Re: [Haskell-cafe] More liberal than liberal type synonyms
On Wed, Dec 07, 2011 at 04:47:47PM +0100, Gábor Lehel wrote: On Wed, Dec 7, 2011 at 1:07 PM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. Thank you, Yves! It is now more clear for me. Still, it seems that ability to use partially applied type synonyms would be very natural (and useful) extension to the language. It would allow to avoid boilerplate code associated with creating really new types instead of just using synonims for existing ones. The problem as I understand it is that partially-applied type synonyms are in effect type level lambdas, and type checking in the presence of type level lambdas requires higher-order unification, which is undecidable in general. Restricted cases might be possible, I'm not an expert in the field. GHC hackers could probably elaborate. [1] http://stackoverflow.com/questions/8248217/are-there-type-level-combinators-will-they-exist-in-some-future [2] http://en.wikipedia.org/wiki/Unification_(computer_science)#Higher-order_unification It's actually type *inference* that requires higher-order unification in the presence of type-level lambdas, not type checking. This might not be a huge deal: we just have to clearly state that enabling -XPartialTypeFunApps means that you may have to provide some explicit type annotations in places where type inference cannot figure things out. We already have extensions like this (e.g. RankNTypes). The bigger problem for the moment is that for various technical reasons, enabling partial applications of type functions can lead to unsoundness (i.e. typechecked programs which nonetheless crash at runtime) in the way that type equality is handled. For more details see http://stackoverflow.com/questions/7866375/why-does-ghc-think-that-this-type-variable-is-not-injective/7950614#7950614 I agree that the ability to use partially applied type synonyms/functions would be natural and useful. I hope we will eventually see a version of GHC which supports it but there are some nontrivial technical issues to be worked out first. -Brent On Wed, Dec 7, 2011 at 3:51 PM, Yves Parès limestr...@gmail.com wrote: Ah, maybe Dan could tell us if it works only with GHC 7. Dmitry, I had your problem many times. The last time was when I saw you could define the ContT monad in terms of Cont (the opposite is done in the mtl). It leads to a simpler code, but you are stucked when trying to define ContT as an instance of MonadTrans: data Cont r a = ... -- [instances of Monad Cont, blah blah blah] type ContT r m a = Cont r (m a) instance MonadTrans (ContT r) where -- This doesn't compile, even if it is logical lift = ... For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. 2011/12/7 Dmitry Kulagin dmitry.kula...@gmail.com Dmitry, does your code work with LiberalTypeSynonyms extention activated? No, the same error: Type synonym `StateA' should have 1 argument, but has been given 0 But I have GHC 6.12.3 Dmitry 2011/12/7 Yves Parès limestr...@gmail.com: This is impossible: in the definition of 'StateT s m a', m must be a monad and then have the * - * kind. So you cannot pass (StateA a), because it has simply the * kind. Dmitry, does your code work with LiberalTypeSynonyms extention activated? 2011/12/7 Øystein Kolsrud kols...@gmail.com You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int
Re: [Haskell-cafe] More liberal than liberal type synonyms
Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.comwrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Thank you for reply, but this variant actually does not compile: StateA and (StateA a) have different kinds. Dmitry Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
This is impossible: in the definition of 'StateT s m a', m must be a monad and then have the * - * kind. So you cannot pass (StateA a), because it has simply the * kind. Dmitry, does your code work with LiberalTypeSynonyms extention activated? 2011/12/7 Øystein Kolsrud kols...@gmail.com You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.comwrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
Dmitry, does your code work with LiberalTypeSynonyms extention activated? No, the same error: Type synonym `StateA' should have 1 argument, but has been given 0 But I have GHC 6.12.3 Dmitry 2011/12/7 Yves Parès limestr...@gmail.com: This is impossible: in the definition of 'StateT s m a', m must be a monad and then have the * - * kind. So you cannot pass (StateA a), because it has simply the * kind. Dmitry, does your code work with LiberalTypeSynonyms extention activated? 2011/12/7 Øystein Kolsrud kols...@gmail.com You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
