Type family equation violates injectivity?

[Carter Schonwald]

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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