Hi Conal, In your case, the `_N` on the argument to NTCo:HasIf[0] is correct -- the `HasIf` class indeed has a nominal parameter. So, it seems that this part, at least, is OK.
What's the -ddump-tc on EP? Is EP's role nominal? (I think so, given what you're saying.) If that's the case, you won't be able to pass the result of NTCo:HasIf[0] to a coercion built from EP. Popping up a level, what are you trying to do here? If EP's role is indeed nominal, then I don't believe there's a fix here, as the coercion it seems you're trying to build may be unsound. (It looks to me like you want something proving `EP (Bool -> Bool -> Bool -> Bool) ~R EP (HasIf Bool)`, but if EP's role is nominal, then this is indeed bogus.) Richard On Apr 14, 2014, at 2:23 PM, Conal Elliott <co...@conal.net> wrote: > Thanks for the pointers! I don't quite know how to get to the form you > recommend from the existing coercion, which is `Simple.NTCo:HasIf[0] > <GHC.Types.Bool>_N`. (Note the `_N`.) I got that coercion in GHC 7.8.2 from > the following code: > > > class HasIf a where > > ifThenElse :: Bool -> a -> a -> a > > > > instance HasIf Bool where > > ifThenElse i t e = (i && t) || (not i && e) > > With `--dump-tc`, I see > > > TYPE SIGNATURES > > TYPE CONSTRUCTORS > > HasIf :: * -> Constraint > > class HasIf a > > Roles: [nominal] > > RecFlag NonRecursive > > ifThenElse :: Bool -> a -> a -> a > > COERCION AXIOMS > > axiom Main.NTCo:HasIf :: HasIf a = Bool -> a -> a -> a > > INSTANCES > > instance HasIf Bool -- Defined at ../test/HasIf.hs:4:10 > > Do I need to convert the nominal coercion I got from GHC > (`Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N` in this case) to a > representational one? If so, how? > My current formulation (tweaked to use `mkSubCo` and `mkAppCo`) is to apply > `mkAppCo (mkSubCo (Refl Nominal ep))` to the given coercion, which then > produces > > > (LambdaCCC.Lambda.EP (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N)))_R > > And still I get "Role incompatibility: expected nominal, got representational > in `Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N)`". > > I also tried wrapping `mkSubCo` around the entire coercion (applying `mkSubCo > . mkAppCo (Refl Nominal ep)`), and I see the same result: > > > (LambdaCCC.Lambda.EP (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N)))_R > > -- Conal > > > On Mon, Apr 14, 2014 at 4:39 AM, Richard Eisenberg <e...@cis.upenn.edu> wrote: > I agree with Simon, but just `Sub` the `<LambdaCCC.Lambda.EP>_N`, not the > whole coercion. > > There are actually two problems here. The one Simon identified, and also the > fact that Simple.NTCo:HasIf[0] produces a representational coercion. How do I > know? Because of the `NT` -- it's a newtype axiom and must produce > representational coercions. Furthermore, unless the newtype definition is > inferred to require a nominal parameter, the newtype would expect a > representational coercion, not the nominal one you are passing. Check the > dump (using -ddump-tc) of the newtype axiom to be sure. > > Though putting a `Sub` on `<EP>` and changing the Refl constructor on > `<Bool>` would work, you would then be violating an invariant of GHC's > Coercion type: that we prefer `TyConAppCo tc ...` over `AppCo (Refl (TyConApp > tc [])) ...`. > > In sum, here is the coercion I think you want: > > (LambdaCCC.Lambda.EP (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_R)))_R > > This is one of the problems with roles -- they are *very* fiddly within GHC, > and it's hard for a non-expert in roles to get them right. > > Have you seen docs/core-spec/core-spec.pdf? That is updated w.r.t. roles and > may be of help. > > Let me know if I can help further! > Richard > > On Apr 14, 2014, at 5:45 AM, Simon Peyton Jones <simo...@microsoft.com> wrote: > >> I think you need a ‘Sub’. >> >> A cast (e `cast` g) requires a representational coercion. I think you have >> given it a (stronger) nominal one. Sub gets from one to t’other. >> >> Simon >> >> From: Glasgow-haskell-users >> [mailto:glasgow-haskell-users-boun...@haskell.org] On Behalf Of Conal Elliott >> Sent: 14 April 2014 06:00 >> To: ghc-devs@haskell.org; glasgow-haskell-us...@haskell.org >> Subject: Help with coercion & roles? >> >> I’m working on a GHC plugin (as part of my Haskell-to-hardware work) and >> running into trouble with coercions & roles. Error message from Core Lint: >> >> Warning: In the expression: >> >> >> LambdaCCC.Lambda.lamvP# >> @ (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool) >> @ (Simple.HasIf GHC.Types.Bool) >> >> "tpl"# >> ((LambdaCCC.Lambda.varP# >> @ (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool) >> "tpl"#) >> >> `cast` (<LambdaCCC.Lambda.EP>_N (Sym (Simple.NTCo:HasIf[0] >> <GHC.Types.Bool>_N)) >> >> ∷ LambdaCCC.Lambda.EP >> (GHC.Types.Bool >> >> → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool) >> >> ~# >> LambdaCCC.Lambda.EP (Simple.HasIf GHC.Types.Bool))) >> >> >> Role incompatibility: expected nominal, got representational >> >> in <LambdaCCC.Lambda.EP>_N (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N)) >> Do you see anything inconsistent/incompatible in the coercions or roles >> above? I constructed the nominal EP Refl coercion, and applied it (AppCo) an >> existing coercion of a simpler type. >> >> Thanks, >> >> -- Conal >> _______________________________________________ >> Glasgow-haskell-users mailing list >> glasgow-haskell-us...@haskell.org >> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs