Haskell无法访问的代码错误？ [英] Haskell Inaccessible code bug?

问题描述

说我有以下（错误）代码。

Say I have the following (erroneous) code.

data A a b where
  APure ::  (A a b)
  AApply :: A (A b c) c

test :: (A a b) -> a -> b
test (APure) a = a
test AApply a = undefined

GHC会给我这个错误：

GHC will then give me this error:

Couldn't match type `b' with `A b1 b'
  `b' is a rigid type variable bound by
      the type signature for test :: A a b -> a -> b
Inaccessible code in
  a pattern with constructor
    AApply :: forall c b. A (A b c) c,
  in an equation for `test'
In the pattern: AApply
In an equation for `test': test AApply a = undefined

这个错误消息是不是完全错误？
错误与AApply无关。

Isn't this error message completely wrong? The error has nothing to do with AApply.

推荐答案

消息完全错了？该错误与 AApply 无关。

不完全。这可能是一个错误，你得到的错误消息，但它不是完全脱离基地。

Not completely. It's arguably a bug that you get that error message, but it's not completely off base.

看完整个东西后看看这些片段。

Look at the whole thing together after looking at the pieces.

test (APure) a = a

说我们有一个函数

test :: A a b -> r -> r

将签名加在一起

test :: (A a b) -> a -> b

和unify，忽略第一个方程式的类型错误，类型精炼为

and unify, ignoring the type error from the first equation, the type is refined to

test :: A r r -> r -> r

然后查看方程式

test AApply a = undefined

精细类型，因为

AApply :: A (A b c) c

需要

c ~ A b c

如果 AApply 是有效的第一个参数。

if AApply were a valid first argument.

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