[Haskell-cafe] number-parameterized types and heterogeneous lists

[Alfonso Acosta]

Inspired in Oleg's ideas, I implemented the packages type-level and
parameterized-data (which includes number-parameterized vectors).


To get an idea about how they work you might want to read their
haddock documentation in hackage:

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/type-level
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parameterized-data

You can get their darcs repositories (including some minor updates) from:

darcs get http://code.haskell.org/type-level
darcs get http://code.haskell.org/parameterized-data

On Fri, Jun 20, 2008 at 8:01 AM, Harald ROTTER <harald.rotter at sagem.com> wrote:
>
> Dear Haskellers,
>
> after reading Oleg Kiselyov's paper on number-parameterized types I started
> to play around with
> the class Digits that encodes decimal numbers in types. The "typed number"
> 10 would e.g. be defined as
>
>      D1 $ D0 $ Sz
>
> I wondered if it would be possible replace the expression above by a
> heterogeneous list like
>
>      [D1,D0]
>
> so I tried to define
>
>      data Digit = forall a b.(Digits a, Digits (b a)) => Digit (a -> b a)
>
> Loading this into ghci yields:
>
> :t Digit D0
>
> <interactive>:1:0:
>    Ambiguous type variable `a' in the constraint:
>      `Digits a' arising from a use of `Digit' at <interactive>:1:0-7
>    Probable fix: add a type signature that fixes these type variable(s)
>
> Removing the type constraints in the definition of "Digit":
>
>      data Digit = forall a b.Digit (a -> b a)
>
> makes it work like this:
>
>      :t Digit D0
>      Digit D0 :: Digit
>
>      :t [Digit D0, Digit D1]
>      [Digit D0, Digit D1] :: [Digit]
>
> "Digit", however, is far too general (it also includes e.g. \x -> [x]), but
> I would like it to be restricted to the Digit class.
>
> Any help is appreciated.
>
> Thanks
>
> Harald.
>
>
> CODE:
>
> module Test where
>
> data D0 a = D0 a
> data D1 a = D1 a
> data D2 a = D2 a
> data D3 a = D3 a
> data D4 a = D4 a
> data D5 a = D5 a
> data D6 a = D6 a
> data D7 a = D7 a
> data D8 a = D8 a
> data D9 a = D9 a
>
> class Digits ds where
>    d2num :: Num a => ds -> a -> a
>
> data Sz = Sz    -- zero size
> instance Digits Sz where
>    d2num _ acc = acc
>
> instance Digits ds => Digits (D0 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc)
> instance Digits ds => Digits (D1 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+1)
> instance Digits ds => Digits (D2 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+2)
> instance Digits ds => Digits (D3 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+3)
> instance Digits ds => Digits (D4 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+4)
> instance Digits ds => Digits (D5 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+5)
> instance Digits ds => Digits (D6 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+6)
> instance Digits ds => Digits (D7 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+7)
> instance Digits ds => Digits (D8 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+8)
> instance Digits ds => Digits (D9 ds) where
>    d2num dds acc = d2num (t22 dds) (10*acc+9)
>
> t22 :: f x -> x
> t22 = undefined
>
> --data Digit = forall a b.(Digits a, Digits (b a)) => Digit (a -> b a)
> data Digit = forall a b.Digit (a -> b a)
>
> -------------------------------------------------------------------------------------------------
>
>
>
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