3.6 Symbols
A symbol is an atomic value that prints like an identifier. An expression that starts with ' and continues with an identifier produces a symbol value.
Examples: |
> 'a |
a |
> (symbol? 'a) |
#t |
For any sequence of characters, exactly one corresponding symbol is interned; calling the string->symbol procedure, or reading a syntactic identifier, produces an interned symbol. Since interned symbols can be cheaply compared with eq? (and thus eqv? or equal?), they serves as a convenient values to use for tags and enumerations.
Symbols are case-sensitive. By using a #ci prefix or in other ways, the reader can be made to case-fold character sequences to arrive at a symbol, but the reader preserves case by default.
Examples: |
> (eq? 'a 'a) |
#t |
> (eq? 'a (string->symbol "a")) |
#t |
> (eq? 'a 'b) |
#f |
> (eq? 'a 'A) |
#f |
> #ci’A |
a |
Any string (i.e., any character sequence) can be supplied to string->symbol to obtain the corresponding symbol. For reader input, any character can appear directly in an identifier, except for whitespace and the following special characters:
( ) [ ] { } " , ' ` ; # | \
Actually, # is disallowed only at the beginning of a symbol, and then only if not followed by %; otherwise, # is allowed, too. Also, . by itself is not a symbol.
Whitespace or special characters can be included in an identifier by quoting them with | or \. These quoting mechanisms are used in the printed form of identifiers that contain special characters or that might otherwise look like numbers.
Examples: |
> (string->symbol "one, two") |
|one, two| |
> (string->symbol "6") |
|6| |
Reading Symbols in Reference: PLT Scheme documents the fine points of the syntax of symbols.
The display form of a symbol is the same as the corresponding string.
Examples: |
> (display 'Apple) |
Apple |
> (display '|6|) |
6 |
The gensym and string->uninterned-symbol procedures generate fresh uninterned symbols that are not equal (according to eq?) to any previously interned or uninterned symbol. Uninterned symbols are useful as fresh tags that cannot be confused with any other value.
Examples: |
> s |
g42 |
> (eq? s 'g42) |
#f |
> (eq? 'a (string->uninterned-symbol "a")) |
#f |
Symbols in Reference: PLT Scheme provides more on symbols.