Debugging a Safe Module

• October 6, 2016
• Haskell  
• debugging  

TL;DR: change -XSafe to -XTrustworthy.

I’ve found myself needing to add some trace statements to System/Environment.hs. The module is marked Safe, so when you try to:

import Debug.Trace (trace)

you get the following error:

Debug.Trace: Can't be safely imported!
The module itself isn't safe.

I don’t know anything about Safe Haskell and I didn’t want to learn at the time. Removing {-# LANGUAGE Safe #-} made the Safe modules depending on System.Environment (the module I was trying to debug) fail to compile, so that wasn’t an option. Turns out you can just bypass this by marking your module as Trustworthy:

{-# LANGUAGE Trustworthy #-}

Well, TIL.