access to fields in nested records

[Robert Ennals]

> DRiFT will let you do that, by deriving get_ and set_ methods, but other 
> than that, no.  THere was discussion about a year ago (I think) about 
> this, with proposed syntax for something, but it never really got off the 
> ground.  You can probably google for it with my name and some other 
> keywords -- it will likely turn up.

Assuming you are referring to the discussing that I think you are referring 
to, I have a half-written write-up of the idea at the following URL:

http://www.cl.cam.ac.uk/users/rje33/records.ps

This is probably a bit more intelligible than the mailing list messages.


I haven't made an attempt to implement this stuff -- I'm too busy frantically 
finishing my PhD.



-Rob



>  - Hal
> 
> On Thu, 6 Nov 2003, Tim Docker wrote:
> 
> > One thing that bugs me about the named record syntax, is that with a datatype:
> > 
> > data T = T {
> >     t_f1 :: X,
> >     t_f2 :: X
> > };
> > 
> > the function t_f1 has the type of an "accessor", ie
> > 
> > 	t_f1 :: T -> X
> > 
> > but there doesn't any tidy way to get at the "mutator" function
> > 
> > 	t_f1' :: X -> T -> T
> > 
> > without actually having to write the following 
> > 
> > 	t_f1' x t = t{t_f1=x}
> > 
> > Or is there?
> > 
> > 
> > Actually mutators of the general form 
> > 
> > 	t_f1' :: (X->X) -> T -> T
> > 	t_f1_ f t = t{t_f1=f (t_f1 t)}
> > 
> > _______________________________________________
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> > 
> 
> -- 
>  Hal Daume III                                   | hdaume at isi.edu
>  "Arrest this man, he talks in maths."           | www.isi.edu/~hdaume
> 
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