[Haskell-cafe] Vector mysteries

[Takayuki Muranushi]

Hi, Fixie,

Sadly, I've heard that data type contexts are "widely considered a
misfeature, and is going to be removed from the language." see

http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/data-type-extensions.html

Moreover, when Row explicitly takes its type arguments, the use of
ScopedTypeVariables lets you do the following:

https://github.com/nushio3/practice/blob/master/show/storable-mutable-vector.hs


Best,

2012/12/3 Fixie Fixie <fixie.fixie at rocketmail.com>:
> Hi
>
> I am trying to implement Show for a storable-mutable-vector.
>
> Does anyone have a clue why the type-system is killing me :-)
>
> Code is below...
>
> Cheers
>
> Felix
>
> ----
>
> {-# LANGUAGE ExistentialQuantification #-}
>
> import Control.Monad (liftM2)
> import qualified Data.Vector.Unboxed as V
> import qualified Data.Vector.Unboxed.Mutable as MV
> import Data.Vector.Storable.Mutable
> import GHC.Prim (RealWorld)
> import Control.Monad.Primitive
> import Control.Monad
> import qualified Data.Vector as VEC
> import qualified Data.Vector.Generic.Mutable as GM
> import Data.Int
> import Data.Typeable.Internal
> import Data.Primitive
>
> data Row = forall m s. (Storable s, MV.Unbox s, Prim s, PrimMonad m) => Row
> (MV.MVector (PrimState m) s)
>
> instance Show Row where
>     show (Row row) = do
>                     let xx = MV.read row 0
>                     "Done"
>
> main :: IO ()
> main = do
>     print "Done"
>
> I get this error:
>
>     Could not deduce (PrimState m ~ PrimState m0)
>     from the context (Storable s, MV.Unbox s, Prim s, PrimMonad m)
>       bound by a pattern with constructor
>                  Row :: forall (m :: * -> *) s.
>                         (Storable s, MV.Unbox s, Prim s, PrimMonad m) =>
>                         MV.MVector (PrimState m) s -> Row,
>                in an equation for `show'
>     NB: `PrimState' is a type function, and may not be injective
>     Expected type: MV.MVector (PrimState m0) s
>       Actual type: MV.MVector (PrimState m) s
>     In the first argument of `MV.read', namely `row'
>     In the expression: MV.read row 0
>
>
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-- 
Takayuki MURANUSHI
The Hakubi Center for Advanced Research, Kyoto University
http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html