Question

我对Haskell很新。今天当我读这本书并练习它的例子时，我收到了一个错误。以下是第3章第57页中的源代码Nullable.hs。

import Prelude hiding (Maybe)

{-- snippet Nullable --}
data Maybe a = Just a
             | Nothing
{-- /snippet Nullable --}

{-- snippet wrappedTypes --}
someBool = Just True

someString = Just "something"
{-- /snippet wrappedTypes --}


{-- snippet parens --}
wrapped = Just (Just "wrapped")
{-- /snippet parens --}

当我输入ghci Nullable.hs时得到：

GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
[1 of 1] Compiling Main             ( Downloads/examples/ch03/Nullable.hs, interpreted )

Downloads/examples/ch03/Nullable.hs:9:11:
    Ambiguous occurrence `Just'
    It could refer to either `Main.Just', defined at Downloads/examples/ch03/Nullable.hs:4:15
                          or `Prelude.Just', imported from Prelude at Downloads/examples/ch03/Nullable.hs:1:0-28

Downloads/examples/ch03/Nullable.hs:11:13:
    Ambiguous occurrence `Just'
    It could refer to either `Main.Just', defined at Downloads/examples/ch03/Nullable.hs:4:15
                          or `Prelude.Just', imported from Prelude at Downloads/examples/ch03/Nullable.hs:1:0-28

Downloads/examples/ch03/Nullable.hs:16:10:
    Ambiguous occurrence `Just'
    It could refer to either `Main.Just', defined at Downloads/examples/ch03/Nullable.hs:4:15
                          or `Prelude.Just', imported from Prelude at Downloads/examples/ch03/Nullable.hs:1:0-28

Downloads/examples/ch03/Nullable.hs:16:16:
    Ambiguous occurrence `Just'
    It could refer to either `Main.Just', defined at Downloads/examples/ch03/Nullable.hs:4:15
                          or `Prelude.Just', imported from Prelude at Downloads/examples/ch03/Nullable.hs:1:0-28
Failed, modules loaded: none.
Prelude>

我认为这个问题是由范围引起的，所以我在“Just”中添加一个前缀，就像这个someBool = Main.Just True一样，再试一次：

[1 of 1] Compiling Main             ( nu.hs, interpreted )
Ok, modules loaded: Main.
*Main> Just 1

<interactive>:1:0:
    No instance for (Show (Maybe t))
      arising from a use of `print' at <interactive>:1:0-5
    Possible fix: add an instance declaration for (Show (Maybe t))
    In a stmt of an interactive GHCi command: print it

现在我可以确认它不仅是由范围错误引起的。但我无法处理......

这样做的方法是什么？任何建议将不胜感激。

Answer 1

不，原始错误是由范围错误引起的，因此当您明确限定它时，您修复了一个错误但引入了另一个错误。

您可以通过在原始代码中添加deriving (Show)行来解决其他错误：

data Maybe a = Just a
         | Nothing
    deriving (Show)

真实世界Haskell示例中的一个模糊错误

1 个答案: