问题描述
例如:
fun example (a:'a list) : list = a
将具有以下签名:
'a list -> 'a list
如果我对其定义不同但内容相同的话怎么办
What if I define it differently but with same content like
fun example (a : ''a list) : list = a
其签名为:
''a list -> ''a list
有什么区别?
推荐答案
像'a这样的普通类型变量可以替换为任意类型.形式''a是所谓的平等类型变量，这意味着它只能由允许在其上使用相等运算符=(或<>)的类型替换.值.
A plain type variable like 'a can be substituted with an arbitrary type. The form ''a is a so-called equality type variable, which means that it can only be substituted by types that admit the use of the equality operator = (or <>) on their values.
例如，此功能:
fun contains(x, []) = false
| contains(x, y::ys) = x = y orelse contains (x, ys)
不能具有类型'a * 'a list -> bool，因为它在x上使用相等.它被赋予了更严格的类型''a * ''a list -> bool.
cannot have type 'a * 'a list -> bool because it uses equality on x. It is given the more restrictive type ''a * ''a list -> bool.
大多数类型允许相等，但某些类型不允许，例如real，exn，尤其是任何函数类型t -> u.记录，元组或数据类型之类的组合类型，如果它们的所有组成部分都相同，则承认相等.
Most types allow equality, but some don't, e.g., real, exn, and in particular, any function type t -> u. Composed types like records, tuples, or datatypes admit equality if all their components do.
侧面说明:Haskell后来将此概念推广到其类型类的概念，该概念允许对类型进行任意的用户定义的约束.相等类型变量由Eq类型类替换.
Side remark: Haskell later generalised this concept to its notion of type classes, which allows arbitrary user-defined constraints of this sort on types. Equality type variables are replaced by the Eq type class.
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