like Prelude.error, but gives the file location
$(err "OH NO!) main:Main main.hs:4:10 OH NO!
like Prelude.undefined, but gives the file location use trace to output the location. this way we still use undefined instead of calling error
$(undef) main:Main main.hs:4:10 undefined err: Prelude.undefined
A version of Debug.Trace.trace that just prints a value. This should be included in Debug.Trace