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--- tutorials:rgg-code-structure [2025/10/21 09:16] – [Modules] MH
+++ tutorials:rgg-code-structure [2025/10/21 12:04] (current) – [XL Blocks - Rules] MH
@@ Line 7: / Line 7: @@
 The java blocks are framed by curly brackets ''{...}'' and the xl blocks by square brackets ''[...]''. These blocks can be embedded into each other recursively and share declared variables.
-===== functions =====
+===== Functions =====
 Even so the function declaration follows the syntax of java, the body of the function can be of both code blocks. In difference to java rgg code allows to start directly with the declaration of functions without creating a class before. (Internally the rgg file is handled as one java class).
@@ Line 137: / Line 137: @@
 Since java 1.6 did not include lamda expression, an own implementation was added to rgg.
 The syntax an the explanation can be found [[groimp-platform:xl-lambda|here]]
-===== XL Blocks =====
-While using XL queries mainly two rules are used: replacement rules(''%%==>%%'') and execution rule (''%%::>%%'').
+===== XL Blocks - Rules =====
+While using XL queries mainly three rules are used:
+  * Graph rules (''%%==>>%%'')
+  * String replacement rules (''%%==>%%'')
+  * Update or Execution rules (''%%::>%%'')
+All rules have in common that we have a left and a right hand side, both are separated by one of the arrows.
+=== Graph rules ===
+Graph rules are indicated by the rule arrow ''%%==>>%%''.
+The right hand side is a semicolon-terminated list of graph statements building the replacing graph for the matched graph. While the left hand side represents a fixed graph pattern, the graph statements can build the replacing graph dynamically, including loops and conditional execution. Thus it is not always possible to find a graphical representation of the right hand side.
+The graph statements use a stack s of nodes and two state variables n – the last created node – and e – the next edge to create – during execution.
+After the execution of all graph statements, a final step is executed (if the execution has not been terminated using break): A matching non-context node instance of the left hand side, which has not been the value of an evaluated node expression of the right hand side, is deleted. Similarly, an edge matching a non-context edge pattern of the left hand side is deleted if it is not specified on the right hand side.
-==== Replacement rules ====
+=== String rules ===
-The concept of an replacement rule is basically that all part of the project graph that are similar to the left part (before ''%%==>%%'') are replaced by the right part( behind ''%%==>%%'').
+The concept of an replacement rule in XL follows the 'Lindenmayer-form' where basically all parts of the project string that are similar to the left part (before ''%%==>%%'') are replaced by the right part ( behind ''%%==>%%'').
 The left part can thereby be any [[tutorials:common_graph_queries|xl-query]].
 The right part (the production) is a collection of Nodes ( instances of either turtle commands, 3d objects or modules) which are linked by different edges. In the example below only successor- and branch-edges are used. A node that is separated to its predecessor by only a white space is added as a sucessor to the predecessor. A Node or a set of nodes framed by square brackets in that is added to the predecessor with a branch-edge.
@@ Line 161: / Line 180: @@
 As shown above it is also possible to forward parameters from the left side to the right (similar to a parametric rewriting rule in formal systems). Moreover it is possible to use ''a:A''  instead of ''A(x)'' and then on the right side ''a.len'' instead of ''x''.
-==== Execution rule ====
+=== Execution rule ===
-The left side of an execution rule is similar to the left side of the production rule. Yet the main difference is that the nodes (or pattern of nodes) found by the query are not replaced. Instead the code on the right side is applied on them.
+The left side of an execution rule is similar to the left side of the production rule. Yet the main difference is that the nodes (or pattern of nodes) found by the query are not replaced. Instead the (Java) code on the right side is applied on them.
 <code java>
@@ Line 183: / Line 202: @@
 ]
 </code>
+There are no additional actions executed, so update rules don’t change the topological properties of the
+host graph per se.