Scheme 48 Manual -- Command levels

Scheme 48 Manual | Contents | In Chapter: Command processor
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## Command levels

If an error, keyboard interrupt, or other breakpoint occurs, or the `,push` command is used, the command processor invokes a recursive copy of itself, preserving the dynamic state of the program when the breakpoint occured. The recursive invocation creates a new command level. The command levels form a stack with the current level at the top. The command prompt indicates the number of stopped levels below the current one: `>` or `:` for the base level and ``n`>` or ``n`:` for all other levels, where `n` is the command-level nesting depth. The `auto-levels` switch described below can be used to disable the automatic pushing of new levels.

The command processor's evaluation package and the value of the `inspect-focus-value` switch are local to each command level. They are preserved when a new level is pushed and restored when it is discarded. The settings of all other switches are shared by all command levels.

  <eof>
  Discards the current command level and resumes running the level down. <eof> is usually control-`D` at a Unix shell or control-`C` control-`D` using the Emacs `cmuscheme48` library.

  `,pop`
  The same as <eof>.

  `,proceed [`exp` ...`]
  Proceed after an interrupt or error, resuming the next command level down, delivering the values of `exp ...` to the continuation. Interrupt continuations discard any returned values. `,Pop` and `,proceed` have the same effect after an interrupt but behave differently after errors. `,Proceed` restarts the erroneous computation from the point where the error occurred (although not all errors are proceedable) while `,pop` (and <eof>) discards it and prompts for a new command.

  `,push`
  Pushes a new command level on above the current one. This is useful if the `auto-levels` switch has been used to disable the automatic pushing of new levels for errors and interrupts.

  `,level [`number`]`
  Pops down to a given level and restarts that level. `Number` defaults to zero.

  `,reset`
  `,reset` restarts the command processor, discarding all existing levels.

Whenever moving to an existing level, either by sending an <eof> or by using `,reset` or the other commands listed above, the command processor runs all of the `dynamic-wind` "after" thunks belonging to stopped computations on the discarded level(s).

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