创建 Haskell 实例声明
这里对 Haskell 完全是菜鸟，可能还有一个更菜鸟的问题。我正在尝试让 ghci 输出正常工作，但陷入了实例声明。我如何声明“(Show (Stack -> Stack))”的实例：
data Cmd = LD Int
| ADD
| MULT
| DUP
deriving Show
type Prog = [Cmd]
type Stack = [Int]
type D = Stack -> Stack
我一直在尝试创建如下声明：
instance Show D where show = Stack
但我所有的尝试都导致了非法实例声明。非常感谢任何帮助和/或参考！
complete noob to Haskell here with probably an even noobier question. I'm trying to get ghci output working and am stuck on instance declarations. How could I declare an instance for "(Show (Stack -> Stack))" given:
data Cmd = LD Int
| ADD
| MULT
| DUP
deriving Show
type Prog = [Cmd]
type Stack = [Int]
type D = Stack -> Stack
I've been trying to create a declaration like:
instance Show D where show = Stack
but all my attempts have resulted in illegal instance declarations. Any help and/or references much appreciated!
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首先，默认情况下，类型同义词（即使用
type定义的内容）在实例声明中是不合法的。不过，有一个 GHC 扩展可以实现这一点。除此之外，在这种特定情况下，
show需要返回一个String；您的实例试图返回一个...类型同义词名称，这甚至从一开始就没有意义，而且还引用Int列表，这是的错误返回类型>显示。最后，
D是一个函数类型——它应该显示什么呢？在大多数情况下，对于函数类型的Show实例，您实际上无法做太多有意义的事情。如果您只想说“这是 D 型”，您可以编写这样的实例：
不过，我不确定这在实践中有多有用。
First of all, by default, type synonyms (that is, stuff defined using
type) aren't legal in instance declarations. There's a GHC extensions to allow this, however.Beyond that, in this specific case,
showneeds to return aString; your instance is trying to return a... type synonym name, which doesn't even make sense to begin with, and besides refers to a list ofInt, which is the wrong return type forshow.Finally,
Dis a function type--what is that supposed toshow, anyway? There's really not much you can meaningfully do with aShowinstance on a function type, in most cases.If you just want it to say "this is type D", you could write an instance like this:
I'm not sure how useful that is in practice, though.