Given any Scheme value, you can determine whether it is a symbol using
#t if obj is a symbol, otherwise return
scm_is_true (scm_symbol_p (val)).
Once you know that you have a symbol, you can obtain its name as a
string by calling
symbol->string. Note that Guile differs by
default from R5RS on the details of
symbol->string as regards
Return the name of symbol s as a string. By default, Guile reads symbols case-sensitively, so the string returned will have the same case variation as the sequence of characters that caused s to be created.
If Guile is set to read symbols case-insensitively (as specified by
R5RS), and s comes into being as part of a literal expression
(see Literal expressions in The Revised^5 Report on Scheme) or
by a call to the
Guile converts any alphabetic characters in the symbol’s name to
lower case before creating the symbol object, so the string returned
here will be in lower case.
If s was created by
string->symbol, the case of characters
in the string returned will be the same as that in the string that was
string->symbol, regardless of Guile’s case-sensitivity
setting at the time s was created.
It is an error to apply mutation procedures like
strings returned by this procedure.
Most symbols are created by writing them literally in code. However it is also possible to create symbols programmatically using the following procedures:
Return a newly allocated symbol made from the given character arguments.
(symbol #\x #\y #\z) ⇒ xyz
Return a newly allocated symbol made from a list of characters.
(list->symbol '(#\a #\b #\c)) ⇒ abc
Return a newly allocated symbol whose characters form the concatenation of the given symbols, arg ....
(let ((h 'hello)) (symbol-append h 'world)) ⇒ helloworld
Return the symbol whose name is string. This procedure can create symbols with names containing special characters or letters in the non-standard case, but it is usually a bad idea to create such symbols because in some implementations of Scheme they cannot be read as themselves.
Return the symbol whose name is str. If Guile is currently reading symbols case-insensitively, str is converted to lowercase before the returned symbol is looked up or created.
The following examples illustrate Guile’s detailed behaviour as regards the case-sensitivity of symbols:
(read-enable 'case-insensitive) ; R5RS compliant behaviour (symbol->string 'flying-fish) ⇒ "flying-fish" (symbol->string 'Martin) ⇒ "martin" (symbol->string (string->symbol "Malvina")) ⇒ "Malvina" (eq? 'mISSISSIppi 'mississippi) ⇒ #t (string->symbol "mISSISSIppi") ⇒ mISSISSIppi (eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #f (eq? 'LolliPop (string->symbol (symbol->string 'LolliPop))) ⇒ #t (string=? "K. Harper, M.D." (symbol->string (string->symbol "K. Harper, M.D."))) ⇒ #t (read-disable 'case-insensitive) ; Guile default behaviour (symbol->string 'flying-fish) ⇒ "flying-fish" (symbol->string 'Martin) ⇒ "Martin" (symbol->string (string->symbol "Malvina")) ⇒ "Malvina" (eq? 'mISSISSIppi 'mississippi) ⇒ #f (string->symbol "mISSISSIppi") ⇒ mISSISSIppi (eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #t (eq? 'LolliPop (string->symbol (symbol->string 'LolliPop))) ⇒ #t (string=? "K. Harper, M.D." (symbol->string (string->symbol "K. Harper, M.D."))) ⇒ #t
From C, there are lower level functions that construct a Scheme symbol from a C string in the current locale encoding.
When you want to do more from C, you should convert between symbols
and strings using
scm_string_to_symbol and work with the strings.
Construct and return a Scheme symbol whose name is specified by the null-terminated C string name. These are appropriate when the C string is hard-coded in the source code.
Construct and return a Scheme symbol whose name is specified by
scm_from_locale_symbol, name must be null
scm_from_locale_symboln the length of name is
specified explicitly by len.
Note that these functions should not be used when name is a
C string constant, because there is no guarantee that the current locale
will match that of the execution character set, used for string and
character constants. Most modern C compilers use UTF-8 by default, so
in such cases we recommend
respectively, but also frees str with
Thus, you can use this function when you would free str anyway
immediately after creating the Scheme string. In certain cases, Guile
can then use str directly as its internal representation.
The size of a symbol can also be obtained from C:
Return the number of characters in sym.
Finally, some applications, especially those that generate new Scheme
code dynamically, need to generate symbols for use in the generated
gensym primitive meets this need:
Create a new symbol with a name constructed from a prefix and a counter value. The string prefix can be specified as an optional argument. Default prefix is ‘ g’. The counter is increased by 1 at each call. There is no provision for resetting the counter.
The symbols generated by
gensym are likely to be unique,
since their names begin with a space and it is only otherwise possible
to generate such symbols if a programmer goes out of their way to do
so. Uniqueness can be guaranteed by instead using uninterned symbols
(see Symbol Uninterned), though they can’t be usefully written out
and read back in.