How to get a stacktrace from SomeException (or any other custom Exception)?

As @Li-yao Xia said, currently there is no way to do what you want, but there are some workrounds, one of which is add the HasCallStack constraint to your function definition:

import GHC.Stack

head' :: HasCallStack => [a] -> a
head' [] = error "empty list"
head' (x:_) = x

foo :: HasCallStack => [a] -> a
foo xs = head' xs

goo :: HasCallStack => [Int] -> Int
goo (0:_) = error "Divid by zero"
goo xs = div 5 (foo xs)

hoo :: HasCallStack => [Int] -> Int
hoo xs = goo xs + 4

Load the above code into ghci, you can trace a call stack:

ghci> foo []
*** Exception: empty list
CallStack (from HasCallStack):
  error, called at CallStack.hs:4:12 in main:Main
  head', called at CallStack.hs:8:10 in main:Main
  foo, called at <interactive>:14:1 in interactive:Ghci1
ghci> hoo [0, 1]
*** Exception: Divid by zero
CallStack (from HasCallStack):
  error, called at CallStack.hs:11:13 in main:Main
  goo, called at CallStack.hs:15:10 in main:Main
  hoo, called at <interactive>:15:1 in interactive:Ghci1

However, this workaround needs explicit code changes to propagate the stack, which is the drawback.

There are also two other ways:

• Profiling: ghc -prof -fprof-auto and ghci -fexternal-interpeter -prof
• DWARF: ghc -g.

For more information, you can see the posts by Simon Marlow, one of the Co-developers of the GHC.

There is no way to do that because SomeException does not record the trace of the exception. This is in fact the subject of an ongoing GHC proposal: https://github.com/bgamari/ghc-proposals/blob/stacktraces/proposals/0000-exception-backtraces.rst

Currently you have to manually wrap exceptions to capture the trace: define your own exception type with room for a trace, and your own throw function to capture traces.