Re: Records in Haskell

On 12/28/11 1:34 PM, Donn Cave wrote:

Quoth Greg Weber,

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On Wed, Dec 28, 2011 at 2:12 PM, Donn Cave wrote: ...

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I would think row polymorphism is a must-have.

Perhaps if you want *extensible* records. If you would like to make some progress with records in the near future rather than keeping records in limbo, I think we really need to give up for the moment on any higher form of abstraction than straight-forward name-spacing.

No, to be clear on that, I haven't given much thought to extensibility per se, I was thinking row polymorphism is a valuable feature on its own, and extensibility just seemed to me to be an implicit side benefit.

Yes, row polymorphism would still be helpful in lack of extensible records. In particular it allows for easily determining principle (structural) types of records; this is desirable from a generic programming perspective even if records on the whole are not structurally typed. That is, we can distinguish the following types data Foo = MkFoo { x :: T } data Bar = MkBar { x :: T } By considering them to desugar into type Foo = { __type :: Foo , x :: T } constructor MkFoo :: T -> Foo pattern MkFoo :: T -> Pattern type Bar = { __type :: Bar , x :: T } constructor MkBar :: T -> Bar pattern MkBar :: T -> Pattern accessor x :: { x :: T , ... } -> x Of course, an actual implementation needn't come up with a phantom argument like __type in order to nominally distinguish structurally identical types. Rather, the existence of the hack shows that it's doable. -- Live well, ~wren