typesetting/quad/qtest/mds/symbols.md

Symbols

A symbol is an atomic value that prints like an identifier preceded with '. 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 serve 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|                         

+[missing] in [missing] documents the fine points of the syntax of symbols.

The write function prints a symbol without a ' prefix. The display form of a symbol is the same as the corresponding string.

Examples:

> (write 'Apple)  
Apple             
> (display 'Apple)
Apple             
> (write '|6|)    
|6|               
> (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:

> (define s (gensym))                     
> s                                       
'g42                                      
> (eq? s 'g42)                            
#f                                        
> (eq? 'a (string->uninterned-symbol "a"))
#f                                        

+[missing] in [missing] provides more on symbols.