AFAIK, there are two ways to get a call stack for debugging in Haskell:
- add
HasCallStackconstraint in the code - compile code with
ghc -prof -fprof-auto-top
My test code:
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
Following is the call stack I get from both of the above two ways with this test code:
$ 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, the call stack from -prof is already good enough and is easier to use. So I'm wondering why the HasCallStack mechanism is added yet. Is there some difference between these two way which will significantly affect the debugging experience?