[Haskell-cafe] Project postmortem II /Haskell vs. Erlang/

[Joel Reymont]

Yes, that _is_ obvious but then puts the burden on the programmer to  
define the getters. It also misses the setters issue entirely.

Each field can definitely be made into a class and records can be  
composed dynamically, HList-style. I think HList is _the_ facility  
for doing this.

How do you create a "record" that has a field dictionary for updates  
and retrievals and stores the order of the fields for serialization?  
I think HList does provide for the order of records since it keeps  
them in a list of sorts.

Can serialization of these HList-style records serialization be  
efficient in this case? Would updating fields be efficient?

I tried composing my records using HList and that made GHC run out of  
memory and bomb out. Simon Peyton-Jones fixed the problem promptly  
but since I was not familiar with the profiler at the time I never  
got to measuring the pickling efficiency. I figured if it's that  
tough on the compiler then maybe I should pick an easier path.

	Thanks, Joel

On Jan 4, 2006, at 8:41 AM, Ketil Malde wrote:

> Isn't the obvious solution to declare a class here?  I.e.
>
>    class HasProfits h where profits :: h -> Word64
>
>    data Pot = Pot { pProfits :: !Word64, pAmounts = ![Word64] }
>
>    instance HasProfits Pot where profits = pProfits

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