, take the following types:
data Expr (t :: k)
data AsHaskell (t :: k)
data BaseType = DBInt | DBText
type family Col (f :: k -> *) (a :: k) :: *
type instance Col Expr (a :: BaseType) = Expr a
type instance Col AsHaskell (a :: BaseType) = BaseTypeAsHaskell a
type family BaseTypeAsHaskell (bt :: BaseType) :: * where
BaseTypeAsHaskell 'DBInt = Int
BaseTypeAsHaskell 'DBText = String
class Lit (exprType :: k) where
lit :: Col AsHaskell exprType -> Expr exprType
instance Lit 'DBInt where lit = ...
instance Lit 'DBText where lit = ...
(I am modelling the interaction with remote relational databases, to
provide a little more context)
Now when I export this, I end up with the following:
Fine, not much needs to change there. Haddock now also gives me specialised
types for instances, but these don't go far enough:
Lit BaseType DBInt
lit :: Col (TYPE Lifted) DBInt (AsHaskell DBInt) exprType
-> Expr DBInt exprType
But this is considerably less helpful:
First, it seems to be broken, as it mentions the exprType variable, when is
Secondly, it mentions all sorts of kinds which are, in my opinion,
unreadable in the current form.
All of these problems would be solved if we just normalised this type
family application as far as possible. What I *really* want to export is:
lit :: Int -> Expr DBInt
This is the type I want my users to be aware of.
Has this been discussed before? If not, how do people feel about this?
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