Crystal 中的 Hash 和 NamedTuple 有什么区别？

Question

这两种语法看起来相当相似，它们的用例也很相似。示例：

# Hash
hash = {"name" => "Crystal", "year" => 2011}
hash["name"] # outputs: Crystal

# NamedTuple
tuple = {name: "Crystal", year: 2011}
tuple[:name] # outputs: Crystal

那么这两个原语到底有什么不同呢？

Answer 1

API 文档已经很好地解释了这一点。来自NamedTuple（我强调）：

A named tuple is a fixed-size, immutable, stack-allocated mapping of a fixed set of keys to values.

You can think of a NamedTuple as an immutable Hash whose keys (which are of type Symbol), and the types for each key, are known at compile time.

还有：

The compiler knows what types are in each key, so when indexing a named tuple with a symbol literal the compiler will return the value for that key and with the expected type. Indexing with a symbol literal for which there's no key will give a compile-time error.

相比之下，Hash:

A Hash is a generic collection of key-value pairs mapping keys of type K to values of type V.

简而言之，散列是一种可以在运行时更改的数据结构，所有 keys/values 都可以具有任何类型，只要它与通用类型参数 K/V 匹配即可。另一方面，命名元组是一种不可变的数据结构，在编译时是完全已知的。如果您访问一个键，编译器就会知道它的类型。拥有一个命名元组非常类似于将键作为具有公共前缀的变量：

foo = {bar: "bar", baz: 1}

foo_bar = "bar"
foo_baz = 1

NamedTuple 只是添加了一些工具来将这些变量用作一个连贯的集合。

Crystal 中的 Hash 和 NamedTuple 有什么区别？

What's the difference between Hash and NamedTuple in Crystal?

syntax

crystal-lang