2008/12/29 Raeck chiu
People sometime will try to represent a quantity-regard-only data structure (such a order-regadless List) using functions instead of ta concrete data structure (like the haskell build-in []), any particular reason for this?
For example,
data Sex = Male | Female classA :: Sex -> Int classA Male = 100 classA Female = 200
when I should prefer the above solution instead of using a concrete data structure such as [] to represent the classA members?
One important difference between Sex -> Int and [(Sex,Int)] is that the first guarantees exactly one Int per Sex and has no underlying order. (That is, [(Male,100),(Female,200)] and [(Female,200),(Male,100)] are distinct lists but represent the same map.)
It seems to be very difficult to change the number of Male or Female if a concrete data structure is not used. Is it possible change the number of Male in classA when represent classA using function?
Here's the simplest way:
update k v map = \k' -> if k' == k then v else map k'
Note that it requires an Eq instance for Sex.
let classA' = update Male 150 classA
in (classA' Male, classA' Female)
=
(150,200)
--
Dave Menendez