I don't think that's exactly what you want, though. name (2::Int)
crashes your program.
I think you really want a data type.
data Person a b = P a b
pid :: Person a b -> a
pid (P a _) = a
pname :: Person a b -> b
pname (P _ b) = b
Now, what it looks like you want is some kind of extensible relation
name 1 = "john"
name 2 = "julie"
etc.
but there's no simple way to specify this as an open function at the
value level. You can make a list of all the people:
people = [ P 1 "john", P 2 "julie" ]
name :: Int -> Maybe String
name x = fmap pname $ safeHead $ filter ((== x) . pid) people
safeHead [] = Nothing
safeHead (x:_) = Just x
It's still not really clear what you are trying to do.
-- ryan
On Mon, Jan 17, 2011 at 9:15 AM, Patrick Browne
A functional dependency seems to allow me to express my rather strange requirement.
class Person i n | i -> n where pid :: i name :: i -> n
instance Person Int String where pid = 1 name(1) = "john"
-- name(pid::Int) will produce john
Thanks for your help
Pat
On 17/01/2011 14:07, Patrick Browne wrote:
On 17/01/2011 13:04, Ketil Malde wrote:
So other PERSONs would have different types, say:
I think the problem is there is just one constant p1 in the class and there needs to be multiple constants in the implementation (one for each person). It seems difficult to specify this using type classes So, some data declaration as you suggest will probably be needed.
Thanks, Pat
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