Ah, maybe Dan could tell us if it works only with GHC 7. Dmitry, I had your problem many times. The last time was when I saw you could define the ContT monad in terms of Cont (the opposite is done in the mtl). It leads to a simpler code, but you are stucked when trying to define ContT as an instance of MonadTrans: data Cont r a = ... -- [instances of Monad Cont, blah blah blah] type ContT r m a = Cont r (m a) instance MonadTrans (ContT r) where -- *This doesn't compile*, even if it is logical lift = ... For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. 2011/12/7 Dmitry Kulagin dmitry.kula...@gmail.com Dmitry, does your code work with LiberalTypeSynonyms extention activated? No, the same error: Type synonym `StateA' should have 1 argument, but has been given 0 But I have GHC 6.12.3 Dmitry 2011/12/7 Yves Parès limestr...@gmail.com: This is impossible: in the definition of 'StateT s m a', m must be a monad and then have the * - * kind. So you cannot pass (StateA a), because it has simply the * kind. Dmitry, does your code work with LiberalTypeSynonyms extention activated? 2011/12/7 Øystein Kolsrud kols...@gmail.com You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. Thank you, Yves! It is now more clear for me. Still, it seems that ability to use partially applied type synonyms would be very natural (and useful) extension to the language. It would allow to avoid boilerplate code associated with creating really new types instead of just using synonims for existing ones. On Wed, Dec 7, 2011 at 3:51 PM, Yves Parès limestr...@gmail.com wrote: Ah, maybe Dan could tell us if it works only with GHC 7. Dmitry, I had your problem many times. The last time was when I saw you could define the ContT monad in terms of Cont (the opposite is done in the mtl). It leads to a simpler code, but you are stucked when trying to define ContT as an instance of MonadTrans: data Cont r a = ... -- [instances of Monad Cont, blah blah blah] type ContT r m a = Cont r (m a) instance MonadTrans (ContT r) where -- This doesn't compile, even if it is logical lift = ... For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. 2011/12/7 Dmitry Kulagin dmitry.kula...@gmail.com Dmitry, does your code work with LiberalTypeSynonyms extention activated? No, the same error: Type synonym `StateA' should have 1 argument, but has been given 0 But I have GHC 6.12.3 Dmitry 2011/12/7 Yves Parès limestr...@gmail.com: This is impossible: in the definition of 'StateT s m a', m must be a monad and then have the * - * kind. So you cannot pass (StateA a), because it has simply the * kind. Dmitry, does your code work with LiberalTypeSynonyms extention activated? 2011/12/7 Øystein Kolsrud kols...@gmail.com You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] More liberal than liberal type synonyms
On Wed, Dec 7, 2011 at 1:07 PM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. Thank you, Yves! It is now more clear for me. Still, it seems that ability to use partially applied type synonyms would be very natural (and useful) extension to the language. It would allow to avoid boilerplate code associated with creating really new types instead of just using synonims for existing ones. The problem as I understand it is that partially-applied type synonyms are in effect type level lambdas, and type checking in the presence of type level lambdas requires higher-order unification, which is undecidable in general. Restricted cases might be possible, I'm not an expert in the field. GHC hackers could probably elaborate. [1] http://stackoverflow.com/questions/8248217/are-there-type-level-combinators-will-they-exist-in-some-future [2] http://en.wikipedia.org/wiki/Unification_(computer_science)#Higher-order_unification On Wed, Dec 7, 2011 at 3:51 PM, Yves Parès limestr...@gmail.com wrote: Ah, maybe Dan could tell us if it works only with GHC 7. Dmitry, I had your problem many times. The last time was when I saw you could define the ContT monad in terms of Cont (the opposite is done in the mtl). It leads to a simpler code, but you are stucked when trying to define ContT as an instance of MonadTrans: data Cont r a = ... -- [instances of Monad Cont, blah blah blah] type ContT r m a = Cont r (m a) instance MonadTrans (ContT r) where -- This doesn't compile, even if it is logical lift = ... For short, type synonyms work for mere aliases, but not for full-fledged type-level non-inductive functions. And sometimes we intuitively want to use them as such. 2011/12/7 Dmitry Kulagin dmitry.kula...