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.doel@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
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