I agree that type classes should not be used for this purpose. That's
part of the reason I support "linguistic overloading" -- to stop the
type class abuse.
Type classes should be used, as you say, when there are algorithms
that work for arbitrary members -- i.e. the type class encodes
structure and associated properties.
Regards,
John A. De Goes
N-BRAIN, Inc.
The Evolution of Collaboration
http://www.n-brain.net | 877-376-2724 x 101
On Feb 19, 2009, at 7:33 AM, Luke Palmer wrote:
On Thu, Feb 19, 2009 at 7:09 AM, John A. De Goes <j...@n-brain.net>
wrote:
On Feb 14, 2009, at 2:29 PM, Luke Palmer wrote:
To me, typeclasses are at their best when you have a real
abstraction to encode.
I agree.
If you are having trouble using a typeclass and need C++-style ad-
hoc overloading, it's likely you are trying to encode a "fake"
abstraction -- one that has only linguistic, rather than
mathematical meaning.
I don't think what you're calling a "linguistic" abstraction is
"fake".
Please ignore the word "fake". I don't want to get into any
subjective arguments based on the connotation of that word.
What I mean to say is, the theory of typeclasses is good at encoding
mathematical abstractions, and bad at encoding linguistic ones.
Take that as you will, but I conjecture that trying to cram
linguistic overloading into a typeclass is generally going to be
painful.
A good rule of thumb is: are there any algorithms which work for an
arbitrary member of this class? I certainly cannot see any for your
flatten example.
I'm not saying that linguistic overloading is a bad thing. You make
good arguments for it, and I find it cleans up code sometimes.
Typeclasses just aren't the right tool for it, and Haskell has no
good tool for it.
In fact, I think it a very interesting research question to come up
with a mechanism that supports linguistic overloading, and interacts
with typeclasses and inference cleanly. The obvious solution (just
look in your namespace for one that matches) has serious drawbacks,
and nothing else is jumping to mind.
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