[mercury-users] Field updates and state variables (more)

[Peter Schachte]

On Wed, Jul 19, 2006 at 01:12:38AM +1000, Nicholas Nethercote wrote:
> Hi,
> 
> Peter Hawkins recently proposed this syntax for updating fields in state 
> variables:
> 
>   map.insert(Key, Value, !S ^ my_map)
> 
> to mean this:
> 
>   map.insert(Key, Value, !.S ^ my_map, Temp),
>   !:S = !.S ^ my_map := Temp,

How about:

    !S ^ my_map ^ elem(Key) := Value

Should work right now, and it's less verbose than the syntax you
suggest.  OBTW, you can also write

    !S ^ my_map ^ elem(Key)

as an expression in place of

    map.lookup(!.S ^ my_map, Key).

I agree that Peter's suggestion would be a natural and useful
extension, but the pseudo-record access syntax is pretty natural and
clean, and emphasizes the symmetry between lookup and insertion.

-- 
Peter Schachte              The only thing necessary for the triumph of evil
schachte at cs.mu.OZ.AU        is for good men to do nothing.
www.cs.mu.oz.au/~schachte/      -- attributed to Edmund Burke 
Phone: +61 3 8344 1338      
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