Re: [clash-language] Avoiding `OtherCon []` unfoldings, restoring definitions from unfoldings

This doesn't quite match my experience. For example, the following toplevel definition gets an `OtherCon []` unfolding: nonEmptySubsequences :: [a] -> [[a]] nonEmptySubsequences [] = [] nonEmptySubsequences (x:xs) = [x] : foldr f [] (nonEmptySubsequences xs) where f ys r = ys : (x:ys) : r as can be seen with: $ ghc -fforce-recomp -fexpose-all-unfoldings -ddump-prep -dsuppress-uniques A.hs -- RHS size: {terms: 37, types: 55, coercions: 0, joins: 0/6} A.nonEmptySubsequences [Occ=LoopBreaker] :: forall a. [a] -> [[a]] [GblId, Arity=1, Unf=OtherCon []] A.nonEmptySubsequences = \ (@ a) (ds [Occ=Once1!] :: [a]) -> ... So this is not a lifted `case`-bound variable, but a bonafide user-originating toplevel definition. And its value also isn't bottom. On Fri, 1 Apr 2022, Christiaan Baaij wrote:

So if I understand correctly, OtherCon is only created here:https://gitlab.haskell.org/ghc/ghc/-/blob/a952dd80d40bf6b67194a44ff71d7bf759... mpiler/GHC/Core/Opt/Simplify.hs#L3071-3077

simplAlt env _ imposs_deflt_cons case_bndr' cont' (Alt DEFAULT bndrs rhs)   = assert (null bndrs) $     do  { let env' = addBinderUnfolding env case_bndr'                                         (mkOtherCon imposs_deflt_cons)                 -- Record the constructors that the case-binder *can't* be.         ; rhs' <- simplExprC env' rhs cont'         ; return (Alt DEFAULT [] rhs') }

What you should know is that in Core case-expressions are actually more like:

case scrut as b of alts

where `b` binds the evaluated result of `scrut.

So if I am to understand the `simplAlt` code correctly, `case_bndr'` is the binder for the evaluated result of `scrut`. And what is recorded in the unfolding is that once we get to the DEFAULT pattern, we know that `case_bndr'` cannot be the constructors in `imposs_deflt_cons` (probably the constructor matched by the other alternatives).

Now... there's also a FloutOut pass, which might have floated that `case_bndr'` to the TopLevel. And I think that is what you're seeing, and I think you can simply ignore them.

Also... another thing that you should know is that -fexpose-all-unfoldings doesn't actually expose *all* unfoldings. Bottoming bindings are never exposed. That's why in the Clash compiler we have the following code when loading core-expressions from .hi fileshttps://github.com/clash-lang/clash-compiler/blob/cb93b418865e244da50e1d2bc8... 61f3f/clash-ghc/src-ghc/Clash/GHC/LoadInterfaceFiles.hs#L473-L481

loadExprFromTyThing :: CoreSyn.CoreBndr -> GHC.TyThing -> Maybe CoreSyn.CoreExpr loadExprFromTyThing bndr tyThing = case tyThing of   GHC.AnId _id | Var.isId _id ->     let _idInfo    = Var.idInfo _id         unfolding  = IdInfo.unfoldingInfo _idInfo     in case unfolding of       CoreSyn.CoreUnfolding {} ->         Just (CoreSyn.unfoldingTemplate unfolding)       CoreSyn.DFunUnfolding dfbndrs dc es ->         Just (MkCore.mkCoreLams dfbndrs (MkCore.mkCoreConApps dc es))       CoreSyn.NoUnfolding #if MIN_VERSION_ghc(9,0,0)         | Demand.isDeadEndSig $ IdInfo.strictnessInfo _idInfo #else         | Demand.isBottomingSig $ IdInfo.strictnessInfo _idInfo #endif         -> do           let noUnfoldingErr = "no_unfolding " ++ showPpr unsafeGlobalDynFlags bndr           Just (MkCore.mkAbsentErrorApp (Var.varType _id) noUnfoldingErr)       _ -> Nothing   _ -> Nothing

i.e. when we encounter a NoUnfolding with a bottoming demand signature, we conjure an absentError out of thin air.

On Fri, 1 Apr 2022 at 10:05, ÉRDI Gergő wrote: Hi,

I'm CC-ing the Clash mailing list because I believe they should have encountered the same problem (and perhaps have found a solution to it already!).

I'm trying to use `.hi` files compiled with `ExposeAllUnfoldings` set to reconstruct full Core bindings for further processing. By and large, this works, but I get tripped up on identifiers whose unfolding is only given as `OtherCon []`. It is unclear to me what is causing this -- some of them are recursive bindings while others are not.

The problem, of course, is that if all I know about an identifier is that it is `OtherCon []`, that doesn't allow me to restore its definition. So is there a way to tell GHC to put "full" unfoldings everywhere in `ExposeAllUnfoldings` mode?

Thanks,         Gergo

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