Instance for `TVar`

[hvr]

This is a left-over issue from #6

As described in #6 (comment) by @glguy defining an instance for TVar is complicated since we'd lose {-# LANGUAGE Safe #-} by import TVar which seems undesirable, given all other instances can be imported safely.

[phadej]

should the instance live in stm package instead? No safety problem then?

[hvr]

@phadej yeah, the only not-so-nice thing about that is that in stm it'd be an orphan

[treeowl]

One option would be to add a Trustworthy module to define the rnf function and import it into Control.DeepSeq. But that seems pretty silly! The right answer is surely to add a safe TVar re-export somewhere in base.

[RyanGlScott]

The right answer is surely to add a safe TVar re-export somewhere in base.

That would be nice. But this needn't stop us from adding an NFData instance for TVar today, since there's a module Control.DeepSeq.BackDoor specifically for the purpose of safely re-exporting things from base that currently can't be imported safely. We'd likely have to perform some surgery on the CPP bounds to accommodate TVar, but it should be possible.

[hvr]

Yeah, this ticket predates the backdoor module by 1 year... so I mostly forgot about this... if anyone wants to take a stab at a PR...? :-)

[spacekitteh]

Anyone?

[mixphix]

Pull requests are welcome.