Anonymous symbols

muSE has a notion of anonymous symbols - symbols which do not have a textual representation - which are useful in situations where you need to keep a set of properties together, and in macros to introduce new variables into the generated expressions. The difference between named and anonymous symbols (apart from the textual representation) is that named symbols persist for the life time of the muSE execution environment whereas anonymous symbols and their property lists are garbage collected when there are no references to them.

Anonymous symbols are created using (new) . You can use an anonymous symbol as the first argument to the get and put functions to edit its property list.

Property lists

Every symbol in muSE has an associated property list that you can query and edit using the get and put functions. For example -

> (put 'kumar 'sister 'hamsa)
(sister . hamsa)
> (get 'kumar 'brother)
()
> (get 'kumar 'sister)
(sister . hamsa)

A symbol's property list is globally available and is not changed by local contexts such as let.

Symbols and values

In muSE, as in all Schemes I guess, named symbols are entities that are uniquely specified by their textual representation - i.e. two symbols with the same name refer to the same internal object, irrespective of context. In muSE, all named symbols are "interned" forever - i.e. they are automatically kept alive for the life of the running environment.

You can bind values to symbols either using the define syntax or using the set! function. There is very little difference between define and set!. They can both assign values to symbols at the lexically top-level, but only define can be used to specify recursive functions. muSE's define syntax is more restrictive than R5RS Scheme in that you can define functions only like this -

(define f (fn (...args..) ...body...))

whereas in standard Scheme you'd define it like this -

(define (f ...args...) ...body...)

It is not an error to define the value of a symbol more than once using define, but it will complain because it is a common source of programming error that indicates that an incorrect assumption is probably being made. set! will not complain, of course, as the intention is clear.

You can get the string name of a symbol using (name sym) and you can intern a symbol given its string representation using (symbol "name").

Differences with standard Scheme
In MzScheme (for example), symbols introduced by define are not closed over when creating functions using lambda. Changing the definition of such a top-level symbol will change the behaviour of the function created using lambda. In muSE, however, fn captures the values of symbols at the time it is being created, including all top-level definitions. fn:, on the other hand, allows its behaviour to be changed after its definition ... even in a local context such as that introduced by let.