Dear Cafe, the new (8.*) call stack feature https://downloads.haskell.org/~ghc/8.0.1-rc2/docs/html/users_guide/glasgow_e... is certainly nice for debugging during development. But how costly is it at runtime? I notice a 5 percent slowdown. That's not a problem if there's an easy way to switch this off for production (without changing the code). Related: how to make code that uses it, compile with older ghcs that don't have it. I made this hack: do not import GHC.Stack.Types, but instead {-# language CPP, MultiParamTypeClasses #-} #if (__GLASGOW_HASKELL__ < 710) {-# language NullaryTypeClasses #-} #endif module Stack ( HasCallStack ) where #if (__GLASGOW_HASKELL__ >= 800) import GHC.Stack.Types #else class HasCallStack instance HasCallStack #endif When I compile with 8.rc2, and change ">= 800" to ">= 900", I am getting the 5 percent speedup mentioned above. But does it really do what I hope it does (remove all runtime overhead that call stacks may have)? When I compile with 7.10.3, I am getting 5 .. 10 percent faster again. My code does nothing fancy (w.r.t. types and libraries), it just uses Data.IntMap heavily. And it has some class Semiring s where zero :: s one :: s plus :: HasCallStack => s -> s -> s times :: HasCallStack => s -> s -> s - J.W.