How do I use CallStack?

Richard Eisenberg eir at
Mon Dec 7 04:43:08 UTC 2015

Hi devs,

I wish to use the new CallStack feature to track call sites of a function. I want my function to print out where it was called from. I do not want to call `error`. How do I do this?

I looked in the release notes. They describe the CallStack feature at an overview, and the docs suggest that it has a Show instance. But the notes don't say where to import CallStack from. I found it in GHC.Stack, but I'm unsure this is the right place to take it from. The GHC release notes also refer me to the ghc-prim release notes. These (which I assume are ghc-prim/ don't discuss CallStack. In any case, when I try to `show ?callstack`, I learn that CallStack is not an instance of Show, or I somehow haven't imported the instance.



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