Friday, March 09, 2018

Safe Library with better Stack Traces

Summary: The safe library now provides error messages with short and informative stack traces on errors.

The Haskell base library contains a number of functions that crash under certain circumstances, e.g. tail []. The safe library attempts to tame these functions, providing tailDef (which uses a default value on []) and tailNote (which gives you a chance to provide some extra information if there is a failure). Since GHC 7.10 there has been support for adding stack traces to exceptions, where if any function with an appropriate annotation calls error it will include some stack trace.

Just over a year ago the safe library introduced a Partial constraint in Safe.Partial to declare that a function is partial, which also provides the annotations for error. The signature of tailNote became:

tailNote :: Partial => String -> [a] -> [a]

This signature has two benefits - it is visible in the documentation and it provides the stack trace if something goes wrong. If you typed tailNote "uhoh" [] in ghci you got:

Prelude Safe> tailNote "uhoh" []
*** Exception: Safe.tailNote [], uhoh
CallStack (from HasCallStack):
  error, called at .\Safe\Util.hs:23:44 in safe-0.3.16-9YcgrXj17kg79mfNx7tCoF:Safe.Util
  fromNoteModule, called at Safe.hs:65:12 in safe-0.3.16-9YcgrXj17kg79mfNx7tCoF:Safe
  fromNote, called at Safe.hs:108:17 in safe-0.3.16-9YcgrXj17kg79mfNx7tCoF:Safe
  tailNote, called at <interactive>:5:1 in interactive:Ghci1

Great - we can see the final line says we were on line 5 of the interactive and ran tailNote. Useful, but with the new version of safe it's even better:

*Main> tailNote "uhoh" []
*** Exception: Safe.tailNote [], uhoh
CallStack (from HasCallStack):
  tailNote, called at <interactive>:1:1 in interactive:Ghci1

We still get the interesting final line, but all the internal details of safe, e.g. the fact that tailNote calls fromNote have disappeared.

To get the stack traces just add Partial to any function you believe to be partial - it's easy. If you are happy to stick with GHC 8.0 and above you can use HasCallStack from GHC.Stack without depending on safe. I am slowly adding annotations to my packages, for example the extra library has Partial annotations.

Supressing the internal functions was a lot more work. I think it's worth it for a package that is all about nice handling of errors, but I probably won't bother in any of my other packages. The change to the code was going from:

tailNote note = fromNote note "tailNote []" . tailMay


tailNote note x = withFrozenCallStack $ fromNote note "tailNote []" $ tailMay x

The main change is we've added withFrozenCallStack, which freezes the call stack at this point and stops new entries from being accumulated inside the bowels of our library. The other change was to eta-expand the definition by adding x on both sides, so that withFrozenCallStack gets to block the actual error, and not merely a function that later produces an error.

Partial constraints are very powerful, and I hope in time they are adopted universally throughout the Haskell ecosystem. One day I hope the Prelude.tail function will also have a Partial constraint.


Unknown said...

Hi Neil. Are there any thoughts about automating the process of adding annotations to each partial function? Or even having stack traces for partial functions by default (or with a compiler switch)? Thanks.

Neil Mitchell said...

Vasiliy: I would love to do this, but don't have time. A function should be Partial if it calls other Partial functions including error, unless it has an annotation saying that it's been hand-verified by a human. I'd love such a tool!