Allowing duplicate instances in GHC 6.4

Keean Schupke k.schupke at
Fri Mar 25 11:06:42 EST 2005

Robert van Herk wrote:

> Hi all,
> I need to use duplicate instances. I read in the documentation on GHC 
> 6.4, that overlapping class instances checks are lazy instead of 
> gready in 6.4. However, my code still gives duplicate instance errors 
> when compiling in GHC 6.4.
> Is the duplicate instance check still gready? Is there a way to 
> overwrite that behaviour?
> Right now, I have two instance of a class Datasource. Datasource 
> allows the user to read (key,value) pairs.
> class Datasource ds k v where
> ...
> Now, I wanted to make a special datasource that combines two 
> datasources, namely
> data JoinedDS left right = JoinedDS left right
> instance (Datasource left k v) => Datasource (JoinedDS left right) k v 
> where
> ...
> instance (Datasource right k v) => Datasource (JoinedDS left right) k 
> v where
> ...
> The idea is that when you combine 2 datasources in one JoinedDS, the 
> user can read both types from the JoinedDS. I do not need to allow to 
> combine 2 different datasources that have the same types of 
> (key,value) pairs, so the duplicate instances will not occur and when 
> they do, this will be by mistake. Hence, the two premisses in the 
> instance declaration will never be fulfilled both at the same time and 
> I do not want a duplicate instance error here.
> Is there a  solution to this problem?
To resolve overlap the HEAD of the instance must be different... Might I 

-- as value depends on source and key, requires functional dependancy
class Datasource s k v | s k -> v ...
data JoinedDS l r = JoinedDS l r
instance (Datasource l k v1,Datasource r k v2) => Datasource (JoinedDS l 
r) k (v1,v2) ...

Now a joined datasource resturns a pair of values instead of a single value.


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