Merge pull request #605 from notoria/debug

Added library(debug)

README.md

@@ -378,6 +378,8 @@ The modules that ship with Scryer Prolog are also called
   write `integer_si(X)` to ensure soundness of your programs.
   "si" stands for *sufficiently instantiated*, and also for
   *sound inference*.
+* [`debug`](src/lib/debug.pl)
+  Various predicates that allow for declarative debugging.
 * [`pio`](src/lib/pio.pl)
   `phrase_from_file/2` applies a DCG nonterminal to the contents of a
   file, reading lazily only as much as is needed. Due to the compact

src/lib/debug.pl

@@ -0,0 +1,22 @@
+% Source: https://stackoverflow.com/a/30791637
+
+:- module(debug, [
+    op(900, fx, $),
+    op(900, fx, $-),
+    op(950, fy, *),
+    (*)/1,
+    ($)/1,
+    ($-)/1
+]).
+
+:- use_module(library(format), [portray_clause/1]).
+
+$-(G_0) :-
+   catch(G_0, Ex, ( portray_clause(exception:Ex:G_0), throw(Ex) ) ).
+
+$(G_0) :-
+   portray_clause(call:G_0),
+   $-G_0,
+   portray_clause(exit:G_0).
+
+*(_).