[Haskell-cafe] how to avoid overlapping instance error?

[S. Alexander Jacobson]

If you are not using them to prevent overlapping instances, then why 
require instance decls at all?  For example, why does 
GHC require an instance decl here:

   instance (Ord x)=>ToSet [] x where toSet = Set.fromList

But not here:

   listToSet x = Set.fromList x

Or I suppose, one could rephrase this question as why not 
simplify instance declarations to be:

   instance ToSet where
      toSet = Set.fromList

And let the typechecker take care of figuring out what instance is 
being specified here?

-Alex-

______________________________________________________________
S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com


On Mon, 28 Feb 2005, Simon Peyton-Jones wrote:

> Unfortunately, the context of an instance decl is not taken into account
> when matching instance decls.  Yes, it would make sense to do so, but
> it'd make the system yet more complicated.
>
> So Show (table item) overlaps with Show ([] item)
>
> Overlap is checked lazily, so if you look for Show (MyTable t) you won't
> get overlap. Only if you ask for Show (table item), where the type
> checker still doesn't know what type 'table' is going to be, do you get
> a problem.
>
> Simon
>
> | -----Original Message-----
> | From: haskell-cafe-bounces at haskell.org
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of S.
> | Alexander Jacobson
> | Sent: 25 February 2005 17:09
> | To: haskell-cafe at haskell.org
> | Subject: [Haskell-cafe] how to avoid overlapping instance error?
> |
> |
> | Currently, the HAppS.DBMS lib requires the user to provide a Show
> | instance for their table types.  An example might be:
> |
> |     instance (Show item) => Show (MyTable item) where
> |         showsPrec d table = showsPrec d $ Set.toList $ myTableSet
> table
> |
> | But the Table class itself defines a toSet function so I think I
> | should be able to do this once for all Table instances:
> |
> |     instance (Show item,Ord item, Table table item p) => Show (table
> item) where
> |         showsPrec d table = showsPrec d $ Set.toList $ toSet table
> |
> | But I get an error telling me I am overlapping with Show [a].
> |
> | Since [] is not an instance of Table, I don't see why there should be
> | an overlap.
> |
> | -Alex-
> |
> | ______________________________________________________________
> | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com
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