uniqAway and collisions

[Bartosz Nitka]

Hello,

Thank you for the paper, it helped with my understanding of how it's
supposed to work.
Simon, could my issue be related to your comment here: [1]?

-- Note [Generating the in-scope set for a substitution]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- If we want to substitute [a -> ty1, b -> ty2] I used to
-- think it was enough to generate an in-scope set that includes
-- fv(ty1,ty2).  But that's not enough; we really should also take the
-- free vars of the type we are substituting into!  Example:
--  (forall b. (a,b,x)) [a -> List b]
-- Then if we use the in-scope set {b}, there is a danger we will rename
-- the forall'd variable to 'x' by mistake, getting this:
--  (forall x. (List b, x, x)
-- Urk!  This means looking at all the calls to mkOpenTvSubst....

Currently the InScope set only contains the free variables of the arguments
when linting type application [2][3][4] and doesn't contain the free
variables of the body that it's substituting in.

The definition of substTyWith is:

substTyWith :: [TyVar] -> [Type] -> Type -> Type
substTyWith tvs tys = ASSERT( length tvs == length tys )
                      substTy (zipOpenTCvSubst tvs tys)


When I changed it to include the free variables of the body my core lint
error went away:

substTyWith :: [TyVar] -> [Type] -> Type -> Type
substTyWith tvs tys ty = ASSERT( length tvs == length tys )
                      substTy (extendTCvInScopeList (zipOpenTCvSubst tvs
tys) (tyCoVarsOfTypeList ty)) ty


It seems unlikely to me that this is the issue, since this code is very
old, but I don't have a better explanation for this and a second pair of
eyes would help.

Thank you,
Bartosz

[1]
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1591-1601
[2]
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/coreSyn/CoreLint.hs;cac0795af33d622e4c6ebae6ae1f206969287088$788
[3]
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1756
[4]
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/TyCoRep.hs;cac0795af33d622e4c6ebae6ae1f206969287088$1623


2016-01-06 8:42 GMT+00:00 Simon Peyton Jones <simonpj at microsoft.com>:

> |  I doubt there's a bug in uniqAway; it's more likely the in scope set
> |  is not correct.
>
> I think Edward is probably right here.
>
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