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POWhen (and when not) to define a Monad
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copied!<p>This is a question relating to the API design practises for defining your own Monad instances for Haskell libraries. Defining Monad instances seems to be a good way to isolate DSL's e.g. <code>Par</code> monad in monad-par, hdph; <code>Process</code> in distributed-process; <code>Eval</code> in parallel etc...</p> <p>I take two examples of haskell libraries, whose purpose is to IO with database backends. The examples I take are <a href="http://hackage.haskell.org/package/riak" rel="nofollow">riak</a> for Riak IO, and <a href="http://hackage.haskell.org/package/hedis" rel="nofollow">hedis</a> for Redis IO.</p> <p>In hedis, a <code>Redis</code> monad <a href="http://hackage.haskell.org/packages/archive/hedis/0.6.2/doc/html/Database-Redis.html#t:Redis" rel="nofollow">is defined</a>. From there, you run IO with redis as:</p> <pre><code>data Redis a -- instance Monad Redis runRedis :: Connection -> Redis a -> IO a class Monad m => MonadRedis m class MonadRedis m => RedisCtx m f | m -> f set :: RedisCtx m f => ByteString -> ByteString -> m (f Status) example = do conn <- connect defaultConnectInfo runRedis conn $ do set "hello" "world" world <- get "hello" liftIO $ print world </code></pre> <p>In riak, things are different:</p> <pre><code>create :: Client -> Int -> NominalDiffTime -> Int -> IO Pool ping :: Connection -> IO () withConnection :: Pool -> (Connection -> IO a) -> IO a example = do conn <- connect defaultClient ping conn </code></pre> <p>The documentation for <code>runRedis</code> says: <em>"Each call of runRedis takes a network connection from the Connection pool and runs the given Redis action. Calls to runRedis may thus block while all connections from the pool are in use."</em>. However, the riak package also implements connection pools. This is done without additional monad instances on top of the IO monad:</p> <pre><code>create :: Client-> Int -> NominalDiffTime -> Int -> IO Pool withConnection :: Pool -> (Connection -> IO a) -> IO a exampleWithPool = do pool <- create defaultClient 1 0.5 1 withConnection pool $ \conn -> ping conn </code></pre> <p>So, the analogy between the two packages boils down to these two functions:</p> <pre><code>runRedis :: Connection -> Redis a -> IO a withConnection :: Pool -> (Connection -> IO a) -> IO a </code></pre> <p>As far as I can tell, the hedis package introduces a monad <code>Redis</code> to encapsulate IO actions with redis using <code>runRedis</code>. In contrast the riak package in <code>withConnection</code> simply takes a function that takes a <code>Connection</code>, and executes it in the IO monad.</p> <p>So, what are the motivations for defining your own Monad instances and Monad stacks? Why has the riak and redis packages differed in their approach to this?</p>

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