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#### 17.5.2 Defining New Simple Commands

If a `defmath` form contains an `interactive` clause, it defines a Calculator command. Actually such a `defmath` results in two function definitions: One, a ‘`calcFunc-`’ function as was just described, with the `interactive` clause removed. Two, a ‘`calc-`’ function with a suitable `interactive` clause and some sort of wrapper to make the command work in the Calc environment.

In the simple case, the `interactive` clause has the same form as for normal Emacs Lisp commands:

    (defmath increase-precision (delta)
      "Increase precision by DELTA."     ; This is the "documentation string"
      (interactive "p")                  ; Register this as a M-x-able command
      (setq calc-internal-prec (+ calc-internal-prec delta)))

This expands to the pair of definitions,

    (defun calc-increase-precision (delta)
      "Increase precision by DELTA."
      (interactive "p")
      (calc-wrapper
       (setq calc-internal-prec (math-add calc-internal-prec delta))))

    (defun calcFunc-increase-precision (delta)
      "Increase precision by DELTA."
      (setq calc-internal-prec (math-add calc-internal-prec delta)))

where in this case the latter function would never really be used! Note that since the Calculator stores small integers as plain Lisp integers, the `math-add` function will work just as well as the native `+` even when the intent is to operate on native Lisp integers.

The ‘`calc-wrapper`’ call invokes a macro which surrounds the body of the function with code that looks roughly like this:

    (let ((calc-command-flags nil))
      (unwind-protect
          (save-current-buffer
            (calc-select-buffer)
            body of function
            renumber stack
            clear Working message)
        realign cursor and window
        clear Inverse, Hyperbolic, and Keep Args flags
        update Emacs mode line))

The `calc-select-buffer` function selects the `*Calculator*` buffer if necessary, say, because the command was invoked from inside the `*Calc Trail*` window.

You can call, for example, `(calc-set-command-flag 'no-align)` to set the above-mentioned command flags. Calc routines recognize the following command flags:

`renum-stack`

  Stack line numbers ‘`1:`’, ‘`2:`’, and so on must be renumbered after this command completes. This is set by routines like `calc-push`.

`clear-message`

  Calc should call ‘`(message "")`’ if this command completes normally (to clear a “Working…” message out of the echo area).

`no-align`

  Do not move the cursor back to the ‘`.`’ top-of-stack marker.

`position-point`

  Use the variables `calc-position-point-line` and `calc-position-point-column` to position the cursor after this command finishes.

`keep-flags`

  Do not clear `calc-inverse-flag`, `calc-hyperbolic-flag`, and `calc-keep-args-flag` at the end of this command.

`do-edit`

  Switch to buffer `*Calc Edit*` after this command.

`hold-trail`

  Do not move trail pointer to end of trail when something is recorded there.

Calc reserves a special prefix key, shift-`Y`, for user-written extensions to Calc. There are no built-in commands that work with this prefix key; you must call `define-key` from Lisp (probably from inside a `calc-define` property) to add to it. Initially only `Y ?` is defined; it takes help messages from a list of strings (initially `nil`) in the variable `calc-Y-help-msgs`. All other undefined keys except for `Y` are reserved for use by future versions of Calc.

If you are writing a Calc enhancement which you expect to give to others, it is best to minimize the number of `Y`-key sequences you use. In fact, if you have more than one key sequence you should consider defining three-key sequences with a `Y`, then a key that stands for your package, then a third key for the particular command within your package.

Users may wish to install several Calc enhancements, and it is possible that several enhancements will choose to use the same key. In the example below, a variable `inc-prec-base-key` has been defined to contain the key that identifies the `inc-prec` package. Its value is initially `"P"`, but a user can change this variable if necessary without having to modify the file.

Here is a complete file, `inc-prec.el`, which makes a `Y P I` command that increases the precision, and a `Y P D` command that decreases the precision.

    ;;; Increase and decrease Calc precision.  Dave Gillespie, 5/31/91.
    ;; (Include copyright or copyleft stuff here.)

    (defvar inc-prec-base-key "P"
      "Base key for inc-prec.el commands.")

    (put 'calc-define 'inc-prec '(progn

    (define-key calc-mode-map (format "Y%sI" inc-prec-base-key)
                'increase-precision)
    (define-key calc-mode-map (format "Y%sD" inc-prec-base-key)
                'decrease-precision)

    (setq calc-Y-help-msgs
          (cons (format "%s + Inc-prec, Dec-prec" inc-prec-base-key)
                calc-Y-help-msgs))

    (defmath increase-precision (delta)
      "Increase precision by DELTA."
      (interactive "p")
      (setq calc-internal-prec (+ calc-internal-prec delta)))

    (defmath decrease-precision (delta)
      "Decrease precision by DELTA."
      (interactive "p")
      (setq calc-internal-prec (- calc-internal-prec delta)))

    ))  ; end of calc-define property

    (run-hooks 'calc-check-defines)

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