如何在 Haskell 堆栈测试中对 Maybe 值使用 putStrLn？

Question

我正在使用 Haskell 堆栈创建程序。我的程序构建正常，但我想编写一个测试文件，以便我可以运行 stack test 在我现有的程序上。我的问题是我的函数测试 returns 一个 Maybe 类型，不能使用语法 putStrLn f 输出。我正在尝试在我的测试文件中编写一个函数，该函数采用 Maybe 值和 returns “Nothing”（如果给定 Nothing）或包含 a 的字符串（如果给定） Just a.

到目前为止，这是我的代码：

printMaybe :: Show a => Maybe a -> String
printMaybe Nothing = "Nothing"
printMaybe (Just a) = show a

并且 main 包含该行 putStrLn $ printMaybe (Nothing)

我的错误信息是：

test\Spec.hs:12:16: error: * Ambiguous type variable a0' arising from a use of printMaybe' prevents the constraint (Show a0)' from being solved. Probable fix: use a type annotation to specify what a0' should be. These potential instances exist: instance Show Ordering -- Defined in GHC.Show' instance Show Integer -- Defined in GHC.Show' instance Show a => Show (Maybe a) -- Defined in GHC.Show' ...plus 22 others ...plus 27 instances involving out-of-scope types (use -fprint-potential-instances to see them all) * In the second argument of ($)', namely `printMaybe (Nothing)' In a stmt of a 'do' block: putStrLn $ printMaybe (Nothing) In the expression: do putStrLn ("====== Tests Start ======") putStrLn $ printMaybe (Nothing) | 12 | putStrLn $ printMaybe (Nothing) | ^^^^^^^^^^^^^^^^^^^^

任何帮助将不胜感激:)

Answer 1

如错误所述，问题不在于您的 printMaybe，而在于您的 do 块中的 printMaybe Nothing，从那时起就不清楚 a 类型是。

因此你可能有一些东西like:K

main = do
    putStrLn "====== Tests Start ======"
    putStrLn (printMaybe <b>Nothing</b>)

但不清楚 Maybe a 类型的 a 是什么 Nothing。你可以给编译器一个提示：

main = do
    putStrLn "====== Tests Start ======"
    putStrLn (printMaybe (Nothing <b>:: Maybe Int</b>))

如何在 Haskell 堆栈测试中对 Maybe 值使用 putStrLn？

How do I use putStrLn on Maybe value in Haskell Stack test?

haskell

maybe