Debug printing inside the typechecker knot

[Ryan Scott]

I've recently found myself wanting to add some pprTrace statements in
typechecker-related code but unable to do so, as the code would pull
on knot-tied TyCons (leading to an infinite loop if actually ran). In
particular, I want to print some code [1] in TcTyClsDecls.tcConDecl:

       ; (ze, tkvs)     <- zonkTyBndrs tkvs
       ; (ze, user_tvs) <- zonkTyBndrsX ze user_tvs
       ; arg_tys <- zonkTcTypesToTypesX ze arg_tys
       ; ctxt    <- zonkTcTypesToTypesX ze ctxt
       ; res_ty  <- zonkTcTypeToTypeX   ze res_ty

       ; let (univ_tvs, ex_tvs, tkvs', user_tvs', eq_preds, arg_subst)
               = rejigConRes tmpl_bndrs res_tmpl tkvs user_tvs res_ty

In [2], goldfire claimed that there is a way to work around this problem now:

    > But you can do this. It just has to be before the final zonk. I
see that rejigConRes is used after the final zonk. But there's nothing
about that function that requires things be in their final state. The
arguments need to be zonked, but they can be zonked by zonkTcType and
friends, not zonkTcTypeToType and friends. So, for debugging: zonk
everything with the functions in TcMType (not those in TcHsSyn). Then
rejigConRes. Then print all the output you like. Then do the final
zonk. This double-zonk is redundant, so it's not good to keep going
forward, but it shouldn't cause other trouble.

Unfortunately, this advice doesn't seem to pan out in practice. I
started off by looking for a TcMType equivalent of this function:

    zonkTcTypeToTypeX :: ZonkEnv -> TcType -> TcM Type

However, I am utterly unable to find anything defined in TcMType with
a similar type signature. Actually, it's worse than that: there isn't
a single mention of ZonkEnv anywhere in TcMType! So as far as I can
tell, this advice is completely unimplementable, unless I'm missing
something obvious. Any tips?

Ryan S.
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[1] https://gitlab.haskell.org/ghc/ghc/blob/51fd357119b357c52e990ccce9059c423cc49406/compiler/typecheck/TcTyClsDecls.hs#L2312-2318
[2] https://phabricator.haskell.org/D4974#137224