haskell - 不在范围内 : type constructor or class ‘∼’

Question

我正在尝试从 Maguire 的 Thinking with Types 中编译代码片段:

{-# LANGUAGE TypeOperators #-}

data Expr_ a
    = (a ∼ Int) => LitInt_ Int
    | (a ∼ Bool) => LitBool_ Bool
    | (a ∼ Int) => Add_ (Expr_ Int) (Expr_ Int)
    | (a ∼ Bool) => Not_ (Expr_ Bool)
    | If_ (Expr_ Bool) (Expr_ a) (Expr_ a)

使用 GHCi 8.10.4 版，我得到以下信息:

λ> :l TypeEquality.hs 
[1 of 1] Compiling Main             ( TypeEquality.hs, interpreted )

TypeEquality.hs:4:10: error:
    Not in scope: type constructor or class ‘∼’
  |
4 |     = (a ∼ Int) => LitInt_ Int
  |          ^

TypeEquality.hs:5:10: error:
    Not in scope: type constructor or class ‘∼’
  |
5 |     | (a ∼ Bool) => LitBool_ Bool
  |          ^

TypeEquality.hs:6:10: error:
    Not in scope: type constructor or class ‘∼’
  |
6 |     | (a ∼ Int) => Add_ (Expr_ Int) (Expr_ Int)
  |          ^

TypeEquality.hs:7:10: error:
    Not in scope: type constructor or class ‘∼’
  |
7 |     | (a ∼ Bool) => Not_ (Expr_ Bool)
  |          ^
Failed, no modules loaded.

是否有可以编译的语言编译指示？

Answer 1

真正的问题是我复制并粘贴了书中的代码，而 ~ 符号被格式化为 similar-but-not-identical ∼ !我可能不会是最后一个犯这个错误的人，所以希望通过搜索错误消息将有人看到这篇文章。

以下将编译:

{-# LANGUAGE GADTs #-}

data Expr_ a
    = (a ~ Int) => LitInt_ Int
    | (a ~ Bool) => LitBool_ Bool
    | (a ~ Int) => Add_ (Expr_ Int) (Expr_ Int)
    | (a ~ Bool) => Not_ (Expr_ Bool)
    | If_ (Expr_ Bool) (Expr_ a) (Expr_ a)