Anonymous records. A solution to the problems of record-system.

[Nikita Volkov]

Yes, TRex indeed looks very similar. I'm quite surprised how I managed to
come up with something that much similar without knowing anything about
TRex.

So do you know anything about why it hasn't been yet implemented?


2013/10/14 Christopher Done <chrisdone at gmail.com>

> So basically, TRex?
>
> On 14 October 2013 18:13, Nikita Volkov <nikita.y.volkov at gmail.com> wrote:
> > Anonymous records. A solution to the problems of record-system.
> >
> > The current record system is notorious for three major flaws:
> >
> > It does not solve the namespacing problem. I.e., you cannot have two
> records
> > sharing field names in a single module. E.g., the following won't
> compile:
> >
> > data A = A { field :: String }
> > data B = B { field :: String }
> >
> > It's partial. The following code will result in a runtime error:
> >
> > data A = A1 { field1 :: String } | A2 { field2 :: String }
> >
> > main = print $ field1 $ A2 "abc"
> >
> > It does not allow you to use the same field name for different types
> across
> > constructors:
> >
> > data A = A1 { field :: String } | A2 { field :: Int }
> >
> > This proposal approaches all the problems above and also a fourth one,
> which
> > is unrelated to the current record system: it allows one to avoid
> > declaration of intermediate types (see details below).
> >
> > Gentlemen, I want you to meet,
> >
> > Anonymous Records
> >
> > When a record-syntax is used in Haskell it's almost always a
> > single-constructor ADT. A question rises then: why use ADT when you don't
> > need its main feature (i.e., the multiple constructors)? This main
> feature
> > is actually the root of the second and the third problem of record-syntax
> > from the list above. In such situations one doesn't actually need ADT,
> but
> > something more like a tuple with ability to access its items by name.
> E.g.:
> >
> > f :: (a :: Int, b :: String) -> String
> > f rec = rec.b ++ show rec.a
> >
> > application:
> >
> > f (a = 123, b = "abc")
> >
> > So now you think "Okay, but how about naming it?". Well, not a problem at
> > all - use the existingtype-construct:
> >
> > type TheRecord = (a :: Int, b :: String)
> >
> > Now, about the avoidance of intermediate types:
> >
> > type Person = (name :: String, phone :: (country :: Int, area :: Int,
> number
> > :: Int))
> >
> > See? No need to declare separate types for inner values. But, of course,
> if
> > you need, you still can:
> >
> > type Phone = (country :: Int, area :: Int, number :: Int)
> > type Person = (name :: String, phone :: Phone)
> >
> > We can nicely access the deeply nested fields, e.g.:
> >
> > personCountryCode :: Person -> Int
> > personCountryCode person = person.phone.country
> >
> > Okay. What about the type ambiguity? E.g., in the following the Person is
> > actually the same type asCompany:
> >
> > type Person = (name :: String, phone :: Phone)
> > type Company = (name :: String, phone :: Phone)
> >
> > Easily solvable with a help of newtype:
> >
> > newtype Person = Person (name :: String, phone :: Phone)
> > newtype Company = Company (name :: String, phone :: Phone)
> >
> > What about ADTs? Again, easy:
> >
> > data Product = Tea (brand :: Company)
> >              | Milk (brand :: Company, fatness :: Float)
> >
> > Now, the beautiful fact:
> >
> > This solution does not conflict with any existing feature of Haskell! As
> the
> > examples show, it easily fits into the language as an extension. It can
> > peacefully coexist with the existing record system of ADTs. Hence a
> complete
> > backwards compatibility with old codebase. There's also a potential for
> many
> > other additional features.
> >
> > Links
> >
> > Source of this proposal.
> >
> >
> > _______________________________________________
> > Haskell-prime mailing list
> > Haskell-prime at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-prime
> >
>
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