比较Haskell中具有模式匹配的两个列表

Question

I am trying to implement a function where I have two lists, the first is of any type and the second is a Boolean, and I want it to return just the first list if it is equal to true. For example:

pickIt [1, 2, 3] [True, False, True]  returns [1, 3]

Here is my code:

pickIt :: [a] -> Bool -> [a]
pickIt (x:xs) (y:ys) = (x, y) : pickIt xs ys
pickIt _ _ = []

I think my type is wrong, but I am completely stumped on how to approach this. Any help, guidance, or a link to go in the right direction would be helpful.

Answer 1

Your type is wrong, you said you have a list of bools (in English) then your type said you have a single Bool. Use [Bool] instead of Bool .

You said (by example) that you want a list of the element from the first list, so [a] , as a result. Then your code return tuples of (a,Bool) (ie see your (x,y) value). Instead test if y is true and only if so cons on x via x: .