How do I use CallStack?

Ben Gamari ben at
Mon Dec 7 10:59:37 UTC 2015

Eric Seidel <eric at> writes:

> Hi Richard,
> Sorry for all of the confusion, it seems the docs do indeed need some
> love!
> On Sun, Dec 6, 2015, at 20:56, Richard Eisenberg wrote:
>> That looks like exactly what I want. Thanks.
>> There remain two mysteries:
>> - I thought that CallStacks were a new feature that would come with GHC
>> 8.0. Yet it seems the datatype is present in base-4.8.x. Even though the
>> docs even say (wrongly, evidently) that it's in base since 4.9.
> They were originally merged into 7.11, but were backported to the
> official 7.10.2 release due to popular demand. It appears the @since
> annotation wasn't updated correspondingly.
>> - That function seems missing in HEAD. Or maybe it moved. A little
>> searching says it *did* move, to GHC.Exception.
> In HEAD we are now using CallStacks for error and undefined, which was
> not the case for the 7.10.2 release. This means the type needs to be
> defined much earlier in base, before we even have enough functionality
> to write a sensible formatter. showCallStack currently lives in
> GHC.Exception because that's where it's used, but that's not a good
> reason... I'll take another look at moving it back to GHC.Stack.

Indeed, this is probably a more logical place for them to live. Recent
experience tells me that there is a great potential for import cycles in this
area, however. I wish you luck ;)

>> Well, my problem is solved. But I think the documentation needs a pass
>> here. And is there a reason not to have a Show instance?
> I usually only use compiler-derived Show instances so that Read
> automatically works, as well as some nice formatting libraries like
> for debugging. For
> pretty-printing like showCallStack I prefer a standalone function or a
> separate type-class.

Sure, I don't see any harm in deriving a Show instances, however. You
may want to derive it with standalone deriving so you can then attach a
Haddock pointing to the pretty version. e.g.,

    data CallStack = ...

    -- | See also 'prettyCallStack'.
    deriving instance Show CallStack

> If the name "showCallStack" suggests the compiler-derived output, we
> could change it to something like "prettyCallStack" or
> "formatCallStack", I don't have a strong opinion there.

I have also struggled with these sorts of naming decisions. The
overloading of the word "show" is a bit problematic. (+1) for "pretty".


- Ben
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