An output_handler handles the results of evaluate,
including the values, graphics, conditions. Each type of output is handled by
a particular function in the handler object.
Function to handle the echoed source code under evaluation.
text
Function to handle any textual console output.
graphics
Function to handle graphics, as returned by
recordPlot.
message
Function to handle message output.
warning
Function to handle warning output.
error
Function to handle stop output.
value
Function to handle the values returned from evaluation. If it
only has one argument, only visible values are handled; if it has more
arguments, the second argument indicates whether the value is visible.
Details
The handler functions should accept an output object as their first argument.
The return value of the handlers is ignored, except in the case of the
value handler, where a visible return value is saved in the output
list.
Calling the constructor with no arguments results in the default handler,
which mimics the behavior of the console by printing visible values.
Note that recursion is common: for example, if value does any
printing, then the text or graphics handlers may be called.