Where I work now, on-call was long ago relegated to nothing more than a routing task. The on-call person takes the initial call, figures out who best can solve it, and contacts the person. The guy on my team who writes only in Javascript and Python would NEVER be in the position to make a critical fix overnight in a Java module. For those of us who work in Java, reading the language is trivial but understanding what the module is supposed to do from a high level is non-trivial. People who work in Haskell daily would be the same.
This is what I tried to call out. A Haskell developer called at 4am would have no more problem reading Haskell code than I would have reading Java code. While a Javascript developer should NOT try to issue a critical patch in a language (s)he doesn't work in, regardless if the language is Haskell, Python, etc. Any organization requiring you to do so is poorly managed.