Restricting maybewires when inserting and updating

[bergmark]

Using the To type family discussed in #11 I've specialized inserts and updates to be

myInsert
  :: ( Default TableMaybeWrapper wires maybewires
     , Default (PPOfContravariant Assocer) maybewires maybewires
     , maybewires ~ To Maybe wires -- Added
     )
  => Connection -> Table wires -> Expr maybewires -> IO ()

myUpdate
  :: ( Default TableExprRunner wires wires'
     , Default (PPOfContravariant Assocer) maybewires maybewires
     , Default TableMaybeWrapper wires maybewires
     , maybewires ~ To Maybe wires -- Added
     )
  => Connection -> Table wires -> ExprArr wires' maybewires -> ExprArr wires' (Wire Bool) -> IO Int64

This improves type inference and seems to always be what I want. Assuming To would make it into opaleye, is there any reason to not restrict these functions?

[bergmark]

With this you don't need to add a type signature around MakeExpr calls.

[tomjaguarpaw]

Ah this would be very helpful! What are the consequences for the API consumer? Does it force them to enable any extensions?

[bergmark]

Nope!

[tomjaguarpaw]

Sounds very cool. Is it possible to substitute To Maybe wires for maybewires in the function signature, or does the equivalence have to be put in the context like that?

[bergmark]

I thought no, but it seems like it works. It's requires replacing maybewires with To Maybe wires everywhere, but I think it's a good change.

[bergmark]

Oh scratch the last part, it's possible to change it on just the top level, just forgot to replace one of the maybewires when I tried it out.

[bergmark]

You still need to type all Nothings inside your type, but you can add a type signature for the entire record for this so you don't need to type each field explicitly, i.e. both fields in User below would be ambiguous without the type signature for u:

insertUser = myInsert userTable $ makeMaybeExpr u
  where
    u :: To Maybe User
    u = User Nothing (Just 1)