Symbols

Symbols are an unusual Lisp-ish type. Most programming languages use symbols exclusively as variables names. In Idio symbols get a Lisp-ish life and a shell-ish life.

Symbols are like tags or names or flags you can pass around in their own right, not associated with anything. Think of an enumerated type in C where the members of the type must resolve to some integral value and subsequently tested for by, say, a switch statement. In Lisp languages you can simply compare the names for equality.

Symbols can are always be compared with eq?. Of interest, the symbol x is the same as the symbol x where-ever it exists in the program.

Symbols becomes variables in a traditional sense when they are bound to values. Subsequently, when the evaluator sees a symbol it will replace the symbol with the value it references.

With our shell hat on, symbols are also the names of programs we expect to be able to find on PATH. So, unlike almost every programming language except shells, Idio does not raise an error when a symbol is used and cannot be resolved to a value, it simply carries on with the symbol.

Eventually, if the value in functional position in a list that Idio wants to invoke is a symbol then it will look for a program with the same name on the PATH.

Nothing is quite perfect, though. Frequently, to ensure the evaluator does not resolve a symbol to its bound value the symbol will be (single) quoted, 'foo, say.

symbols.idio

;; we've not used foo before so it should be a symbol but it's a good
;; habit to quote symbols just in case
sym := 'foo

printf "sym eq? 'foo: %s\n" (eq? sym 'foo)
printf "sym eq? 'bar: %s\n" (eq? sym 'bar)
printf "sym eq? 'foo: %s\n" (eq? sym 'foo)

;; again, we haven't used the symbol touch yet
printf "symbol? touch: %s\n" (symbol? touch)

;; the symbol touch becomes the program touch
touch "words.txt"

;; similarly ls becomes the program ls and -l is converted to a simple
;; string argument
ls -l "words.txt"

$ idio symbols
sym eq? 'foo: #t
sym eq? 'bar: #f
sym eq? 'foo: #t
symbol? touch: #t
-rw-rw-r--. 1 idf idf 0 Oct 27 16:56 words.txt

Last built at 2024-10-13T06:11:44Z+0000 from 77077af (dev) for Idio 0.3