[m-rev.] for review: improve purity documentation

[Fergus Henderson]

On 13-Mar-2002, Peter Schachte <schachte at cs.mu.OZ.AU> wrote:
> > doc/reference_manual.texi:
> > 	Improve the documentation regarding purity.
> 
> This looks good to me, except one nit:
> 
> >  @item semipure
> >  Semipure predicates are just like pure predicates, except that their
> > -behaviour may be affected by the invocation of impure predicates.  That
> > -is, they are sensitive to the state of the computation other than as
> > -reflected by their input arguments, though they do not affect the state
> > -themselves.
> > +declarative semantics may be affected by the invocation of impure
> > +predicates.  That is, they are sensitive to the state of the computation
> > +other than as reflected by their input arguments, though they do not
> > +affect the state themselves.
> 
> Haven't we said in the past that impure and semipure predicates have
> no declarative semantics, only operational semantics?

No.  Impure predicates have no declarative semantics.
But semipure predicates have a "contextual" declarative semantics.
As the reference manual says:

 |    Semipure goals can be given a "contextual" declarative semantics.
 | They cannot have any side-effects, so it is expected that, given the
 | context in which they are called (relative to any impure goals in the
 | program), their declarative semantics fully captures the intent of the
 | programmer.  Thus a semipure goal has a perfectly consistent declarative
 | semantics, until an impure goal is reached.  After that, it has another
 | (possibly different) declarative semantics, until the next impure goal
 | is executed, and so on.  Mercury compilers must respect this contextual
 | nature of the semantics of semipure goals; within a single context, a
 | compiler may treat a semipure goal as if it were pure.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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