[Haskell-cafe] Type level variants

[Shao Cheng]

You can use DataKinds extension and define a singleton type for
ReadOnly/WriteOnly. The code roughly looks like:

data ReadWrite = ReadOnly | WriteOnly

newtype Conn (a :: ReadWrite) = ...

openConn :: MonadIO m => Sing a -> m (Conn a)

In the implementation of openConn you can pattern match on Sing a and
recover ReadOnly/WriteOnly both on term/type level. The singletons package
provide utilities to ease writing this style of code.

On Tue, Feb 27, 2018, 10:56 PM Brian Hurt <bhurt at spnz.org> wrote:

> I'm looking for pointers on how to do something.  What I'm trying to do: I
> want to define a newtype wrapper for database connections with a phantom
> type to control whether the connection is read-only or read-write.  So I
> have:
>
>     newtype Conn a = Conn { unConn :: Connection }
>
>     data ReadOnly = ReadOnly
>
>     data ReadWrite = ReadWrite
>
>     -- Simplifying here
>     openConn :: MonadIO m => a -> Conn a
>
>     query :: (MonadIO m, ToRow r, FromRow s) => Conn a -> Query -> r -> m
> [s]
>
>     execute :: (MonadIO m, ToRow r) => Conn a -> Query -> r -> m Int64
>
> But I want to be able to restrict the type a to be either ReadOnly or
> ReadWrite.  Solutions I've come up with so far are:
>
> - Don't bother.  Later function calls put enough of constraint on the
> types that it isn't really necessary.  Or rather, ReadWrite is necessary,
> but ReadOnly isn't.
>
> - Define some type class that ReadWrite and ReadOnly implement, but don't
> export the body of the typeclass from the module, preventing other people
> from implementing it for other types.
>
> - Some sort of trickiness with closed type families that I haven't worked
> out yet.
>
> Are their alternatives I haven't considered yet?
>
> Thanks.
>
> Brian
>
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