How to graphically represent Meta characters

Question

Given that the variable x contains the numeric representation of the key "Control-Z", say

(setq x ?\C-z), 

one may graphically represent its contents by issuing the command

(char-to-string x), 

producing the usual representation of "Control-Z", namely "^Z". I would like to acheive a similar effect in case x contained the numeric representation of "Meta-Z", namely

(setq x ?\M-z).

However my system represents "Meta-Z" as the number 134217850, and (char-to-string x) now causes the error (wrong-type-argument characterp 134217850). I am aware that the graphical representation of "Meta-Z" is not as standard as for "Control-Z", since the latter is part of the ASCII code while the former isn't, but anything legible, such as "M-Z" would be better than the big number 134217850.

My question is thus: which standard elisp function should I use in order to transform numerical representations of Meta characters into a legible string?

Answer 1

Properly speaking, ?\C-z is the read syntax of the character. Internally, the Lisp object is an integer. Nonetheless, characters in strings may have different character codes [2]. In other words, a similar read syntax does not necessarily produce the same Lisp object.

You can see in the following example that the character codes are different: 134217818 and 218.

ELISP> ?\M-Z
134217818
ELISP> "\M-Z"
"\332"
ELISP> ?\332
218

Characters inserted in text must be in the range of 0 to 4194303. Extra characters, typed using modifier keys, are used to represent keyboard inputs. [1]

In summary, you cannot use a string (read syntax) to represent a meta character. For a conversion function, look at [3].

(single-key-description ?\M-z)

References

• [1] Elisp manual, section 2.3.3. Character Type.
• [2] Elisp manual, section 2.3.8.3 Nonprinting Characters in Strings.
• [3] Elisp manual, section 24.5 Describing Characters for Help Messages.