如何在Haskell中实现三个列表元素组合的自定义求和字符串输出

Hey, let's sort this out for you! You can absolutely achieve the formatted output you want—here are a couple of clean approaches, including one that uses the string concatenation pattern you mentioned.

Approach 1: Modify smallTriplets to Generate Formatted Strings Directly #

Instead of returning a list of tuples, we'll have the function build the exact string lines you need right in the list comprehension. We'll also keep the let sum = x + y + z binding to avoid recalculating the total:

main = do 
    let smallTriplets xs ys zs = [ "Sum is " ++ show x ++ "+" ++ show y ++ "+" ++ show z ++ " = " ++ show sum
                                 | x <- xs, y <- ys, z <- zs
                                 , let sum = x + y + z
                                 , sum <= 10 ]
    mapM_ putStrLn (smallTriplets [1,2] [3,4] [5,6])

This uses your preferred string concatenation style, and the sum binding keeps the code cleaner and more efficient by reusing the calculated total.

Approach 2: Separate Data Generation from Formatting #

If you want to keep the original smallTriplets function (to reuse the tuple data elsewhere), split the logic into two parts—generate valid triplets first, then format them with a helper function:

main = do 
    -- Original triplet generation logic, untouched
    let smallTriplets xs ys zs = [ (x, y, z) | x <- xs, y <- ys, z <- zs, x + y + z <= 10]
        -- Helper to convert a triplet to your desired string format
        formatTriplet (x,y,z) = "Sum is " ++ show x ++ "+" ++ show y ++ "+" ++ show z ++ " = " ++ show (x + y + z)
    -- Print each formatted line
    mapM_ (putStrLn . formatTriplet) (smallTriplets [1,2] [3,4] [5,6])

This is ideal if you need to work with the raw triplet data for other tasks later on.

Both Approaches Produce Your Desired Output: #

Sum is 1+3+5 = 9
Sum is 1+3+6 = 10
Sum is 1+4+5 = 10
Sum is 2+3+5 = 10

内容的提问来源于stack exchange，提问作者Ahmad Ismail