[Haskell-cafe] Typeable state map

[Neil Brown]

Hi,

You have a problem with your function getPhi.  It has type:

getPhi :: forall a e m. (MutableMatrix a e m, Typeable a, Typeable e, 
Floating e, Ord e) => VarStateT m ()

where MutableMatrix is:

class (PrimMonad m) => MutableMatrix a e m | a -> e m where

No matter how you call getPhi, you can't specify the 'a' and 'e' types 
-- they don't appear in the real part of the type signature, and the 
fundep on the MutableMatrix class is not sufficient to determine 'a' or 
'e' from 'm'.  This means that there is no way for the compiler to pick 
a type for 'a' or 'e' when getPhi is called, and there is no way to make 
the 'a' and 'e' for getPhi be the same as the 'a' and 'e' for compute.  
getPhi needs to know what type it is attempting to cast the dynamic 
state into and at the moment you can't tell it.

The solutions that I can think of are either to pass dummy parameters of 
type 'a' and 'e' to allow you to specify the type, or to add a fundep 
like m -> a e (but I don't think that one holds?).

Thanks,

Neil.

On 10/11/10 13:10, Vladimir Matveev wrote:
> Hi,
> I want to use something like (Control.Monad.StateT (Data.Map.Map
> String Dynamic) m). So I wrote VarStateT datatype and corresponding
> classes (Control/Monad/VarState.hs in [1]). Then I tried to use it in
> my program (Math/Eigenvalues.hs in [1]). But compiler refused to
> compile it with many type errors. I thought that it would be
> sufficient to use ScopedTypeVariables and write all type annotations.
> It helped slightly, many messages were gone, but now compiler throws
> an error I don't know how to solve.
>
> ghc --make  -o out/eigenvalues -outputdir build Main.hs
> [3 of 4] Compiling Math.Eigenvalues ( Math/Eigenvalues.hs,
> build/Math/Eigenvalues.o )
>
> Math/Eigenvalues.hs:14:30:
>      Could not deduce (MutableMatrix a e m)
>        from the context (MutableMatrix a2 e2 m,
>                          Typeable a2,
>                          Typeable e2,
>                          Floating e2,
>                          Ord e2)
>        arising from a use of `getPhi' at Math/Eigenvalues.hs:14:30-35
>      Possible fix:
>        add (MutableMatrix a e m) to the context of
>          the type signature for `compute'
>      In the second argument of `(>>)', namely `getPhi'
>      In the first argument of `(>>)', namely `maxElem a>>  getPhi'
>      In the first argument of `(>>)', namely
>          `maxElem a>>  getPhi>>  buildB'
>
> Math/Eigenvalues.hs:14:40:
>      Could not deduce (MutableMatrix a1 e1 m)
>        from the context (MutableMatrix a2 e2 m,
>                          Typeable a2,
>                          Typeable e2,
>                          Floating e2,
>                          Ord e2)
>        arising from a use of `buildB' at Math/Eigenvalues.hs:14:40-45
>      Possible fix:
>        add (MutableMatrix a1 e1 m) to the context of
>          the type signature for `compute'
>      In the second argument of `(>>)', namely `buildB'
>      In the first argument of `(>>)', namely
>          `maxElem a>>  getPhi>>  buildB'
>      In the first argument of `(>>=)', namely
>          `maxElem a>>  getPhi>>  buildB>>  buildA'
>
> How to sort this out?
>
> [1] http://dl.dropbox.com/u/1415321/prog/eigenvalues.tar.gz
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