[mercury-users] Handy Programming tool for Mercury.

[Richard A. O'Keefe]

	For example, piping the following through state2m
		* type foo(T)
		* 	a	int
		* 	b	char	readonly
		* 	c	string	mutable
		* 	d	T	private
	you get
		:- pred set_a(foo, int, foo).
		:- mode set_a(in, in, out) is det.

I have two comments.
First, when I've done things like this by hand in Prolog, I have
found a *four* argument version
	a_varies(Old_Foo, Old_A, New_Foo, New_A)
extraordinarily useful.  For example, to increment a
field, you'd do
	a_varies(F0, A0, F, A), succ(A0, A)
which is of course bidirectional.
It's not clear to me whether something like this could work
in Mercury, given that whichever mode of a_varies/4 is chosen
you want to fill in the changed field _after_ building the
rest of the record.  Of course, if it were split into two
unifications,
    (since
	a_varies(foo(A0,B,C,D), A0, foo(A,B,C,D), A)
     therefore
	a_varies(F0, A0, F, A), succ(A0, A)
     is equivalent to
	F0 = foo(A0,B,C,D), F = foo(A,B,C,D), succ(A0, A)
    )
then Mercury _would_ be able to handle it by reordering.

The other comment is that I would have thought that
	set_a(New_A_Value, Old_Foo, New_Foo)
would have fitted into DCG notation rather better, so that
one could do
	change_all(X) -->
	    set_a(1),
	    set_b('c'),
	    set_c("dee"),
	    set_d(X).

--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------