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:claus.reinke@talk21.com] 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

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