31.19.9 Defining and Using Fields

A field is a range of consecutive characters in the buffer that are identified by having the same value (comparing with eq) of the field property (either a text-property or an overlay property). This section describes special functions that are available for operating on fields.

You specify a field with a buffer position, pos. We think of each field as containing a range of buffer positions, so the position you specify stands for the field containing that position.

When the characters before and after pos are part of the same field, there is no doubt which field contains pos: the one those characters both belong to. When pos is at a boundary between fields, which field it belongs to depends on the stickiness of the field properties of the two surrounding characters (see Sticky Properties). The field whose property would be inherited by text inserted at pos is the field that contains pos.

There is an anomalous case where newly inserted text at pos would not inherit the field property from either side. This happens if the previous character’s field property is not rear-sticky, and the following character’s field property is not front-sticky. In this case, pos belongs to neither the preceding field nor the following field; the field functions treat it as belonging to an empty field whose beginning and end are both at pos.

In all of these functions, if pos is omitted or nil, the value of point is used by default. If narrowing is in effect, then pos should fall within the accessible portion. See Narrowing.

Function: field-beginning &optional pos escape-from-edge limit

This function returns the beginning of the field specified by pos.

If pos is at the beginning of its field, and escape-from-edge is non-nil, then the return value is always the beginning of the preceding field that ends at pos, regardless of the stickiness of the field properties around pos.

If limit is non-nil, it is a buffer position; if the beginning of the field is before limit, then limit will be returned instead.

Function: field-end &optional pos escape-from-edge limit

This function returns the end of the field specified by pos.

If pos is at the end of its field, and escape-from-edge is non-nil, then the return value is always the end of the following field that begins at pos, regardless of the stickiness of the field properties around pos.

If limit is non-nil, it is a buffer position; if the end of the field is after limit, then limit will be returned instead.

Function: field-string &optional pos

This function returns the contents of the field specified by pos, as a string.

Function: field-string-no-properties &optional pos

This function returns the contents of the field specified by pos, as a string, discarding text properties.

Function: delete-field &optional pos

This function deletes the text of the field specified by pos.

Function: constrain-to-field new-pos old-pos &optional escape-from-edge only-in-line inhibit-capture-property

This function constrains new-pos to the field that old-pos belongs to—in other words, it returns the position closest to new-pos that is in the same field as old-pos.

If new-pos is nil, then constrain-to-field uses the value of point instead, and moves point to the resulting position in addition to returning that position.

If old-pos is at the boundary of two fields, then the acceptable final positions depend on the argument escape-from-edge. If escape-from-edge is nil, then new-pos must be in the field whose field property equals what new characters inserted at old-pos would inherit. (This depends on the stickiness of the field property for the characters before and after old-pos.) If escape-from-edge is non-nil, new-pos can be anywhere in the two adjacent fields. Additionally, if two fields are separated by another field with the special value boundary, then any point within this special field is also considered to be on the boundary.

Commands like C-a with no argument, that normally move backward to a specific kind of location and stay there once there, probably should specify nil for escape-from-edge. Other motion commands that check fields should probably pass t.

If the optional argument only-in-line is non-nil, and constraining new-pos in the usual way would move it to a different line, new-pos is returned unconstrained. This used in commands that move by line, such as next-line and beginning-of-line, so that they respect field boundaries only in the case where they can still move to the right line.

If the optional argument inhibit-capture-property is non-nil, and old-pos has a non-nil property of that name, then any field boundaries are ignored.

You can cause constrain-to-field to ignore all field boundaries (and so never constrain anything) by binding the variable inhibit-field-text-motion to a non-nil value.