Haskell 列出组合 function

Question

I want to write a recursive function that gets two lists + a requirement as input and outputs all possible tuples with one element each from the 1st and 2nd list that meet the requirement.

It should look something like this:

combine [1,2,3] [5,6,7] (\ab -> a+b > 7) -> [(1,7),(2,6),(2,7),(3,5),(3,6),(3,7)].

I currently just have:

combine:: [a] -> [b] -> [(a, b)] 
combine [] ys = []
combine xs [] = []
combine (x:xs) (y:ys) = (x,y) : combine xs ys 

but it doesn't filter for anything.

Answer 1

That makes sense, since your input does not filter for anything. You should add an extra parameter here:

combine:: [a] -> [b] ->  -> [(a, b)]
combine [] ys  = []
combine xs []  = []
combine (x:xs) (y:ys) 
  | … = …
  | otherwise = …

here p is thus a function that takes an a and a b and returns a Bool , depending on the outcome you thus fire one of the two guards. I leave filling in the … parts as an exercise.

If you want to produce all possible combinations for x and y for which the condition holds, list comprehension is a better tool. You can then work with:

combine:: [a] -> [b] ->  -> [(a, b)]
combine xs ys  = [ … | … <- xs, … <- ys, … ]