[Haskell-cafe] HasCallStack - runtime costs?

Jonas Scholl anselm.scholl at tu-harburg.de
Mon Mar 7 18:02:55 UTC 2016

I think the documentation for HasCallStack is not quite correct. As far
as I understood, head and tail are not changed to contain call stacks as
this would indeed carry a performance-penalty. Maybe the documentation
should call the function myHead to distinguish it from the library
function. The user-manual also uses errorWithCallStack, but I think base
calls this function just error and adds errorWithoutStackTrace (at least
thats what I find in

On 03/07/2016 06:21 PM, Johannes Waldmann wrote:
> Eric wrote earlier:
>> If you import a function with a HasCallStack constraint
>> there's no way to disable the overhead ...
>> For non-recursive functions like head and tail,
>> the overhead is probably not large enough to be noticeable.
> Now I'm confused: should this mean that head and tail
> are "functions with a HasCallStack constraint"?
> If I ask ghci for their type, it just says head :: [a] -> a
> and that's also what's in the source
> https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/libraries/base-
> but the Callstack explanation in the user manual
> does show a different version of head
> https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/users_guide/glasgow_exts.html#hascallstack
> I am not particularly concerned about head and tail here
> but generally about how to find out whether my code
> is taking a performance penalty.
> I think the documentation (user manual Section 9.31)
> is rather brief about the semantics. It speaks about
> solving "HasCallStack" constraints - but they could also
> be propagated? There is secondary documentation like
> https://ghc.haskell.org/trac/ghc/wiki/ExplicitCallStack
> but it does not tell what is actually implemented,
> or I don't see it.
> - J.W.
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