Symbols are a double-edged thing in Scheme. While seeming completely natural they can cause substantial confusion for beginners.
The Guile reference states that “Symbols in Scheme are widely used in three ways: as items of discrete data, as lookup keys for alists and hash tables, and to denote variable references.” This sounds pretty complicated, but I hope to make that become clear soon.
On a first level you can see symbols as “names for something”. Symbols are quite similar to strings in so far as they are sequences of characters - but written without additional quotation marks. And in some contexts in LilyPond they can even be used interchangeably. But still they are something different.
First of all symbols are not self-evaluating. Earlier we learnt about self-evaluating expressions in Scheme, so for example
4 evaluates to the value
"Hello" evaluates to the string with content
Hello. A symbol on the other hand doesn't evaluate to itself but always denotes something else - it is a symbol for something. When Scheme encounters the symbol
Hello it will treat it as the reference to a variable whose name is
Hello and will then evaluate to the value of that variable.
guile> 4 4 guile> "Hello" "Hello" guile> Hello ERROR: Unbound variable: Hello ABORT: (unbound-variable)
In this case there is no variable with the name
Hello, which triggers this error. But earlier we saw how that works when a respective variable exists:
guile> red (1.0 0.0 0.0)
red is a symbol (defined in LilyPond) that evaluates to the value of a variable which represents a list of three numbers.
Often we need the symbol “itself” to pass along as data. To achieve this we make use of quoting - a way to tell Scheme that a symbol does not denote something else. There are two equivalent ways to express this:
guile> (quote red) red guile> 'red red
quote is a procedure that takes one argument - a symbol -, and prevents the evaluation of that symbol. This is a somewhat confusing concept, and therefore we have a dedicated section on quoting.
There is much more to symbols than can be said in this short introduction, and we will come back to the topic whenever necessary. In need of more detailed information one can head for the section in the Guile reference.
Presumably there are substantial aspects still missing from this page.