既然我们在 GHC 中已经有了 ghc -prof -fprof-auto-top ，为什么还要提供 `HasCallStack` 机制？

Question

AFAIK，有两种方法可以在 Haskell 中获取用于调试的调用堆栈：

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

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this测试代码从以上两种方式得到的调用栈：

$ 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-prof 的调用堆栈已经足够好并且更易于使用。所以我想知道为什么还要添加HasCallStack 机制。这两种方式之间是否存在一些会显着影响调试体验的差异？

Answer 1

HasCallStack 有几个基本优势：

1. 它更轻量级，不需要-prof，因此不需要重新编译（因为已分析的代码与未分析的代码具有不同的 ABI）

2. 根据您放置 HasCallStack 约束的位置和使用 withFrozenCallStack 的位置，您可以更好地控制调用堆栈中包含的内容，以防止无关/内部细节出现在跟踪中

3. 它允许您使用 getCallStack 从程序本身内部访问调用堆栈，因此您可以将其合并到用于日志记录、异常等的消息中