Historical precedent

Emacs commands normally read their arguments through the minibuffer, after they have been called. For example, if I type 'C-x C-f' (find-file) to open a file, Emacs says "Find file:" in the minibuffer and allows me to enter a file name. I type the name in after I typing 'C-x C-f.'

However, there is one exception: A command may take a numerical "prefix argument," which can be specified in various ways. Often, but not always, this argument will specify the number of times the command is to be executed.

Vi uses " to specify registers as a prefix; PUIs have the current selection, which is a prefix to many commands like copying or searching ("search only in the selected area").

Specification

"The prefix argument" is an ordered set of cells that a binding may use. (By "ordered set" I mean a list on which each element may appear only once; this is similar to a zz rank, except that it cannot loop.) The user interface should show the current prefix argument in a reasonable way.

Currently, the prefix argument determined as follows:

• If a search according to search_bindings--benja has just been finished (according to the PEG, by pressing the space bar twice), and this search has returned more than zero cells, these cells are the prefix argument. (The order is defined by the search policy, configurable by the user in an as-yet-undefined way).
• Else, if there any cells are marked, they are the prefix argument. (Marks are ordered; this is outside the scope of this PEG, but normally the cell that was marked first is the first in the list.)
• Else, there is no prefix argument.

Bindings usually use the prefix argument in one of two ways:

• "Prefix or other:" If there is no prefix argument, the cell accursed in the other window is used instead. Examples: Connect, Put (see containment--benja), Go (the 'g' key).
• "Prefix or this:" If there is no prefix argument, the cell accursed in this window is used instead. Examples: Hop, Break, Clone, Delete, Excise, New Cell.

Additionally, toggling the mark (the 'm' key) should work on a cell returned by a search, but toggling the mark on all marked cells doesn't make much sense-- so we need a third case here. There may be more special cases, but the above are the usual ones.

In the future, it should also be possible to set the prefix argument through at least two more mechanisms:

• Entering a cell number. (If you've typed '734,' the cell of that number is the prefix argument.)
• Building an expression, using menus. It should be possible to build expressions like "all emails on gzz-commits which are newer than three days" easily, using the zaubertrank. (Once such an expression is built, it'll become the prefix argument. For example, building the above expression, then hitting 't' and a direction will clone all those emails in a single rank.)

The details of these are out of scope for this PEG; a future PEG will deal with them. Once they are implemented, the prefix argument will be determined as follows:

• The latest search result, cell number or expression, if any.
• Else, the marked cells, if any.
• Else, there is no prefix argument.

That means if you enter a cell number, then perform a search, the cell number will be thrown away and the search will be the prefix argument; if you perform a search and then enter a cell number, the search will be thrown away; and so on.