Type family equation violates injectivity?

Fri Jan 4 14:23:08 UTC 2019

Yeah, I later found it impacted one of my own pieces of code too, in that I
needed to make still further type families injective.

I do think that a lot of vectors current module structure reflects a desire
for injectivity coupled with historical a lack of mechanism for
guaranteeing it.

Mess up on my part for sure.  :)

On Fri, Jan 4, 2019 at 8:11 AM Boespflug, Mathieu <m at tweag.io> wrote:

> Hi Carter,
>
> thanks for looking into this. We were initially surprised to see a
> breaking change in a point release, but no biggy. It's pretty hard to offer
> strong stability guarantees without automated tooling to catch this kind of
> thing, and these things happen. We'll patch up HaskellR shortly.
>
> Best,
>
>
> On Sun, 30 Dec 2018 at 01:06, Carter Schonwald <carter.schonwald at gmail.com>
> wrote:
>
>> To be clear : I’m annoyed with myself that this impacted a package that
>> depends on vector, but this does seem to be the case that the newest bug
>> fix release for vector actually revealed a broken design for the vector
>> instances / data types in the inline-r package.
>>
>> To;dr — I suggest patching inline-r to remove the s parameter in its
>> immutable vector data types
>>
>> On Sat, Dec 29, 2018 at 6:48 PM Carter Schonwald <
>> carter.schonwald at gmail.com> wrote:
>>
>>> so i took a look .. (also the inline-r devs seem to have done a hackage
>>> revision so you wont hit that issue in your current setup if you do a cabal
>>> update ..)
>>> and it seems like the type definitions in inline-r are kinda bogus  and
>>> you should get them patched ...
>>>
>>> the MVector type class, and related type families, all assume your
>>> mutable type has the last two arguments as the io/state token and then the
>>> element type
>>>
>>> eg
>>> basicLength :: v s a -> Int
>>> <http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Int.html#t:Int>
>>>
>>>
>>> i looked at
>>> https://github.com/tweag/HaskellR/blob/1292c8a9562764d34ee4504b54d93248eb7346fe/inline-r/src/Data/Vector/SEXP.hs#L346-L374
>>> and
>>>
>>>
>>>
>>> as a point of grounding this chat
>>> the injective type familly in question is defined by the follwoing
>>>
>>>
>>> --#if MIN_VERSION_base(4,9,0)type family Mutable <http://hackage.haskell.org/package/vector-0.12.0.2/docs/src/Data.Vector.Generic.Base.html#Mutable> (v <http://hackage.haskell.org/package/vector-0.12.0.2/docs/src/Data.Vector.Generic.Base.html#local-6989586621679032525> :: * -> *) = (mv <http://hackage.haskell.org/package/vector-0.12.0.2/docs/src/Data.Vector.Generic.Base.html#local-6989586621679032526> :: * -> * -> *) | mv -> v#elsetype family Mutable (v :: * -> *) :: * -> * -> *#endif
>>>
>>> anyways, it looks like the Pure / immutable vector data type in inline-r
>>> has a spurious state token argument in its definition that shouldn't be
>>> there, OR there need to be two "s" params in inline-r instead of the one
>>>
>>> heres the full code i linked to in question
>>>
>>>
>>> -- | Mutable R vector. Represented in memory with the same header as
>>> 'SEXP'
>>>
>>> -- nodes. The second type parameter is phantom, reflecting at the type
>>> level the
>>> -- tag of the vector when viewed as a 'SEXP'. The tag of the vector and
>>> the
>>> -- representation type are related via 'ElemRep'.
>>> data MVector s ty a = MVector
>>> { mvectorBase :: {-# UNPACK #-} !(SEXP s ty)
>>> , mvectorOffset :: {-# UNPACK #-} !Int32
>>> , mvectorLength :: {-# UNPACK #-} !Int32
>>> }
>>> -- | Internal wrapper type for reflection. First type parameter is the
>>> reified
>>> -- type to reflect.
>>> newtype W t ty s a = W { unW :: MVector s ty a }
>>> instance (Reifies t (AcquireIO s), VECTOR s ty a) => G.MVector (W t ty)
>>> a where
>>>
>>> data Vector s (ty :: SEXPTYPE) a = Vector
>>>     { vectorBase :: {-# UNPACK #-} !(ForeignSEXP ty) , vectorOffset ::
>>> {-# UNPACK #-} !Int32 , vectorLength :: {-# UNPACK #-} !Int32
>>> }
>>>
>>>
>>> type instance G.Mutable (W t ty s) = Mutable.W t ty
>>> Anyways, the fix here is to remove the s param from the Pure version of
>>> W and "Sexp Vector"
>>>
>>>
>>>
>>> On Sat, Dec 29, 2018 at 6:16 PM Carter Schonwald <
>>> carter.schonwald at gmail.com> wrote:
>>>
>>>> were you using the same version of vector in both setups?
>>>>
>>>> in the most recent  vector release  we made mutable type family
>>>> injective in the vector package for ghc's that support it ...
>>>>
>>>> On Sat, Dec 29, 2018 at 1:50 PM Dominick Samperi <djsamperi at gmail.com>
>>>> wrote:
>>>>
>>>>> When I use v8.6.3 of GHC under Ubuntu to install the inline-r package
>>>>> I get the error "Type family equation violates injectivity
>>>>> annotation," and
>>>>> a type variable on the LHS cannot be inferred from the RHS, due to
>>>>> the lack of injectivity (I suppose).
>>>>>
>>>>> On the other hand, v8.0.2 of GHC (shipped with Haskell Platform under
>>>>> Ubuntu) does not have this problem (it has other problems).
>>>>>
>>>>> Has something changed in the latest version of the compiler that might
>>>>> cause this? Possible work-around?
>>>>>
>>>>> FYI, the line that triggers the error is:
>>>>> type instance G.Mutable (W t ty s) = Mutable.W t ty
>>>>>
>>>>> The variable that cannot be inferred is 's'.
>>>>>
>>>>> Thanks,
>>>>> Dominick
>>>>> _______________________________________________
>>>>> ghc-devs mailing list
>>>>> ghc-devs at haskell.org
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>>
>>>> _______________________________________________
>> ghc-devs mailing list
>> ghc-devs at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20190104/e6d1b0bd/attachment.html>