From your blog post: Another solution is to use type synonyms to flip the parameter order around and write
type StateT' m s a = StateT s m a
That in combination with the TypeSynonymInstances extension would allow us to write instance MonadState (StateT' m),
This isn't true, actually; (StateT' m) isn't a valid instance even with TypeSynonymInstances; type synonyms must be fully applied. TypeSynonymInstances just lets you do things like
type List' x = (Int, [x]) instance Monoid (List' x) where mempty = (0, mempty) mappend (a,b) (c,d) = (a + c, b ++ d)
This is the same as if you wrote the "instance" line as
instance Monoid (Int, [x])
the compiler is just allowing you to put a fully applied type synonym
in the instance and applying it for you before creating the instance.
-- ryan
On Sun, Jan 11, 2009 at 1:40 PM, Martijn van Steenbergen
Hello everybody,
I think I'm finally beginning to understand how type synonym families [1] work, in the way that works best for me: running into a problem to which type synonyms offer a solution.
I wrote down my train of thoughts [2] and I was hoping you could give me some feedback: are there any mistakes in my conclusions? Did I miss any obvious corollaries? Are the points I make valid?
Also, I'm hoping it will be valuable to read for those wanting to understand type synonym families.
Thank you in advance,
Martijn.
[1] http://www.haskell.org/haskellwiki/GHC/Type_families#Detailed_definition_of_... [2] http://martijn.van.steenbergen.nl/journal/type-synonym-families/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe