Type family equation violates injectivity?

[Carter Schonwald]

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
>>>
>>
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