In most of the places where strings are used, we conceptualize the string as containing text characters—the same kind of characters found in buffers or files. Occasionally Lisp programs use strings that conceptually contain keyboard characters; for example, they may be key sequences or keyboard macro definitions. However, storing keyboard characters in a string is a complex matter, for reasons of historical compatibility, and it is not always possible.
We recommend that new programs avoid dealing with these complexities
by not storing keyboard events in strings containing control
characters or the like, but instead store them in the common Emacs
format as understood by key-valid-p.
If you read a key sequence with read-key-sequence-vector (or
read-key-sequence), or access a key sequence with
this-command-keys-vector (or this-command-keys), you can
transform this to the recommended format by using key-description.
The complexities stem from the modifier bits that keyboard input characters can include. Aside from the Meta modifier, none of these modifier bits can be included in a string, and the Meta modifier is allowed only in special cases.
The earliest GNU Emacs versions represented meta characters as codes
in the range of 128 to 255. At that time, the basic character codes
ranged from 0 to 127, so all keyboard character codes did fit in a
string. Many Lisp programs used ‘\M-’ in string constants to stand
for meta characters, especially in arguments to define-key and
similar functions, and key sequences and sequences of events were always
represented as strings.
When we added support for larger basic character codes beyond 127, and additional modifier bits, we had to change the representation of meta characters. Now the flag that represents the Meta modifier in a character is 2**27 and such numbers cannot be included in a string.
To support programs with ‘\M-’ in string constants, there are special rules for including certain meta characters in a string. Here are the rules for interpreting a string as a sequence of input characters:
Functions such as read-key-sequence that construct strings of
keyboard input characters follow these rules: they construct vectors
instead of strings, when the events won’t fit in a string.
When you use the read syntax ‘\M-’ in a string, it produces a code in the range of 128 to 255—the same code that you get if you modify the corresponding keyboard event to put it in the string. Thus, meta events in strings work consistently regardless of how they get into the strings.
However, most programs would do well to avoid these issues by following the recommendations at the beginning of this section.