ttools: allow environment-sensitive parameter reporting

New interface DynamicTask introduced for tasks with parameter lists
that are sensitive to the environment.  This allows the help system
to be more helpful at runtime.

ttools/src/main/uk/ac/starlink/ttools/task/DynamicTask.java

@@ -0,0 +1,34 @@
+package uk.ac.starlink.ttools.task;
+
+import uk.ac.starlink.task.Environment;
+import uk.ac.starlink.task.Parameter;
+import uk.ac.starlink.task.TaskException;
+
+/**
+ * Extends the Task interface by methods which allow task parameters to be
+ * queried based on a (at least partially) populated Environment.
+ * By default, Tasks report the Parameters they use so that the on-line
+ * help system can supply that information to users at runtime.
+ * However, for some tasks the parameters in use depend on the value of
+ * other parameters.  This interface allows tasks to make that information
+ * available.  The method implementations are considered to be on a
+ * best-efforts basis, it is not guaranteed that they will be able to
+ * report all the parameters.  This information is only used for user help.
+ *
+ * @author   Mark Taylor
+ * @since    19 Sep 2014
+ */
+public interface DynamicTask {
+
+    /**
+     * Attempts to find a parameter with a given name that might be used
+     * by this task in the content of the given environment.
+     * This ought not to result in additional prompts to the user.
+     *
+     * @param   env  execution environment
+     * @param   paramName  requested parameter name
+     * @return   parameter with the given name, or null
+     */
+    Parameter getParameterByName( Environment env, String paramName )
+            throws TaskException;
+}

ttools/src/main/uk/ac/starlink/ttools/task/LineInvoker.java

@@ -252,17 +252,17 @@ else if ( arg.equals( "-stderr" ) && it.hasNext() ) {
                 task = taskFactory_.createObject( taskName );
                 String[] taskArgs = (String[])
                                     argList.toArray( new String[ 0 ] );
+                LineWord[] words = new LineWord[ taskArgs.length ];
+                for ( int i = 0; i < taskArgs.length; i++ ) {
+                    words[ i ] = new LineWord( taskArgs[ i ] );
+                }
+                env.setWords( words );
                 String helpText = helpMessage( env, task, taskName, taskArgs );
                 if ( helpText != null ) {
                     out.println( "\n" + helpText );
                     return 0;
                 }
                 else {
-                    LineWord[] words = new LineWord[ taskArgs.length ];
-                    for ( int i = 0; i < taskArgs.length; i++ ) {
-                        words[ i ] = new LineWord( taskArgs[ i ] );
-                    }
-                    env.setWords( words );
                     long start = System.currentTimeMillis();
                     Executable exec = task.createExecutable( env );
                     String[] unused = env.getUnused();
@@ -421,7 +421,8 @@ private void logParameterValues( String taskName,
      * @return  help text, or null
      */
     private String helpMessage( LineTableEnvironment env, Task task,
-                                String taskName, String[] taskArgs ) {
+                                String taskName, String[] taskArgs )
+            throws TaskException {
         for ( int i = 0; i < taskArgs.length; i++ ) {
             String arg = taskArgs[ i ];
             String helpFor = null;
@@ -460,6 +461,15 @@ else if ( helpFor != null ) {
                     }
                 }
 
+                /* If that fails, look for environment-sensitive parameters. */
+                if ( task instanceof DynamicTask ) {
+                    Parameter param =
+                        ((DynamicTask) task).getParameterByName( env, helpFor );
+                    if ( param != null ) {
+                        return getParamHelp( env, taskName, param );
+                    }
+                }
+
                 /* If that fails, treat the special case of plotting parameters
                  * which have per-table or per-subset suffixes.
                  * This allows you to get help for parameter xdata without