Using the trace trick needs to guard on False: myfun a b | trace ("myfun " ++ show a ++ " " ++ show b) False = undefined myfun a b = ... See here for some

Using the trace trick needs to guard on False: myfun a b | trace ("myfun " ++ show a ++ " " ++ show b) False = undefined myfun a b = ... See here for some other helpful trace functions: https://hackage.haskell.org/package/base-4.8.1.0/docs/Debug-Trace.html https://hackage.haskell.org/package/base-4.8.1.0/docs/Debug-... There is even a traceStack which will print a call stack if available.