Collapsing HsForAllTy, again

[Alan & Kim Zimmerman]

And once the splitLHsForAllTy is sorted, this

tc_inst_head :: HsType Name -> TcM TcType
tc_inst_head (HsForAllTy _ _ hs_tvs hs_ctxt hs_ty)
  = tcHsTyVarBndrs hs_tvs $ \ tvs ->
    do { ctxt <- tcHsContext hs_ctxt
       ; ty   <- tc_lhs_type hs_ty ekConstraint    -- Body for forall has
kind Constraint
       ; return (mkSigmaTy tvs ctxt ty) }

results in

libraries/base/Data/Monoid.hs:217:23:
    Illegal constraint: Alternative f => Monoid (Alt f a)
    In the instance declaration for ‘Alternative f => Monoid (Alt f a)’

Alan

On Fri, Apr 10, 2015 at 1:11 PM, Alan & Kim Zimmerman <alan.zimm at gmail.com>
wrote:

> It looks like
>
> splitLHsForAllTy
>     :: LHsType name
>     -> (LHsTyVarBndrs name, HsContext name, LHsType name)
> splitLHsForAllTy poly_ty
>   = case unLoc poly_ty of
>         HsParTy ty                -> splitLHsForAllTy ty
>         HsForAllTy _ _ tvs cxt ty -> (tvs, unLoc cxt, ty)
>         _                         -> (emptyHsQTvs, [], poly_ty)
>         -- The type vars should have been computed by now, even if they
> were implicit
>
> needs a recursive call for the HsForAllTy case, it now generates
>
> libraries/base/Data/Monoid.hs:217:10:
>     Malformed instance: forall f a. Alternative f => Monoid (Alt f a)
>
>
> Alan
>
> On Fri, Apr 10, 2015 at 10:13 AM, Simon Peyton Jones <
> simonpj at microsoft.com> wrote:
>
>>  Look at how instance declarations are parsed. If you look at Parser.y
>> you’ll see that for
>>
>>
>>
>> instance (Eq a, Eq b) => Eq (a,b)
>>
>>
>>
>> we get (in effect)
>>
>>
>>
>> mkImplicitHsForAllTy (mkQualifiedHsForAllTy (Eq a, Eq b) (Eq (a,b))
>>
>>
>>
>> The outer mkImplicit.. is to ensure that there is always, in the end, a
>> HsForAllTy around the whole thing, even around
>>
>> instance Eq a
>>
>> say.
>>
>>
>>
>> But we don’t actually want two nested HsForAllTys.  mk_forall_ty
>> collapsed the two.
>>
>>
>>
>> But you don’t want that either.  So I think you should make
>> mkImplictHsForAllTy do the test instead.  Its goal is to wrap a HsForallTy
>> if there isn’t one already. So
>>
>>
>>
>> mkImplicitHsForAllTy (HsForAllTy exp tvs cxt ty)
>>
>>   = HsForAllTy exp’ tvs cxt ty
>>
>>   where
>>
>>     exp’ = case exp of
>>
>>              Qualified -> Implicit
>>
>>              _         -> exp
>>
>> mkImplicitHsForAllTy ty = mkHsForAllTy Implicit  []  (L loc _) ty
>>
>>
>>
>> should do the job.
>>
>>
>>
>> Incidentally, mkImplicitHsForAllTy should not take a ctxt argument.  If
>> you have a non-empty context, use mkQualifiedHsForAllTy.  That means that
>> in Convert you’ll need to use
>>
>>    mkHsForAllTy Implicit ctxt ty’
>>
>> instead of mkImplicitHsForAllTy
>>
>>
>>
>> Simon
>>
>>
>>
>> *From:* Alan & Kim Zimmerman [mailto:alan.zimm at gmail.com]
>> *Sent:* 10 April 2015 08:02
>> *To:* Simon Peyton Jones
>> *Cc:* ghc-devs at haskell.org
>> *Subject:* Re: Collapsing HsForAllTy, again
>>
>>
>>
>> If I replace it with
>>
>>
>> mkHsForAllTy :: HsExplicitFlag -> [LHsTyVarBndr RdrName] -> LHsContext
>> RdrName -> LHsType RdrName -> HsType RdrName
>> -- Smart constructor for HsForAllTy
>> -- mkHsForAllTy exp tvs (L _ []) ty = mk_forall_ty exp tvs ty
>> mkHsForAllTy exp tvs (L _ []) ty = HsForAllTy exp Nothing (mkHsQTvs tvs)
>> (L noSrcSpan []) ty
>> mkHsForAllTy exp tvs ctxt     ty = HsForAllTy exp extra   (mkHsQTvs tvs)
>> cleanCtxt        ty
>>   where -- Separate the extra-constraints wildcard when present
>>         (cleanCtxt, extra)
>>           | (L l HsWildcardTy) <- ignoreParens (last (unLoc ctxt)) =
>> (init `fmap` ctxt, Just l)
>>           | otherwise = (ctxt, Nothing)
>>         ignoreParens (L _ (HsParTy ty)) = ty -- TODO:AZ We lose the
>> annotation here
>>         ignoreParens ty                 = ty
>>
>>
>> I get the following errors in the stage 2 compile (only first 3 shown
>> here)
>>
>>
>> libraries/ghc-prim/GHC/Classes.hs:52:19:
>>     Malformed instance: (Eq a, Eq b) => Eq (a, b)
>>
>> libraries/ghc-prim/GHC/Classes.hs:53:19:
>>     Malformed instance: (Eq a, Eq b, Eq c) => Eq (a, b, c)
>>
>> libraries/ghc-prim/GHC/Classes.hs:54:19:
>>     Malformed instance: (Eq a, Eq b, Eq c, Eq d) => Eq (a, b, c, d)
>>
>>
>>   Alan
>>
>>
>>
>> On Fri, Apr 10, 2015 at 12:14 AM, Simon Peyton Jones <
>> simonpj at microsoft.com> wrote:
>>
>>  Hmm.  I’m not sure what the motivation is either.  Try dropping it out
>> and see if anything goes wrong.
>>
>>
>> Simon
>>
>>
>>
>> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Alan
>> & Kim Zimmerman
>> *Sent:* 09 April 2015 22:15
>> *To:* ghc-devs at haskell.org
>> *Subject:* Collapsing HsForAllTy, again
>>
>>
>>
>> With the help of Jan Stolarek I tracked down the HsForAllTy flattening to
>>  `HsTypes.mk_forall_ty`.
>>
>> This function takes any nested HsForAllTy's where the top one does not
>> have a context defined, and collapses them into a single one.
>>
>> I do not know what the motivation for this is, and if it perhaps speeds
>> up or simplifies further compilation.
>>
>> But now that API Annotations have arrived, making sure we do not lose the
>> annotations for the sub-HsForAllTy  causes significant gymnastics in the
>> parser [1].
>>
>> So my question is, is there a good reason to continue doing this, given
>> the trade-off in parser complexity.
>>
>> Regards
>>
>>   Alan
>>
>> [1]  https://phabricator.haskell.org/D833
>>
>>
>>
>>
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20150410/cc00a8bb/attachment.html>