Changes between Version 2 and Version 3 of TFvsFD


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Timestamp:
Jun 5, 2012 7:20:31 PM (23 months ago)
Author:
illissius
Comment:

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  • TFvsFD

    v2 v3  
    8282}}} 
    8383 
    84 Type synonyms can manipulate constraint kinds but can not use 
    85 them. The standard encoding of the above class from FD to 
    86 superclass equalities does not work. 
     84The standard encoding of the above class from FD to 
     85superclass equalities does not work because type instances 
     86can't have type variables on the RHS which weren't introduced 
     87in the LHS: 
    8788 
    8889{{{ 
    8990class (f :<$>: a) ~ b =>  FMap (f :: * -> * -> Constraint) a b 
    9091  where type f :<$>: a 
    91 instance f a b => FMap f (HJust a) b 
    92   where type f :<$>: (HJust a) = HJust b 
     92instance f a b => FMap f (HJust a) (HJust b) 
     93  where type f :<$>: (HJust a) = HJust b -- error, b not available 
    9394}}} 
    9495