The trace-mood

The trace-mood can be entered by the user by means of a trace-command (<#8099#>t<#8099#>). It can also be entered from a program by a call of <#8100#>assert<#8100#> (see appendix A.10). In the trace-mood, which is indicated by <#8101#>Trace command, please...<#8101#> on the status line, the executor lists every unit before executing it, and asks for a command as follows:

• If the unit is simple (i.e. not a invocation of a procedure, refinement or operator and not a control structure), it is executed. If this unit yields a result, this is displayed.
• If the unit is such an invocation or a control structure, it is listed and the executor waits for one of the following commands:
  • <#8104#>b <#8104#> backtrace-command

    The Elan Programming Environment switches to the backtrace-mood.

  • <#8105#>c <#8105#> continue-command

    The remainder of the program is executed without trace.

  • <#8106#>e <#8106#> edit-command

    Execution is terminated, and the definition that contains the current unit is offered for local editing.

  • <#8107#>n <#8107#> next-command

    The body of the current unit is executed in trace mode.

  • <#8108#>p <#8108#> previous-command

    All the remaining units (entered by the last next-command) are executed (without trace), if the body yields a result this is displayed and the trace-mood is re-entered.

  • <#8109#>q <#8109#> quit-command

    Return to the command mode.

  • <#8110#>s <#8110#> show-command

    A known name (may be abbreviated) is asked and the definition and the definition of that name is displayed.

  • <#8111#>x <#8111#> execute-command

    The body is executed (without trace), its result (if any) is displayed and trace-mood is re-entered.

At the end of each body the executor waits again for a valid trace-command.