// Example of Prolog-style declarative programming in XL
// Print all paths to the screen in a directed acyclic graph

// Import the backtracking engine and basic text I/O
import D = DECLARATIVE_PROGRAMMING_SUPPORT
import IO = XL.TEXT_IO

// The equivalent of the Prolog program
{prolog} D.declarations PrintAllPaths is

   // Clauses (the type is D.clause)
   using D
   with
      clause path[X, X] is
         IO.Write X
      clause path[X, Z] is
         edge Y, Z; path X, Y; IO.Write Z
      clause allPaths[X, Z] is
         path X, Z; IO.WriteLn; D.fail

   // Facts. Syntactically, these are procedure calls to 'edge'.
   // Semantically, the {prolog} pragma modifies the meaning of the
   // current declaration, so that the 'edge' instruction is seen
   // as a truth to verify for 'PrintAllPaths' to be true.
   edge A, B
   edge A, C
   edge A, D
   edge B, E
   edge C, E
   edge C, F
   edge D, F
   edge F, G
   edge G, E


// Invoking PrintAllPaths from conventional code
procedure Test() is

   // Check if we can find a solution to 'allPaths'
   D.Verify PrintAllPaths.allPaths

   // Run solutions one by one (as an iterator)
   with D.name X, Y
   for PrintAllPaths.allPaths[X, Y] loop
      IO.WriteLn "Currently exploring X=", X, " and Y=", Y