[Haskell-cafe] Re: Implementing fixed-sized vectors (using datatype algebra?)

[Alfonso Acosta]

OK I'll include the module after I change the things mentioned.

BTW, I finally have an initial version of the parameterized-data package:

Darcs repository:

http://code.haskell.org/parameterized-data

Haddock documentation:

http://code.haskell.org/~fons/parameterized-data/doc/

Any comments/suggestions would be appreciated.

To fix the problem of the "vector" constructor I included a Template
Haskell variant. It is quite simple to use:

$ ghci -XTemplateHaskell
Prelude> :m +Data.Param
Prelude Data.Param> $(vectorTH [1 :: Int, 2, 3, 4])
Prelude Data.Param> :t $(vectorTH [1 :: Int, 2, 3, 4])
(vectorTH [1 :: Int, 2, 3, 4]) :: (Num t) => FSVec Data.TypeLevel.Num.Reps.D4 t

Note that the size parameter (type-level decimal numeral) is correct.

It would be nice to be able to use a Quasiquoter [1] (available in GHC
HEAD) to enable pattern matching and custom syntax to Vector literals.
However, I was bitten by a polymorphism problem when implementing it:
see [2] for details

[1] http://www.haskell.org/ghc/dist/current/docs/users_guide/template-haskell.html#th-quasiquotation
[2] http://www.haskell.org/pipermail/template-haskell/2008-February/000655.html

On Wed, Feb 20, 2008 at 2:26 AM, Wolfgang Jeltsch
<g9ks157k at acme.softbase.org> wrote:
> Am Mittwoch, 20. Februar 2008 00:39 schrieben Sie:
>
> > Why are the value-level reflecting functionsimplemented as type-class
>  > methods? It makes the code more verbose and I don't see any advantage
>  > compared to simply defining a function per class. Let me show you an
>  > example:
>  >
>  > This is your implementation of Not:
>  >
>  >     class (Boolean boolean, Boolean boolean') =>
>  >           Not boolean boolean' | boolean -> boolean', boolean' -> boolean
>  > where not :: boolean -> boolean'
>  >
>  >     instance Not False True where
>  >         not _ = true
>  >
>  >     instance Not True False where
>  >         not _ = false
>  >
>  > This is how I would do it:
>  >
>  >     class (Boolean boolean, Boolean boolean') =>
>  >           Not boolean boolean' | boolean -> boolean', boolean' -> boolean
>  > where
>  >
>  >     instance Not False True
>  >     instance Not True False
>  >
>  >    not :: Not a b => a -> b
>  >    not = undefined
>
>  Your definition of the not function uses the implementation detail that false
>  and true are actually undefined.  My implementation of the not function also
>  works if false and true are defined to be something different.  Of course,
>  false and true are in the same library, so we know this implementation detail
>  and could make use of it.
>
>
>  > Furthermore, why did you choose to use Boolean instead of simply Bool?
>
>  To avoid a name clash.  Feel free to change this.
>
>  Note that False and True don't cause a name clash since they live in a
>  namespace different from the one Prelude's False and True live in.
>
>  > Cheers,
>  >
>  > Fons
>
>
>
>  Best wishes,
>  Wolfgang
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