[Haskell-cafe] Type level variants

[Brian Hurt]

No, and thank you and astrohavoc at gmail.com for pointing it out.  I was
going "There *has to be* a better solution to this".

On Tue, Feb 27, 2018 at 3:10 PM, Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> On Tue, Feb 27, 2018 at 02:53:33PM +0000, Brian Hurt 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:
> [...]
> > Are their alternatives I haven't considered yet?
>
> Have you considered
>
>    {-# LANGUAGE DataKinds #-}
>
>    data Read = ReadOnly | ReadWrite
>
> ?
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