``Orphan instances'' should be avoided anyway.

Malcolm Wallace malcolm.wallace at cs.york.ac.uk
Wed Aug 13 04:29:03 EDT 2008

>  We already have named instances:
> data ShowDict alpha = ShowDict alpha {
>    namedShows :: alpha -> String -> String
>    }

This is not valid Haskell.  Either all components of the data  
structure are named fields, or none.  It is not possible to have a  
mixture of the two, as here.

In addition, there are many classes it is not currently possible to  
simulate using this technique, because the types go significantly  
beyond Haskell'98.  I believe you need both Rank-2 Types and  
Polymorphic Components to achieve comparable expressivity.  Fuller  
details on the Haskell-Prime site:


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