Alfonso, Thanks! For didactic purposes I will defer looking into your code. It does not always help to know the correct solution :)
Bests, --A On Mon, Jun 23, 2008 at 11:26 AM, Alfonso Acosta <[EMAIL PROTECTED]> wrote: > 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 > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
