Added ifthen() function reference

content/reference/functions/ifthen.markdown

@@ -0,0 +1,76 @@
+---
+layout: default
+title: ifthen
+aliases:
+  - "/reference-functions-ifthen.html"
+---
+
+{{< CFEngine_function_prototype(...) >}}
+
+**Description:** Evaluate each pair of arguments as a (`class`, `value`) tuple, returning the `value` of the first pair whose `class` is set.
+
+If none are set, the variable is not defined.
+
+**Arguments**:
+
+The `ifthen` function is like `ifelse()` without the default (`else`)
+value. It must have an even number of arguments (at least 2). Any other
+number is a fatal error. The first argument of each pair is a class
+expression, like the `expression` attribute of a classes promise, so it
+can be more than a class name, for example `Tuesday.linux.!verbose`.
+
+Generally, if `ifthen` were called with arguments `(a1, a2, b1, b2)`,
+the behavior expressed as pseudo-code is:
+
+```
+if a1 then return a2
+else-if b1 then return b2
+else return nothing
+```
+
+When no class expression is true, the function returns nothing, so the
+variable is not defined by that promise. This is useful when there is
+no sensible default value, for example to leave a variable to be
+defined by another promise or to be checked with `isvariable()`. With
+`ifelse()`, you have to provide a placeholder default value and check
+for it.
+
+**Example:**
+
+```cf3
+bundle agent __main__
+{
+  classes:
+    "webserver" expression => "any";
+
+  vars:
+    # The value of the first true class expression is used.
+    "port" string => ifthen("dbserver", "5432", "webserver", "80");
+
+    # No class expression is true, so the variable is not defined.
+    "unset" string => ifthen("dbserver", "5432", "mailserver", "25");
+
+  reports:
+    "port is $(port)";
+    "unset is not defined" if => not(isvariable("unset"));
+}
+```
+
+Output:
+
+```
+R: port is 80
+R: unset is not defined
+```
+
+**Notes:**
+
+- Unlike `ifelse()` with exactly three arguments, `ifthen()` follows the
+  general rule for functions: it is not evaluated while any of its
+  arguments contain unresolved variables.
+
+**See also:** `ifelse()`, `isvariable()`
+
+**History:**
+
+- Introduced in 3.29.0