Functor fmap，模式匹配函数值，haskell [英] Functor fmap, pattern match function values, haskell

问题描述

我具有以下类型，并希望将其设置为仿函数:

I have the following type, and would like to make it a Functor:

newtype SubsM a = SubsM {runSubsM :: Context -> Either Error (a, Env)}

到目前为止，我知道了

instance Functor SubsM where
    fmap f (SubsM a)  =  SubsM (\s->(Right((f a),(fst s))))

我收到一个错误，因为a不是期望的类型，我的问题是我该如何在左侧匹配一个模式?

I get an error because a is not the expected type, my question is how do i pattern match a on the left-hand side?

推荐答案

您可以使用case在Either Error (a, Env)上进行模式匹配:

You can pattern match on the Either Error (a, Env) with case:

instance Functor SubsM where
  fmap f (SubsM cf) = SubsM $ \c ->  case (cf c) of
    Left err -> Left err
    Right (v, env) -> Right (f v, env)

在Left情况下，您传播错误；在Right情况下，将结果对拆包，并将f应用于第一个元素.

In the Left case you propagate the error, in the Right case you unpack the resulting pair and apply f to the first element.

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