具有不同字段名称的记录的 haskell 变体
鉴于此：
data Foo = Bar { name :: String } | Baz { nickname :: String }
函数 name 和 nickname 似乎都是 Foo ->; 类型。 String：
:t name
name :: Foo -> String
:t nickname
nickname :: Foo -> String
但是，定义不完整，因为以下两个表达式都会引发模式匹配错误：
name $ Baz { nickname = "Bob" }
nickname $ Bar { name = "Fred" }
是否可以完成 name 和 nickname 的定义，即类似：
name Baz { nickname = n } = ...
nickname Bar { name = n } = ...
在拥抱中尝试此操作会产生诸如“变量名的多个声明”之类的错误。
Given this:
data Foo = Bar { name :: String } | Baz { nickname :: String }
Both the functions name and nickname seem to be of type Foo -> String:
:t name
name :: Foo -> String
:t nickname
nickname :: Foo -> String
However, the definitions are incomplete since both of the following expressions will raise pattern match errors:
name $ Baz { nickname = "Bob" }
nickname $ Bar { name = "Fred" }
Is it possible to complete the definitions of name and nickname, i.e. something like:
name Baz { nickname = n } = ...
nickname Bar { name = n } = ...
Trying this in hugs yields errors like "Multiple declarations for variable name".
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不，这是不可能的。字段名称是就范围而言的顶级函数，因此不能重新定义或扩展。让它们成为全部功能就像让
head成为一个功能一样不可能。No, it's not possible. The field names are top-level functions with respect to scope and thus cannot be redefined or extended. It's as impossible to make them total functions as it is to make
headone.尝试 -XDataKinds< /a> 扩展名，它允许您将数据构造函数提升为种类。
请注意，您将需要 12 天前发布的 GHC 7.4.1。现在是问这个问题的好时机！
Try the -XDataKinds extension, it allows you to promote data constructors to be kinds.
Note that you'll need GHC 7.4.1, which was released just 12 days ago. This is a good time to be asking this question!