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-4.9.0.... 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_e... 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.