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 ?

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 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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