[racket] problems with the term 'symbol'

Thanks! --Christian

Am 13.10.2014 um 19:41 schrieb Jens Axel Søgaard:
> 2014-10-13 17:53 GMT+02:00 Christian Wagenknecht <c.wagenknecht at hszg.de>:
>> With regard of Racket's symbols I have a problem with the consistency of the
>> terminology as follows.
>> A symbol in Scheme and maybe in earlier Racket versions is considered as an
>> identifier. For example xyz is a symbol, whereas 'xyz avoids the evaluation
>> of xyz.
> A symbol is not an identifier. An identifier is a name. Typically identifiers
> are represented as syntax objects which besides the actual name also
> captures source location information (such as line and column numbers).
>
> A symbol is "almost" a string.
>
>        (string->symbol "foo")  produces a symbol spelled foo
>        "foo"                             produces a string
>
> For languages with mutable strings the operation string=? takes time
> proportional
> to the length of the strings. Consider this program:
>
>      (define a "foo")
>      (do-something)
>      (define b "foo")
>      (string=? a b)
>
> If strings are mutable, then (do-something) might have changed "foo" into "fox".
> The call (string=? a b) thus needs to compare all characters in a and b.
>
> If on the other hand strings are immutable, then (define a "foo") can store the
> characters f, o, and o in a string in a memory location p. The variable b in
> (define b "foo") can now share the string, and simply point to the
> same location p.
> In other words: strings with the same spelling will only be allocated once.
> A comparison (string=? a b) can no be reduced to a simple pointer comparison.
>
> Symbols are immutable strings with the guarantee that only one symbol of a
> given spelling will be allocated. In order words comparing symbols for
> equality is O(1).
>
> You can create a symbol with the spelling foo in several ways. For example
>          (string->symbol "foo")
> but, since that is cumbersome to write, one can use the abbreviation
>          'foo
>
>
> Identifiers are names that appear in source code. They can be used for example
> to name variables.
>
> When programs need to process source code, these names need to be represented
> as a concrete value. In Racket identifiers are not represented as
> symbols but as syntax
> objects. The syntax object contains a symbol that represent the
> spelling of the name,
> but it also contains lexical information.
>
> /Jens Axel
>
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>> In current version the little ' (normally as shorthand for quote) belongs to
>> the symbol. For example 'xyz is a symbol. A symbol is obviously considered
>> as a quoted identifier, at least syntactically.
>>
>> However, when using a symbol as part of an expression the prepending '
>> disappears. For example: (vector 1 'xyz), consisting of a number and a
>> symbol evaluates to '#(1 xyz). But xyz is not a symbol but an identifier.
>> Therefore to say that this vector belongs of a number and a symbol is no
>> longer valid. (vector-ref '#(1 xyz) 1) returns 'xyz, which is correct,
>> however, this is also visually a difference where there is none.
>>
>> Could you help me to get it right, please?
>>
>>
>> ____________________
>>    Racket Users list:
>>    http://lists.racket-lang.org/users
>>
>
>


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