[Haskell-cafe] Re: Equality constraints and RankNTypes - how do I assist type inference

[Sjoerd Visscher]

This works:

-- Tensor product of two vector spaces
type family Tensor u v :: *
type instance Tensor (Vect k a) (Vect k b) = Vect k (TensorBasis a b) 

On Aug 21, 2010, at 9:15 AM, DavidA wrote:

> Brandon S Allbery KF8NH <allbery <at> ece.cmu.edu> writes:
> 
>> 
>>> type Tensor u v =
>>>    (u ~ Vect k a, v ~ Vect k b) => Vect k (TensorBasis a b) -- **
>> 
>> IIRC this actually substitutes as
>> 
>>  (forall k a b. (u ~ Vect k a, v ~ Vect k b) => Vect k (TensorBasis a b))
>> 
>> and the implicit forall will generally mess things up because it won't be
>> floated out to the top level.  (Or in other words, constraints in type
>> declarations don't generally do what you intend.)
>> 
> 
> Thanks. Is it possible to elaborate a bit - I still don't really understand
> what goes wrong. And is there any way to fix it?
> 
> 
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--
Sjoerd Visscher
http://w3future.com