为什么要提供“ HasCallStack”机制，因为我们在GHC中已经有“ ghc -prof -fprof-auto-top”？

Question

AFAIK，有两种方法可以获取调用堆栈以在Haskell中进行调试：

1. HasCallStack在代码中添加约束
2. 用编译代码 ghc -prof -fprof-auto-top

我的测试代码：

import GHC.Stack

-- | a wrapper function to make "last" from base traceable
last' :: HasCallStack => [a] -> a
last' xs = case xs of [] -> error "abuse last"; _ -> last xs

-- | a untraceable partial function
foo :: [Int] -> Int
foo xs = last' xs + 1

-- | a untraceable partial function
-- , which looks like traceable, but it's call stack is cut off by "foo"
bar :: HasCallStack => [Int] -> Int
bar xs = foo xs

-- | an empty list
-- , which is also an improper input of "last'"
xs :: [Int]
xs = []

-- | the program is expected to print a call stack telling how "bar" is called
-- , but we can only know that "foo" called "last'" improperly from the printed call stack
main :: IO ()
main = print $ bar xs

以下是我通过上述两种方式通过此测试代码获得的调用堆栈：

$ ghc -prof -fprof-auto call-stack-cut-off.hs
$ ./call-stack-cut-off
call-stack-cut-off: abuse last
CallStack (from HasCallStack):
  error, called at call-stack-cut-off.hs:5:29 in main:Main
  last', called at call-stack-cut-off.hs:9:10 in main:Main
CallStack (from -prof):
  Main.last' (call-stack-cut-off.hs:5:1-60)
  Main.foo (call-stack-cut-off.hs:9:1-21)
  Main.bar (call-stack-cut-off.hs:14:1-15)
  Main.main (call-stack-cut-off.hs:24:1-21)
  Main.CAF (<entire-module>)

IMO，来自的调用堆栈-prof已经足够好，并且更易于使用。所以我想知道为什么HasCallStack要添加该机制。这两种方式之间是否存在一些差异，这些差异会严重影响调试体验？

Answer 1

HasCallStack有几个基本优点：

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1. 它\xe2\x80\x99s 更轻量级，不需要-prof，因此它\xe2\x80\x99t 不需要重新编译（因为分析代码与非分析代码具有不同的 ABI）

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2. 它使您可以根据您放置HasCallStack约束的位置以及用于withFrozenCallStack防止不相关/内部详细信息显示在跟踪中的位置，更好地控制调用堆栈中包含的\xe2\x80\x99s

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3. 它允许您使用 访问程序本身内部的调用堆栈getCallStack，因此您可以将其合并到消息中以进行日志记录、异常等

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