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 <[EMAIL PROTECTED]> 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) > > ------------------------------------------------------------------------------------------------- > > > > " Ce courriel et les documents qui y sont attaches peuvent contenir des > informations confidentielles. Si vous n'etes pas le destinataire escompte, > merci d'en informer l'expediteur immediatement et de detruire ce courriel > ainsi que tous les documents attaches de votre systeme informatique. Toute > divulgation, distribution ou copie du present courriel et des documents > attaches sans autorisation prealable de son emetteur est interdite." > > " This e-mail and any attached documents may contain confidential or > proprietary information. If you are not the intended recipient, please advise > the sender immediately and delete this e-mail and all attached documents from > your computer system. Any unauthorised disclosure, distribution or copying > hereof is prohibited." > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
