style questino: where to put instances

[Hal Daume III]

I know you're all probably pretty tired of hearing from me, but I have a
style question.

Let's say I'm defining a hashmap.  I have a module HashMap which defines a
class Hashable a where hash :: a -> Int or something like that.  I have a
common datastructure called a Foo which I want to be able to hash.  Foo
and a bunch of accompanying functions are defined in module Foo.  Where
should I put the instance Hashable Foo?

(1) In Foo.hs
    pros : seems somewhat appropriate since it's basically defining
               a function on Foo and if I don't export the constructors
               for Foo, this is the only place it can go
    cons : means i have to import HashMap into Foo even though I'm not
               actually using a HashMap.  Sure I can only import the
               part I need.

(2) In HashMap.hs
    pros : when you import HashMap you get all necessary instances
    cons : Foo is application specific, HashMap is not; reduces
               reusability

    <-- I don't like option (2)

(3) In Instances.hs  -- i.e., create a new file for instances
    pros : modular
    cons : what if FOo doesn't export constructors?  requires
               importing lots of files which often slows down
               compilation since a change to one things means
               *everything* has to recompile

(4) something I haven't thought of

I know edison tends to follow (3); in general I follow (1), but sometimes
I run in to problems of mutually dependant modules this way.

How do people resolve this problem?

 - Hal

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume