That looks nice, just unfortunate you need to cast to ::Float in homer2?Age::Float. I don't see why this is needed, but I must say I don't understand your code completely yet, working on that :)
Also, wouldn't this approach be less performant? Or is GHC that good that ist compiles away all the overhead? >----- Oorspronkelijk bericht ----- >Van: Claus Reinke [mailto:[EMAIL PROTECTED] >Verzonden: dinsdag, juni 19, 2007 01:37 PM >Aan: haskell-cafe@haskell.org >Onderwerp: Re: [Haskell-cafe] Useful IDE features - "implement instance" > >> Just another wild idea which I might find useful, but is more like >> refactoring, is to convert the fields of a record to get/set type-classes, >> and refactor all usages of those fields. > >you could use a preprocessor (DrIFT, Data.Derive) to derive the >instances, but you need to share the class declarations between all >client modules. > >> ------------------- >> data Person = Person { name :: String, age :: Float } >> >> main = print $ name p ++ " is " ++ show (age p) ++ " years old" >> where p = Person { name = "Homer", age = 41 } >> ------------------- >>.. > >alternatively, you could generalise this a bit, so that there is only one >class for all combinations of records, fields, and field value types, and >then generalise it further so that you only need one pair of instances >to define selection and update for all records. that kind of operates at >the borders of the language, so you lose portability (the nicest version >is ghc-only; nearly all language extensions used are also supported >by hugs, but with a slightly different interpretation). you'd still need >to share the label types between all client modules. > >claus > >---------------------------------------------------------- >{-# OPTIONS_GHC -fallow-undecidable-instances #-} >{-# OPTIONS_GHC -fallow-overlapping-instances #-} >{-# OPTIONS_GHC -fglasgow-exts #-} > >infixl ? >infixr <:,:< > >------------------- poor man's records > >-- record extension >-- (ghc only: infix constructor; for hugs, use (,) instead) >data fieldValue :< record = fieldValue :< record > >-- field selection (?) and update (<:) >-- needs overlapping instances to recurse down record extensions >-- for hugs: drop the functional dependency, use more type annotations >class Has field value record | field record -> value where > (?) :: record -> field -> value > (<:) :: (field,value) -> record -> record > >-- if the first field matches, we're done >instance Has field value ((field,value) :< record) where > ((_f,v) :< _) ? f = v > (f,v) <: ((_f,_) :< r) = ((f,v) :< r) > >-- otherwise, try again, with the remaining record >instance Has field value record => Has field value ((f,v) :< record) where > ((f',v') :< r) ? f = r ? f > (f,v) <: ((f',v') :< r) = ((f',v') :< ( (f,v)<:r ) ) > >-- some field labels >data Name = Name >data Age = Age > >------------------- a generic version, no separate Person type or instances > >type Person1 = (Name,String) :< (Age,Float) :< () > >homer :: Person1 >homer = (Name,"Homer") :< (Age,41) :< () > >test1 = print $ homer?Name ++ " is " ++ show(homer?Age) ++ " years old" > >------------------- a more down-to-earth version, closer to the original > >data Person = Person String Float > >instance Has Name String Person where > (Person name age) ? Name = name > (Name,newName) <: (Person name age) = Person newName age > >instance Has Age Float Person where > (Person name age) ? Age = age > (Age,newAge) <: (Person name age) = Person name newAge > >defaultPerson = Person "" 0 > >homer2 = (Name,"Homer2") <: (Age,42::Float) <: defaultPerson > >test2 = print $ homer2?Name ++ " is " ++ show(homer2?Age::Float) ++ " years >old" > >------------------- > >main = test1 >> test2 > > > >_______________________________________________ >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
