You were on the right track with your first approach. The only issue is
that Haskell requires that all type synonyms are fully saturated. You
defined your type synonym to take two parameters, `m` and `r`, so
Haskell complained when you only partially applied it to just `m` in the
type for `processHandshake`.
Fortunately, the solution is really simple: just leave out the `r` in
the definition of `ServerState`:
type ServerState m = Parser ByteString (ErrorT
ServerPipelineException m)
That's equivalent, but more generally useful, since you can now apply it
to only `m`.
This is also the same reason that `pipes` defines `Producer` like this:
type Producer b = Proxy X () () b
... and not like this:
type Producer b m r = Proxy X () () b m r
On 7/1/14, 10:46 PM, Kyle Van Berendonck wrote:
Hi,
I'm using mtl and pipes-parse.
I have the following:
type ServerState m r = Parser ByteString (ErrorT
ServerPipelineException m) r
When I try and put it in a pipe though, like so:
processHandshake :: Monad m => Pipe InboundPacketHandshake
OutboundPacketHandshake (ServerState m) ()
I get this error:
Type synonym ‘ServerState’ should have 2 arguments, but has been
given 1
In the type signature for ‘processHandshake’:
processHandshake :: Monad m =>
Pipe InboundPacketHandshake
OutboundPacketHandshake (ServerState m) ()
But, if I bring the () inside the ServerState, I get:
The third argument of ‘Pipe’ should have kind ‘* -> *’,
but ‘ServerState m ()’ has kind ‘*’
In the type signature for ‘processHandshake’:
processHandshake :: Monad m =>
Pipe InboundPacketHandshake
OutboundPacketHandshake (ServerState m ())
I'm confused. How do I layer arbitrary numbers of transformers inside
pipes?
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