[Haskell-cafe] Re: Parametrized monads

Miguel Mitrofanov miguelimo38 at yandex.ru
Tue Jun 16 01:18:46 EDT 2009

And the third one I forgot at first: so far, all code I wrote could be  
easily ported to Hugs.

On 16 Jun 2009, at 02:00, Miguel Mitrofanov wrote:

> Probably.
> I have two objections against using type families. Both are pretty  
> much theoretical.
> First, it seems to me that using type families would require some  
> other extensions. Multi-parameter type classes are OK, but, in my  
> experience, the road from them to the darkness of undecidable  
> instances is quite short, and I don't feel very safe on these  
> grounds. Maybe it's just me, but I can't help imagining myself  
> adding just one element to the already long list, and the compiler  
> throwing tons of cryptic error messages on my head because the  
> derivation of necessary instances is just too long. It happened  
> before, and I still don't know how to prevent it.
> Secondly, type families are one thing that I have no idea of how to  
> abstract if needed. I know how to abstract a usual type class - by  
> passing the dictionary explicitly, for example; there are some more  
> sophisticated methods like higher-level monad - but it seems almost  
> impossible to abstract a class that can have an associated data  
> type. I don't see any reason to do this - yet; and for me,  "yet" is  
> enough to make me nervous.
> On 16 Jun 2009, at 01:25, Ashley Yakeley wrote:
>> Miguel Mitrofanov wrote:
>>> Suppose I want to create a specific monad as a combination of  
>>> monad transformers - something like "StateT smth1 (ReaderT smth2  
>>> Identity)". As you can see, each transformer is parametrized with  
>>> a type of kind *. I want to abstract these parameters, so that  
>>> instead of "StateT smth..." I can write something like
>>> Zip (ConsT StateT (ConsT ReaderT NilT)) (ConsA smth1 (ConsA smth2  
>>> NilA)) Identity
>>> and it would be a type isomorphic to the first one. I mean, I want  
>>> (ConsT StateT (ConsT ReaderT NilT))" to be a separate entity of  
>>> fixed kind, so that I can, say, create a class instance for it or  
>>> something.
>>> I'd be quite happy if list length appears as a separate parameter,  
>>> like
>>> Zip (Succ (Succ Zero)) (ConsT ...
>>> I would NOT be happy with something like
>>> Zip (List_2 StateT ReaderT) (Arg_2 smth1 smth2)
>>> If haskell had polymorphic kinds, I'd be able to do it easily;  
>>> unfortunately, it doesn't have them.
>> I think the "type families" extension can do this.
>> -- 
>> Ashley Yakeley
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