如何只从元组列表中获取我想要的元组？ [哈斯克尔]

Question

I'm trying to set up a list of tuples from a list of Integers in this form: (a,b) a <- [1..4] and b <- xs. But I keep getting all the different combinations of the statement.

okTup :: [Int] -> [(Int,Int)]
okTup xs = [(i,j) | i <- [1..4], j <- xs]

Input: okTup [3,1,4,2]

What I get is this: [(1,3),(1,1),(1,4),(1,2),(2,3),(2,1),(2,4),(2,2),(3,3),(3,1),(3,4),(3,2),(4,3),(4,1),(4,4),(4,2)]

But I only want this: [(1, 3 ),(2, 1 ),(3, 4 ),(4, 2 )]

Answer 1

That's zip :

okTup xs = zip [1..] xs

If you prefer to do this with a list comprehension, or you want to extend an existing comprehension, you can use the ParallelListComp extension, which allows you to write:

okTup xs = [(i, j) | i <- [1..] | j <- xs]

(Note the vertical bar | instead of a comma , .)

Of course, this essentially just uses zip under the hood.