[Haskell-cafe] Puzzling "Reduction stack overflow" error message

[CASANOVA Juan]

"Have you tried "-freduction-depth=0"?  Does that "terminate"?"

I had not before you said it. While there is a long tower of constraints, there are no recursive ones that I know of that might drastically increase the depth so I was inclined, by the heuristic that normally just going deeper doesn't solve this kind of problems, not to do it.

But I have now, since when you said it I realized it was definitely something to try.

I have tried with -freduction-depth=20000, and it still fails (with the same Eq OFunction constraint).
With -freduction-depth=1000000 or -freduction-depth=0, it does not terminate within a minute or two.

So it does look like there is some infinite depth going on.

Thank you for your suggestion anyway. Any more ideas? Mostly I am looking for potential explanations for why it would have an issue with infinite reduction with a constraint that's trivially satisfiable, and then I can try to rationalize that and see how it could apply to my case.

Juan Casanova.
________________________________
From: Haskell-Cafe <haskell-cafe-bounces at haskell.org> on behalf of Viktor Dukhovni <ietf-dane at dukhovni.org>
Sent: 24 January 2021 05:06
To: haskell-cafe at haskell.org <haskell-cafe at haskell.org>
Subject: Re: [Haskell-cafe] Puzzling "Reduction stack overflow" error message

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On Sun, Jan 24, 2021 at 03:15:24AM +0000, CASANOVA Juan wrote:

> The following function I have is the one that throws the error:
>
> resolve_to_constraints_metacnf :: SOMetaSignature -> SOMetaCNF -> Computation (Maybe SOMetaUnifSystem)
> resolve_to_constraints_metacnf sig cnf = result
> where
> f1 = (ADDirect <$>) :: SOMetaliteral -> SOMetaUnifLiteral;
> f2 = (f1 <$>) :: [SOMetaliteral] -> [SOMetaUnifLiteral];
> f3 = (f2 <$>) :: [[SOMetaliteral]] -> [[SOMetaUnifLiteral]];
> ucnf = f3 cnf :: [[SOMetaUnifLiteral]];
> resolved = res_computeresolve SOResGreedyFactorH ucnf :: StateT uv Computation (Maybe SOMetaUnifSystem);
> runstated = runStateT resolved (UnifVar 0);
> result = fst <$> runstated
>
> The error starts as follows:
>
> Reduction stack overflow; size = 201
>       When simplifying the following type: Eq OFunction

This error is reported by:

    solverDepthErrorTcS :: CtLoc -> TcType -> TcM a
    solverDepthErrorTcS loc ty
      = setCtLocM loc $
        do { ty <- zonkTcType ty
           ; env0 <- tcInitTidyEnv
           ; let tidy_env     = tidyFreeTyCoVars env0 (tyCoVarsOfTypeList ty)
                 tidy_ty      = tidyType tidy_env ty
                 msg
                   = vcat [ text "Reduction stack overflow; size =" <+> ppr depth
                          , hang (text "When simplifying the following type:")
                               2 (ppr tidy_ty)
                          , note ]
           ; failWithTcM (tidy_env, msg) }
      where
        depth = ctLocDepth loc
        note = vcat
          [ text "Use -freduction-depth=0 to disable this check"
          , text "(any upper bound you could choose might fail unpredictably with"
          , text " minor updates to GHC, so disabling the check is recommended if"
          , text " you're sure that type checking should terminate)" ]

Have you tried "-freduction-depth=0"?  Does that "terminate"?

--
    Viktor.
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