Haskell 与 ContT、callCC 混淆，当
继续寻求理解 ContT 和朋友们的意义。请考虑下面的（荒谬但说明性的）代码：
v :: IO (Either String [String])
v = return $ Left "Error message"
doit :: IO (Either String ())
doit = (flip runContT return) $ callCC $ \k -> do
x <- liftIO $ v
x2 <- either (k . Left) return x
when True $ k (Left "Error message 2")
-- k (Left "Error message 3")
return $ Right () -- success
该代码无法编译。但是，如果将 when 替换为下面带注释的 k 调用，则它可以编译。这是怎么回事？
或者，如果我注释掉 x2 行，它也会编译。 ???
显然，这是原始代码的精炼版本，因此所有元素都有其用途。感谢有关发生的情况以及如何解决问题的解释性帮助。谢谢。
Continuing quest to make sense of ContT and friends. Please consider the (absurd but illustrative) code below:
v :: IO (Either String [String])
v = return $ Left "Error message"
doit :: IO (Either String ())
doit = (flip runContT return) $ callCC $ \k -> do
x <- liftIO $ v
x2 <- either (k . Left) return x
when True $ k (Left "Error message 2")
-- k (Left "Error message 3")
return $ Right () -- success
This code does not compile. However, if the replace the when with the commented k call below it, it compiles. What's going on?
Alternatively, if I comment out the x2 line, it also compiles. ???
Obviously, this is a distilled version of the original code and so all of the elements serve a purpose. Appreciate explanatory help on what's going on and how to fix it. Thanks.
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这里的问题与
when和either的类型有关，与 ContT 无关：第二个参数需要是
m ()类型code> 对于一些 monadm。因此，可以像这样修改代码的when行：以使代码编译。这可能不是您想要做的，但它给了我们一个关于可能出现错误的提示：
k的类型已被推断为when难以接受的类型。现在对于
either签名：请注意，either的两个参数必须是产生相同类型结果的函数。这里return的类型由x的类型决定，而x的类型又由v上的显式签名确定。因此，(k . Left)位必须具有相同的类型；这又将k的类型修复为（GHC 确定的），这与
when的期望不兼容。但是，当您注释掉
x2行时，它对类型检查器的代码视图的影响将被消除，因此k不再被强制为不方便的类型并且是免费的假设when书中合适的类型。这样，代码就可以编译了。最后一点，我使用 GHCi 的断点工具在两种情况下获取了
k的确切类型——我还不够专业，无法用手写出它们并以任何方式保证他们的正确性。 :-) 使用:break ModuleName line-number column-number来尝试一下。The problem here has to do with the types of
whenandeither, not anything particular to ContT:The second argument needs to be of type
m ()for some monadm. Thewhenline of your code could thus be amended like so:to make the code compile. This is probably not what you want to do, but it gives us a hint as to what might be wrong:
k's type has been inferred to be something unpalatable towhen.Now for the
eithersignature: notice that the two arguments toeithermust be functions which produce results of the same type. The type ofreturnhere is determined by the type ofx, which is in turn fixed by the explicit signature onv. Thus the(k . Left)bit must have the same type; this in turn fixes the type ofkat (GHC-determined)This is incompatible with
when's expectations.When you comment out the
x2line, however, its effect on the type checker's view of the code is removed, sokis no longer forced into an inconvenient type and is free to assume the typewhich is fine in
when's book. Thus, the code compiles.As a final note, I used GHCi's breakpoints facility to obtain the exact types of
kunder the two scenarios -- I'm nowhere near expert enough to write them out by hand and be in any way assured of their correctness. :-) Use:break ModuleName line-number column-numberto try it out.