#9301: Rank-2 constraints are assigned kind * -------------------------------------+------------------------------------ Reporter: heisenbug | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Description changed by heisenbug: Old description:

Compile this program with GHC-HEAD (I have 7.9.20140605, but I guess 7.8.3 will also do): {{{ {-# LANGUAGE RankNTypes #-}

strangelet :: (forall t . Num t) => () strangelet = \_ -> () }}}

Very strangely it gets accepted! {{{ *Main> :t strangelet strangelet :: (forall t. Num t) -> () }}} I suspect that the type checker assigns kind `*` to rank-2 constraints. Of course it should assign a `Constraint` kind.

What I ultimately want is to declare: {{{ foo :: (Monad m, forall n . Num (m n)) => m n foo = ... }}} ..or such. Would that be unreasonable?

NB: 7.4.2 rejects it (even with `-XConstraintKinds`). I did not try 7.6.3.

New description: Compile this program with GHC-HEAD (I have 7.9.20140605, but I guess 7.8.3 will also do): {{{ {-# LANGUAGE RankNTypes #-} strangelet :: (forall t . Num t) => () strangelet = \_ -> () }}} Very strangely it gets accepted! {{{ *Main> :t strangelet strangelet :: (forall t. Num t) -> () }}} I suspect that the type checker assigns kind `*` to rank-2 constraints. Of course it should assign a `Constraint` kind. What I ultimately want is to declare: {{{ foo :: (Monad m, forall n . Num (m n)) => m n foo = ... }}} ...or such. Would that be unreasonable? NB: 7.4.2 rejects it (even with `-XConstraintKinds`). I did not try 7.6.3. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9301#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9301: Rank-2 constraints are assigned kind * -------------------------------------+------------------------------------ Reporter: heisenbug | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by heisenbug): Ooops, just repeated this experiment with 7.9.20140712 and now I get: {{{ strangelet.hs:3:16: Illegal constraint: forall t. Num t In the type signature for ‘strangelet’: strangelet :: (forall t. Num t) => () Failed, modules loaded: none. }}} So that type gets rejected. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9301#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9301: Rank-2 constraints are assigned kind * -------------------------------------+------------------------------------ Reporter: heisenbug | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): The trouble is that I don't know how to do type inference in the presence of polymorphic constraints. It is not a small change. It would be a useful feature. I've wanted it since at least 2000 (see [http://research.microsoft.com/en-us/um/people/simonpj/papers/derive.htm Derivable Type Classes]). Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9301#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler