Tuples or Record

[Derek Elkins]

On Mon, 18 Aug 2003 16:21:46 +0100
Keith Wansbrough <Keith.Wansbrough@cl.cam.ac.uk> wrote:

> > Hello,
> > When I have started my project, I use a Tuples but i would know if
> > it is possible to create a record such C or Ocaml provide. I mean
> > creating a structure  where variables are accessible by a '.' or
> > something like that.
> 
> Yes.  Like this:
> 
>   data Tree a = Node { key :: Int,
>                        val :: a,
>                        left, right :: Tree a }
>               | Nil deriving Show
>   

hmm, maybe I shouldn't say this, but record syntax does not preclude
positional syntax.  You can still pattern match and construct Node as
if it was defined Node Int a (Tree a) (Tree a), i.e. 
Node 1 "foo" Nil Nil
or f (Node 0 _ _ _) = ...
Of course, this brings back all the issues of positional notation, so
you may not want to use this... um, ever.
However, one issue with records is that it's possible (or rather easier)
to make partially defined records.  E.g. Node { key = 5 } is legal, and
any attempt to use val/left/right will cause a run-time error.  So this
might be a use for the alternate positional syntax.  Code that uses the
positional syntax to build records will break when fields are added to
the record rather than go on creating broken records (of course, GHC at
least, produces a warning for uninitialized fields).