Re: [GHC] #9607: Programs that require AllowAmbiguousTypes in 7.8

24 Feb 2015

      #9607: Programs that require AllowAmbiguousTypes in 7.8
-------------------------------------+-------------------------------------
        Reporter:  jstolarek         |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.8.3
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
-------------------------------------+-------------------------------------

Comment (by trevorcook):

 My apologies if I am off base, but I think I have an example.

 I'm making a visualization server that handles the rendering of 3d*
 graphics. The server listens for messages regarding "entity" primitives
 and current point of view. For every entity it adds it handshakes to the
 client with a unique id, so that the client can update the entity if
 needed. The basic service deals with primitive shapes, and the idea is to
 develop client services which can translate between higher level
 abstractions and the visualization primitives. For example, a plotting
 utility might be a client which exposes an "axes" primitive, which in turn
 relates to a set of visualization primitives used for tic marks, axis
 lines, and labels.

 I'm organizing the architecture to be based on the below Client and Server
 type classes. Clients create commands and further processing based on the
 responses to the commands. Servers listen to commands, do something, and
 eventually return responses. The classes attempt to separate out the key
 protocol/visualization components from the actual implementation. The
 forwarding action below pastes together clients and servers, and will
 hopefully be reusable for making concrete services based on different
 messaging technologies, and for implementations for different
 visualization domains. It seems to require AmbiguousTypes, though I don't
 understand why.

 {{{#!haskell
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}

 module CSFwd  where

 import VisServ.Base
 import Data.Monoid (mconcat,Monoid)

 -- | Imported from VisServ.Base
 --type family ComResp cmd

 -- |triggered by some data, trig, a client request yields a list of
 commands, cmd,
 -- and functions which consume the resulting response.
 class ClientAction cmd trig a where
   clientRequest :: trig -> [(cmd, ComResp cmd -> a)]

 -- |triggered by some command, cmd, a server does some reaction, react,
 and
 -- given subsequent data, a, (presumably somehow derived from react) will
 -- respond to the command, ComResp cmd.
 class ServerAction cmd react a where
   serverResponse ::  cmd -> (react, a -> ComResp cmd)

 -- The idea with forwarding action hinges to a large part around the
 server response
 -- (a->c->d). The idea being that whatever implementation of this server
 will have
 -- some data, a, available which needs to be combined with, c, to yield
 data, d,
 -- necessary for a response to the original metaCmd request. The data, c,
 can be
 -- created based on replys to visualization commands  So, given the input
 functions
 -- which create the client trigger and initial data b, this function
 handles the
 -- forwarding of visualization commands based on some other commands.
 forwardingAction :: forall a b c d trig metaCmd . ( Monoid c
                     , ServerAction metaCmd (a->c->d) d
                     , ClientAction VisCom trig (b->c) )
                  =>  (metaCmd -> a -> trig) -> (metaCmd -> b)
                  -> metaCmd -> a
                  -> ([VisCom],[VisResp] -> d, d -> ComResp metaCmd)
 forwardingAction toTrig tob metaCmd a = (cliCmds,servReact',cmdResp)
   where
     (servReact::(a->c->d),cmdResp::d -> ComResp metaCmd) = serverResponse
 metaCmd
     cmdActs :: [(VisCom,VisResp -> b -> c)]
     cmdActs = clientRequest . toTrig metaCmd $ a
     (cliCmds,replResps) = unzip cmdActs
     b = tob metaCmd
     replResp' :: [VisResp] -> c
     replResp' resps = mconcat $ zipWith (\f resp-> f resp b) replResps
 resps
     servReact' :: [VisResp] -> d
     servReact' resps = servReact a (replResp' resps)

 }}}

 Error:

 {{{
 src/CSFwd.hs:33:21:
     Could not deduce (ClientAction VisCom trig (b -> c0))
       arising from the ambiguity check for ‘forwardingAction’
     from the context (Monoid c,
                       ServerAction metaCmd (a -> c -> d) d,
                       ClientAction VisCom trig (b -> c))
       bound by the type signature for
                  forwardingAction :: (Monoid c,
                                       ServerAction metaCmd (a -> c -> d)
 d,
                                       ClientAction VisCom trig (b -> c))
 =>
                                      (metaCmd -> a -> trig)
                                      -> (metaCmd -> b)
                                      -> metaCmd
                                      -> a
                                      -> ([VisCom], [VisResp] -> d, d ->
 ComResp metaCmd)
       at src/CSFwd.hs:(33,21)-(38,67)
     The type variable ‘c0’ is ambiguous
     In the ambiguity check for:
       forall a b c d trig metaCmd.
       (Monoid c, ServerAction metaCmd (a -> c -> d) d,
        ClientAction VisCom trig (b -> c)) =>
       (metaCmd -> a -> trig)
       -> (metaCmd -> b)
       -> metaCmd
       -> a
       -> ([VisCom], [VisResp] -> d, d -> ComResp metaCmd)
     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
     In the type signature for ‘forwardingAction’:
       forwardingAction :: forall a b c d trig metaCmd. (Monoid c,
                                                         ServerAction
 metaCmd (a -> c -> d) d,
                                                         ClientAction
 VisCom trig (b -> c)) =>
                           (metaCmd -> a -> trig)
                           -> (metaCmd -> b)
                              -> metaCmd -> a -> ([VisCom], [VisResp] -> d,
 d -> ComResp metaCmd)

 }}}

 *Technically not real 3D. I'm using some projective transforms to create
 2D vector graphics based on the diagrams front end. I use a simple
 ordering over the transformed shapes to determine the order they are glued
 to the page.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9607#comment:7
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler