number-parameterized types and heterogeneous lists

20 Jun 2008

      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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