style questino: where to put instances
Hal Daume III
Tue, 23 Apr 2002 10:07:17 -0700 (PDT)
I know you're all probably pretty tired of hearing from me, but I have a
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
<-- 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 Daume III
"Computer science is no more about computers | firstname.lastname@example.org
than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume