Richard I'm confused too. The 'go' function in 'expandTypeSynonyms' didn't use to take a substitution, but now it does. I think you intend that go subst ty = expand (substTy subst ty) where 'go' is the new 'go' and 'expand' is 'expandTypeSynonyms'. Correct? So you are avoiding repeated traversal by carrying a substitution in with you. It would be good to write down this invariant. OK so suppose we have new_go subst (T ty1 ty2) where type T a = rhs. (Notice T has arity 1 but is applied to two args.) Then we this should be equivalent to expand (T (subst(ty1)) (subst(ty2)) which is expand (rhs[a -> subst ty1] subst(ty2)) So instead of go subst (TyConApp tc tys) | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys = let subst' = unionTCvSubst subst (mkTopTCvSubst tenv) in go subst' (mkAppTys rhs tys') I'd expect to see go subst (TyConApp tc tys) | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc (map (go subst) tys) = mkAppTys (go (mkTopTCvSubst tenv) rhs) tys' Notice that I'm applying 'go subst' to 'tys' before giving the args to 'expandSynTyCon_maybe'. Does that look right? What you have looks entirely inexplicable. Simon | -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Ömer | Sinan Agacan | Sent: 06 January 2016 16:34 | To: ghc-devs <ghc-devs@haskell.org> | Subject: expandTypeSynonyms panics after kind equality patch | | My branch panicking during stage 2 build and when I tried to debug I | realized the panicking function is `unionTCvSubst`, when called by | `expandTypeSynonyms`. | In my branch I'm doing some type-based transformations and I'm using | `expandTypeSynonyms` on type of identifiers for that. | | According to git blame logs, the function `unionTCvSubst` was added | with kind equality patch. The patch made this change in | `expandTypeSynonyms`: | | - = go (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys') | + = let subst' = unionTCvSubst subst (mkTopTCvSubst tenv) in | + go subst' (mkAppTys rhs tys') | | Which is what's causing the panic. The exact place it's panicking | during the | stage2 build is when I call `expandTypeSynonyms` on type `TvSubstEnv` | (i.e. in my transformation I get type of an id and it turns out to be | TvSubstEnv, when I call `expandTypeSynonyms` on this type it panics) | | I figured this much but I don't understand type system details, so, | does anyone here have any ideas what's going wrong here? Richard? | | Note that this panic happens even though I don't do any changes in | types - I'm just looking at the types for some transformations but no | changes on the types are done. | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.h | askell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc- | devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c2d9763ba60bc4a7 | 845d608d316b74402%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=hRlHPbg | girbpZXO1Ei6jN%2bRV7UUl98pFg%2bMHGVzxtmU%3d