[Haskell-cafe] Re: Foralls in records

[Adde]

Matthew Brecknell <haskell <at> brecknell.org> writes:

> Since the concrete type has been forgotten, there's no way to get it
> back. You can't write a function that exposes the forgotten type, so
> getConnection is basically a lost cause. When you write "getConnection
> :: Transaction c", you are saying that the function is fully polymorphic
> in c. In other words, you are allowing the caller of getConnection to
> specify the type of connection. But you can't do that, because you
> already gave the connection a concrete type when you constructed the
> TransactionT. You might think you could get away with "getConnection ::
> Connection c => Transaction c", but this is still too polymorphic.
> 
> So what can you do? You can pattern-match on a TransactionT, provided
> you don't allow the existentially-quantified type to escape the scope of
> the pattern-match. In this case, all you know is that the type conforms
> to the Connection class, so you must use methods from the Connection
> class within that scope to consume the quantified type.


Thanks, using pattern matching to avoid mentioning the type didn't even cross 
my mind. 
You are correct in assuming that I thought I could get away 
with "getConnection :: Connection c => Transaction c". To be honest, I still 
don't understand why it's too polymorphic. To me it says that it'll return a 
Transaction parameterised by a type confirming to the Connection interface, 
even though the concrete type is long lost.