[mercury-users] Libraries and DCG convenience...

[Peter Ross]

On 21-Jun-1999, Luke Evans <Luke.Evans at seagatesoftware.com> wrote:
> Threading the graph variables through all this is painful and seems (IMHO)
> to be exactly what DCG's were designed for.   Of course, I can write
> wrappers for the standard predicates which reorder the arguments to present
> the <Graph state before> and <Graph state after> arguments at the end of the
> argument list, but this becomes a real overhead to using the standard
> library.
> 
You can use higher-order functions to avoid writing wrapper functions.

:- module main.

:- interface.

:- import_module io.

:- pred main(io__state::di, io__state::uo) is det.

:- implementation.

:- import_module graph, std_util.

main -->
	{ graph__init(G0) },
	{ create_graph(G0, G) },
	io__write(G).

:- pred create_graph(graph(int), graph(int)) is det.

create_graph -->
	switch(graph__det_insert_node, 1, Node1),
	switch(graph__det_insert_node, 2, Node2),
	switch(graph__det_insert_node, 3, Node3),
	switch(graph__det_insert_edge, Node1, Node2, unit, _Arc12),
	switch(graph__det_insert_edge, Node2, Node3, unit, _Arc23).

:- pred switch(pred(T, A, B, T), A, B, T, T).
:- mode switch(pred(in, in, out, out) is det, in, out, in, out) is det.

switch(P, A, B, T0, T) :-
	P(T0, A, B, T).

:- pred switch(pred(T, A, B, C, D, T), A, B, C, D, T, T).
:- mode switch(pred(in, in, in, in, out, out) is det,
		in, in, in, out, in, out) is det.

switch(P, A, B, C, D, T0, T) :-
	P(T0, A, B, C, D, T).
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