假设我有这种数据类型:
data SomeDataType a = SomeDataType a
我希望向用户展示它的表示形式(在控制台输出中),因此我需要一个“漂亮打印”函数。我不想使用show
,因为它会返回一个表达式,而我只想将我的类型的唯一字段的值转换为字符串。
我期望这种行为:
>>> let myintdata = SomeDataType (22::Int)
>>> putStrLn $ prettyPrint myintdata
22
>>> let alice = SomeDataType "Alice"
>>> let bob = SomeDataType "Bob"
>>> putStrLn $ prettyPrint alice ++ " loves " ++ prettyPrint bob
Alice loves Bob
所以我是这样实现的:
prettyPrint :: Show a => SomeDataType a -> String
prettyPrint (SomeDataType x) = show x
对于数字来说,它工作得很好,但是字符串会被引用和转义:
>>> let alice = SomeDataType "Alice"
>>> let bob = SomeDataType "Bob"
>>> putStrLn $ prettyPrint alice ++ " loves " ++ prettyPrint bob
"Alice" loves "Bob"
此外,我希望完全控制未来将不同内容类型转换为字符串的方式。因此,我准备创建自己的类型类!它看起来像这样:
{-# LANGUAGE FlexibleInstances #-}
data SomeDataType a = SomeDataType a
class PrettyPrint a where
prettyPrint :: a -> String
instance {-# OVERLAPPABLE #-} PrettyPrint a where
-- I don't care about this right now,
-- let's learn how to print strings without quotes first!
prettyPrint = const "Stupid Robot"
instance PrettyPrint String where
prettyPrint = id
instance Show a => PrettyPrint (SomeDataType a) where
prettyPrint (SomeDataType x) = prettyPrint x
我对第一次测试感到满意:
>>> putStrLn $ prettyPrint "No quotes!"
No quotes!
但是当我尝试美化打印我的数据类型时,总是会调用通用实例而不是String的实例:
>>> let alice = SomeDataType "Alice"
>>> let bob = SomeDataType "Bob"
>>> putStrLn $ prettyPrint alice ++ " loves " ++ prettyPrint bob
Stupid Robot loves Stupid Robot
目前我怀疑有一种完全不同的方法来解决这个“漂亮打印”问题。是这样吗?还是我在代码中忽略了一些简单明显的错误?