ghc-call-stack-extras- Extra utilities for HasCallStack

Copyright(c) 2018 David Feuer
LicenseBSD-style (see the file LICENSE)
Safe HaskellNone



Currently this module only supports adding notes to call stacks, but it may offer more features later.



callStackNote :: HasCallStack => String -> (HasCallStack => a) -> a Source #

Add a note to the current call stack. This note will be included in the stack trace in case of an error. In the event that the note itself throws an exception, a placeholder will be shown instead.


Suppose we've written

f :: HasCallStack => (HasCallStack => Int -> Int -> Int) -> Int -> Int
f g x = callStackNote ("x = " ++ show x) $ g 5 x

quotTrace :: HasCallStack => Int -> Int -> Int
quotTrace _ 0 = error "divide by zero"
quotTrace x y = x quot y

calling print $ f quotTrace 0 will print something like

Test: divide by zero
CallStack (from HasCallStack):
  error, called at Test.hs:11:17 in main:Main
  quotTrace, called at Test.hs:14:18 in main:Main
  g, called at Test.hs:8:44 in main:Main
  callStackNote (x = 0)
    , called at Test.hs:8:9 in main:Main
  f, called at Test.hs:14:16 in main:Main