从类型'T a`转换为`T b`而不需要样板 [英] Convert from type `T a` to `T b` without boilerplate

问题描述

因此，我有一个AST数据类型，它有大量的情况，这是通过注解类型进行参数化的。

  data Expr a = Plus a Int Int 
 | ... 
 | Times a Int Int 
  

我有注释类型 S 和 T ，还有一些函数 f :: S - > Ť。我想通过我的转换将 Expr S 转换为 Expr T f 对每个 S 发生在Expr值内。

有没有办法使用SYB或泛型来做到这一点，并避免在每种情况下模式匹配？这似乎是这种适合的类型。我只是对SYB不够熟悉，不知道具体的实现方式。 解析方案

听起来你想要一个 Functor 实例。这可以由GHC使用 DeriveFunctor 扩展名自动获得。

So, I have an AST data type with a large number of cases, which is parameterized by an "annotation" type

data Expr a = Plus a Int Int
    | ...
    | Times a Int Int

I have annotation types S and T, and some function f :: S -> T. I want to take an Expr S and convert it to an Expr T using my conversion f on each S which occurs within an Expr value.

Is there a way to do this using SYB or generics and avoid having to pattern match on every case? It seems like the type of thing that this is suited for. I just am not familiar enough with SYB to know the specific way to do it.

解决方案

It sounds like you want a Functor instance. This can be automatically derived by GHC using the DeriveFunctor extension.

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