Editing Eiffel (programming language) (section)

===Exception handling===
[[Exception handling]] in Eiffel is based on the principles of design by contract. For example, an exception occurs when a routine's caller fails to satisfy a precondition, or when a routine cannot ensure a promised postcondition. In Eiffel, exception handling is not used for [[control flow]] or to correct data-input mistakes.

An Eiffel exception handler is defined using the {{mono|rescue}} keyword. Within the {{mono|rescue}} section, the {{mono|retry}} keyword executes the routine again. For example, the following routine tracks the number of attempts at executing the routine, and only retries a certain number of times:

<syntaxhighlight lang="eiffel">
connect_to_server (server: SOCKET)
      -- Connect to a server or give up after 10 attempts.
    require
        server /= Void and then server.address /= Void
    local
        attempts: INTEGER
    do
        server.connect
    ensure
      connected: server.is_connected
    rescue
        if attempts < 10 then
            attempts := attempts + 1
            retry
        end
    end
</syntaxhighlight>

This example is arguably flawed for anything but the simplest programs, however, because connection failure is to be expected. For most programs a routine name like {{mono|attempt_connecting_to_server}} would be better, and the postcondition would not promise a connection, leaving it up to the caller to take appropriate steps if the connection was not opened.