Haskell refer to list of list element-CodePudding

--This line count how many 0 are in the list

hasZero :: [Int] -> Bool
hasZero x = 0 < sum [1 | y <- x, y == 0]

--In this line I want to count how many empty list are in the list, but I got compiling erorr.

hasEmpty :: [[a]] -> Bool
hasEmpty x = 0 < [1 | y <- [x], y == []]

CodePudding user response：

There are 2 issues in your second method - you are missing sum which does the count in the first one, and you are wrapping passed list into another one, it should be y <- x as in the first one:

hasEmpty :: [[a]] -> Bool
hasEmpty x = 0 < sum [1 | y <- x, null y]

Which can be rewritten as:

hasEmpty :: [[a]] -> Bool
hasEmpty = any null

CodePudding user response：

In the first example, you summed the elements of the list your list comprehension generated. In the second, you haven't done this. You also don't need to put x in brackets.

hasEmpty :: Eq a => [[a]] -> Bool
hasEmpty x = 0 < sum [1 | y <- x, y == []]

This is a peculiar way to accomplish this goal. The more idiomatic way (aside from using the existing Prelude function any) would be to use pattern matching and recursion.

hasZero :: [Int] -> Bool
hasZero [] = True
hasZero (0:_) = True
hasZero (_:xs) = hasZero xs

Using any:

hasZero :: [Int] -> Bool
hasZero = any (== 0)