Tom Pledger wrote:
Marcin 'Qrczak' Kowalczyk writes: [...]
My new record scheme proposal does not provide such lightweight extensibility, but fields can be added and deleted in a controlled way if the right types and instances are made.
Johan Nordlander must be on holiday or something, so I'll deputise for him. :-)
No holiday in sight, I'm afraid :-) I just managed to resist the temptation of throwing in another ad for O'Haskell. But since my name was brought up...
O'Haskell also has add-a-field subtyping. Here's the coloured point example (from http://www.cs.chalmers.se/~nordland/ohaskell/survey.html):
struct Point = x,y :: Float
struct CPoint < Point = color :: Color
Regards, Tom
Notice though that O'Haskell lacks the ability delete fields, which I think is what Marcin also proposes. I've avoided such a feature in O'Haskell since it would make the principal type of an expression sensitive to future type declarations. For example, assuming we have f p = p.x , its principal type would be Point -> Float if only the type definitions above are in scope, but OneDimPoint -> Float in another scope where some type OneDimPoint is defined to be Point with field y deleted. -- Johan