为什么 Data.String.IsString 类型类只定义一种转换？
为什么 Haskell 基础包只定义 IsString 类来进行从 String 到 'like-string' 值的转换，而不定义从 'like- string' 值转换为String？
该类应定义为：
class IsString a where
fromString :: String -> a
toString :: a -> String
ref: http://hackage.haskell.org/packages/archive/base/4.4.0.0/doc/html/Data-String.html
Why does the Haskell base package only define the IsString class to have a conversion from String to 'like-string' value, and not define the inverse transformation, from 'like-string' value to String?
The class should be defined as:
class IsString a where
fromString :: String -> a
toString :: a -> String
ref: http://hackage.haskell.org/packages/archive/base/4.4.0.0/doc/html/Data-String.html
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原因是恕我直言，
IsString的主要目的是用于 Haskell 源代码（或 (E)DSL）中的字符串文字 - 另请参阅 Paradise：通过 OverloadedStrings 语言扩展嵌入 Haskell 中的两阶段 DSL），其方式与其他多态文字的工作方式类似（例如，通过fromRational表示浮点文字，或通过fromInteger表示整数文字）术语
IsString可能有点误导，因为它表明该类型-class 表示类似字符串的结构，而它实际上只是表示在 Haskell 源代码中具有引用字符串表示形式的类型。The reason is IMHO that
IsString's primary purpose is to be used for string literals in Haskell source code (or (E)DSLs -- see also Paradise: A two-stage DSL embedded in Haskell) via theOverloadedStringslanguage extension in an analogous way to how other polymorphic literals work (e.g. viafromRationalfor floating point literals orfromIntegerfor integer literals)The term
IsStringmight be a bit misleading, as it suggests that the type-class represents string-like structures, whereas it's really just to denote types which have a quoted-string-representation in Haskell source code.如果您想使用 toString :: a -> String，我想你只是忘记了
show :: a ->; String，或更准确地说是Show a =>显示::a->字符串。如果您想对具有
:: a -> 的类型进行操作String和:: String -> a，您可以简单地将这些类型类约束放在函数上。我们仔细注意，我们避免定义具有一组函数的类型类，这些函数也可以分为两个子类。因此，我们不会将
toString放入IsString中。最后，我还必须提到
Read，它提供了Read a => 。字符串 ->一个。您可以使用read和show进行非常简单的序列化。IsString中的fromString有不同的用途，它与语言编译指示OverloadedStrings一起使用，那么你可以非常方便地插入类似"This 的代码不是字符串"::Text。 （Text是字符串的（高效）数据结构）If you desire to use
toString :: a -> String, I think you're simply forgetting aboutshow :: a -> String, or more properlyShow a => show :: a -> String.If you want to operate on a type both having a
:: a -> Stringand:: String -> a, you can simply put those type-class constraints on the functions.We carefully note that we avoid defining type classes having a set of functions that can as well be split into two subclasses. Therefor we don't put
toStringinIsString.Finally, I must also mention about
Read, which providesRead a => String -> a. You usereadandshowfor very simple serialization.fromStringfromIsStringhas a different purpose, it's useful with the language pragmaOverloadedStrings, then you can very conveniently insert code like"This is not a string" :: Text. (Textis a (efficient) data-structure for Strings)