[m-rev.] for review: state vars syntax sugar

[Peter Schachte]

Ralph Becket wrote:

> What about running DCGs "backwards"?  We might want to do the same with
> state variables.

Touché!  And for the X>>dead mode, doesn't X have to be unique?
And there is also an implied mode, and the mostly unique versions.

But they're still pretty orthogonal:

Forward	modes: 	       X>>Final, Initial>>X
Backward modes:        Initial>>X, X>>Final

For normal versions:   Initial=free
For implied versions:  Initial=X

Normal:                Final=X,
Destructive:           Final=dead and X must be unique
Unique:                Final=unique and X must be unique
Mostly destructive:    Final=mostly_unique and X must be mostly_unique
Mostly unique:         Final=mostly_dead and X must be mostly_unique

If that covers all useful cases, how about this for a syntax:

  !(X)                 means X>>X, free>>X
  !destructive         means unique>>dead, free>>unique
  !mostly_destructive  means mostly_unique>>mostly_dead, free>>mostly_unique
  !input(X)            means X>>X, X>>X
  !backward(X)         means B, A if !(X) means A, B

So most of the time, the mode for a state var would just be !ground, or
occasionally !destructive, which look pretty good to me.  Also note that if
Mercury ever supports ui modes, then !unique means (ui, uo).

-- 
Peter Schachte              I worry that 10 or 15 years from now, [my child]
schachte at cs.mu.OZ.AU        will come to me and say 'Daddy, where were you
www.cs.mu.oz.au/~schachte/  when they took freedom of the press away from
Phone: +61 3 8344 1338      the Internet?' -- Mike Godwin
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