7. defining symbols

Another element of code in alv that we haven’t discussed in detail yet are symbols. Symbols (like trace, import* or math/+) are names that serve as placeholders for previously defined values. When code is evaluated, symbols are looked up in the current scope and replaced with the corresponding value found there.

When an alv file starts running, a number of symbols are defined in the default scope: These are the builtins mentioned above, and of which we have already been using trace, import, and import*.

To define a symbol yourself, the def builtin is used. It takes the symbol as its first, and the value to associate as its second parameter. After a symbol is defined, the name becomes an alias that behaves like the value itself. For example, we can use def to associate the result of our calculation with the symbol result, and then refer to it by that symbol in the trace operator:

(import* math)

(def result (+ 1 2))
(trace result)

Symbols need to start with a letter or one of the following special characters:

- + * /
_ . , =
! ? % $
> < ~

After the first character, numbers are also allowed. There are two types of symbols that are treated specially: symbols containing a slash (math/+), and symbols starting and ending with asterisks (*clock*):

• Symbols containing slashes (except at beginning and end of the symbol) are split into multiple symbols, and looked up recursively in the scope. For example, math/+ is found by first looking for a value for the symbol math, and then looking for the symbol + in that value. If the value for the symbol math is not a scope, an error is thrown.
• Symbols starting and ending with asterisks are called dynamic symbols and are looked up in a different way inside user-defined functions. This will be covered in detail later.
• The two special formats can be mixed: when evaluating *hello*/world, alv will look for the symbol world within the scope found by dynamically resolving *hello*.