join :: Eq a => [(a,b)] -> [(a,c)] -> [(a,(b,c))]
join a1 a2 = [ (a,(b,c)) | (a,b) <- a1, c <- [ c | (a',c) <- a2, a == a' ] ]
infixr 5 ⋈
(⋈) :: Eq a => [(a,b)] -> [(a,c)] -> [(a,(b,c))]
(⋈) = join
-- for example:
[("a",1),("b",2),("c",3)] ⋈ [("a",'a'),("b",'b'),("c",'c')]
Monday, March 10, 2008
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7 comments:
The a == a' bit means this is a
natural join, i.e. using an
equality predicate. Why not
make the join parameteric in the
test predicate?
Surely you've nothing better to do.
-- Paul
Hi,
I understand the function u wrote but I am having trouble to create function using data type Maybe.
join :: Eq => [(a,b)] -> [(a,c)] -> [(a,b,Maybe c)]
Can you help? Thanks.
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