Haskell字符串/输出操作

Question

我确实有以下代码：

suffixes :: [a] -> [[a]]
suffixes [] = [[]]
suffixes l@(_:t) = l : suffixes t

prefixes :: [a] -> [[a]]
prefixes [] = [[]]
prefixes l@x = l : prefixes (init x)

menu :: Char -> [a] -> Either String [[a]]
menu 'p' l = Right (prefixes l)
menu 's' l = Right (suffixes l)
menu x _ = Left ("(" ++ show x ++ ")" ++ "is not supported, use (p)refix or (s)uffix")

我确实有以下测试功能：

testMenuP = "Expected Right [[1,2],[1],[]]; menu 'p' [1,2] returned " ++ show (menu 'p' [1,2] :: Either String [[Int]])

testMenuS = "Expected Right [[1,2],[2],[]]; menu 's' [1,2] returned " ++ show (menu 's' [1,2] :: Either String [[Int]])

testMenuC = "Expected Left \"(d) is not supported, use (p)refix or (s)uffix\"; menu 'd' [1,2] returned " ++ show (menu 'd' [1,2] :: Either String [[Int]])

testMenu = putStr (testMenuP ++ "\n" ++ testMenuS ++ "\n" ++ testMenuC ++ "\n")

我现在的问题是，在输出字符串(如测试函数testMenuC中所示)时，如何去掉Char 'd‘中的引号。

Answer 1

您可以将菜单的部分替换为：

menu x _ = Left ("(" ++ [x] ++ ")" ++ "is not supported, use (p)refix or (s)uffix")

甚至是

menu x _ = Left . mconcat $ ["(", [x], ")", "is not supported, use (p)refix or (s)uffix"]