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+= Conditional Expressions
+
+Conditions are evaluated when parsing xref:if.adoc[if] and
+xref:elsif.adoc[elsif] statements.  These conditions allow the server to
+make complex decisions based on one of a number of possible criteria.
+
+.Syntax
+[source,unlang]
+----
+if ( condition ) { ...
+
+elsif ( condition ) { ...
+----
+
+Conditions are expressed using the following syntax:
+
+[options="header"]
+|=====
+| Syntax | Description
+| xref:attr.adoc[&Attribute-Name] | Check for attribute existence.
+| xref:condition/return_code.adoc[rcode] | Check return code of a previous module.
+| xref:condition/operands.adoc[data] | Check value of data.
+| xref:condition/cmp.adoc[lhs OP rhs] | Compare two kinds of data.
+| xref:condition/para.adoc[( condition )] | Check sub-condition
+| xref:condition/not.adoc[! condition] | Negate a conditional check
+| xref:condition/and.adoc[( condition ) && ...] | Check a condition AND the next one
+| xref:condition/or.adoc[( condition ) \|\| ...] | Check a condition OR the next one
+|=====
+
+
+.Examples
+[source,unlang]
+----
+if ( &User-Name == "bob" ) {
+    ...
+}
+
+if ( &Framed-IP-Address == 127.0.0.1 ) {
+    ...
+}
+
+if ( &Calling-Station-Id == "%{sql:SELECT ...}" ) {
+    ...
+}
+----
+
+== Load-time Syntax Checks
+
+The server performs a number of checks when it loads the configuration
+files.  Unlike version 2, all of the conditions are syntax checked
+when the server loads.  This checking greatly aids in creating
+configurations that are correct.  Where the configuration is
+incorrect, a descriptive error is produced.
+
+This error contains the filename and line number of the syntax error.
+In addition, it will print out a portion of the line that caused the
+error and will point to the exact character where the error was seen.
+These descriptive messages mean that most errors are easy to find and fix.
+
+== Load-time Optimizations
+
+The server performs a number of optimizations when it loads the
+configuration files.  Conditions that have static values are
+evaluated and replaced with the result of the conditional comparison.
+
+.Example
+[source,unlang]
+----
+if ( 0 == 1 ) {
+    ...
+}
+----
+
+The condition `0 == 1` is static and will evaluate to `false`.  Since
+it evaluates to `false`, the configuration inside of the `if`
+statement is ignored.  Any modules referenced inside of the `if`
+statement will not be loaded.
+
+This optimization is most useful for creating configurations that
+selectively load (or not) certain policies.  If the condition above
+was used in version 2, then the configuration inside of the `if` statement
+would be loaded, even though it would never be used.
+
+// Copyright (C) 2020 Network RADIUS SAS.  Licenced under CC-by-NC 4.0.
+// Development of this documentation was sponsored by Network RADIUS SAS.