@gmail.com Dmitry, does your code work with LiberalTypeSynonyms extention activated? No, the same error: Type synonym `StateA' should have 1 argument, but has been given 0 But I have GHC 6.12.3 Dmitry 2011/12/7 Yves Parès limestr...@gmail.com: This is impossible: in the definition of 'StateT s m a', m must be a monad and then have the * - * kind. So you cannot pass (StateA a), because it has simply the * kind. Dmitry, does your code work with LiberalTypeSynonyms extention activated? 2011/12/7 Øystein Kolsrud kols...@gmail.com You should be able to write something like this: type StateB a b = StateT SomeOtherState (StateA a) b Best regards, Øystein Kolsrud On Wed, Dec 7, 2011 at 11:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: Hi Dan, I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I am sorry, if this is offtopic, but it seemed to me that the problem is realted to partially applied type synomyms you described. Thanks! Dmitry On Tue, Dec 6, 2011 at 10:59 PM, Dan Doel dan.d...@gmail.com wrote: Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mvh Øystein Kolsrud ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org
Re: [Haskell-cafe] More liberal than liberal type synonyms
On Wed, Dec 7, 2011 at 5:48 AM, Dmitry Kulagin dmitry.kula...@gmail.com wrote: I am still pretty new in Haskell, but this problem annoys me already. If I define certain monad as a type synonym: type StateA a = StateT SomeState SomeMonad a Then I can't declare new monad based on the synonym: type StateB a = StateT SomeOtherState StateA a The only way I know to overcome is to declare StateA without `a': type StateA = StateT SomeState SomeMonad But it is not always possible with existing code base. I'm afraid my proposal doesn't make this work. You could perhaps define StateB, but when you expand in a type you get: StateB a = StateT SomeOtherState StateA a which has a partially applied StateA, and is rejected. The only way to make this work is to eta reduce StateA manually, or make GHC recognize when a synonym can be eta reduced in this way (which might be both possible and useful as a separate proposal). My extension fell within the liberal type synonym space, which says that if you have: F G where F and G are both synonyms, and G is partially applied, then it is okay as long as expansion of F (and any subsequent expansions) cause G to become fully applied. My extension of this is just to allow partial application inside aliases as long as it meets these same criteria. The reason to disallow partially applied type aliases is that they make inference pretty much impossible, unless you only infer them in very limited circumstances perhaps. And if you can't get inference of them, you probably need to start having explicit annotations to tell the type checker what you want to happen, which has some of its own complications with the way quantifiers work in GHC and such. It'd cascade into some thorny issues. Hopefully that covers all the other subsequent stuff folks have been talking about. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] More liberal than liberal type synonyms
Greetings, In the process of working on a Haskell-alike language recently, Ed Kmett and I realized that we had (without really thinking about it) implemented type synonyms that are a bit more liberal than GHC's. With LiberalTypeSynonyms enabled, GHC allows: type Foo a b = b - a type Bar f = f String Int baz :: Bar Foo baz = show because Bar expands to saturate Foo. However, we had also implemented the following, which fails in GHC: type Foo a b = b - a type Bar f = f (Foo Int) (Foo Int) type Baz f g = f Int - g Int quux :: Bar Baz quux = id That is: type synonyms are allowed to be partially applied within other type synonyms, as long as similar transitive saturation guarantees are met during their use. I don't know how useful it is, but I was curious if anyone can see anything wrong with allowing this (it seems okay to me after a little thought), and thought I'd float the idea out to the GHC developers, in case they're interested in picking it up. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
