Idio Reader

The reader’s role is to consume UTF-8 source code and construct an Abstract Syntax Tree suitable for the evaluator.

A complete Idio expression is a whole line (up to a newline) or however many more lines is required to complete any matching parentheses, braces, brackets or double-quoted strings.

You cannot have multiple expressions on one line.

The matching part of the reader gives rise to a standard form, say:

 1define (add a b) "
 2add `a` to `b`
 3...
 4" {
 5  ;; calculate a result in a local variable
 6  r := a + b
 7
 8  ;; return the result
 9  r
10}

Here the starting " on line 1 means the documentation string is not complete until the matching " on line 4 where the { before the end of line means the body is not complete until the matching } on line 10.

An Idio expression of multiple words is always a list. So, even though the example didn’t start and end with a parenthesis, the AST for that expression will be

(define (name formals) docstr body)

Similarly:

ls -l
(ls -l)

are identical. Multiple words always create a list.

*

Whilst not a problem for the reader, single words are problematic:

ls

With our shell hats on we want to run the external program ls. However, Idio is a programming language and programming language idioms win out.

Consider line 9 in the above example:

  • do you intend to return r, the value from the addition of a and b?

  • do you want to run the external program that is called whatever the value of r is?

  • do you want to run the external program called r?

Idio will return the value of r, here, the addition of a and b.

Idio will similarly return the value of ls from our single word example. ls could be a variable, in which case you’ll get its value but it could be undefined and therefore result in itself, a symbol, ls.

Neither of which is very helpful in executing the program ls. To force a single word to be executed you must wrap it in parentheses:

(ls)

This forces Idio to interpret the expression as a list of a single symbol. A list will be evaluated as a “function” call where, here, the function name, a plain symbol, means the VM will seek an external program from the current PATH.

*

The reader will construct simple values as it sees them in the source code, numbers, strings, symbols, etc. and return those values in the AST.

The AST itself will be lists of lists of lists. The printed form of such lists and simple values looks, by and large, like the original source code.

Last built at 2024-12-21T07:10:43Z+0000 from 62cca4c (dev) for Idio 0.3.b.6