Wednesday, January 20, 2010

Lua's syntax

In search of inspiration for language syntax style I remembered Lua. It takes the third major path apart from curly braces (C) and off-side rule (Python). It is thus reminiscent of the Algol family, especially Ada, UNIX shell syntax or Karel (at least some incarnations thereof).

Lua is (mostly) free-form and uses what I call in my head (inappropriately, perhaps) implicit block delimiters. Where a declaration of some program element in C might look like

declaration {

in Lua it is instead


What I like about the latter case is that programmer is freed of the choice whether or not to use curly braces to delimit a single statement in control flow expressions such as if, for, while. Still, the language remains free-form and the code looks visually balanced to my eyes. For some reason I feel the cosmic balance disturbed if the beginning end end statement are not both marked, with the same indentation. In Python the blocks feel– incomplete– without the final delimiter. Of course, which style has better signal-to-noise ratio, can be disputed.

Here, Lua is somewhat more concise than Ada or Fortran where we annotate what kind of block we are ending (e.g. if ... then ... else ... end if) in hope of helping the programmer match the delimiters more easily (and to help the compiler spot bracketing errors).

One design element of Lua syntax that I don't really like is the freedom to or not to terminate any statement with a semicolon. Instead a statement can be terminated with a newline. On one hand this allows us to avoid using punctuation unless necessary, on the other hand the language is not fully free-form. Quoting from the reference manual, the code

a = f

would be mistaken for two statements. Gorgeous ^_^.

In Ada all statements (and other program elements) are terminated with a semicolon. The Prosody IM software written in Lua seems to have adopted a coding style where every simple statement is terminated with a semicolon, while any compound statement or element ending with the end keyword is not.

Conclusion: While Lua syntax style need not be perfect, it demonstrates that even this syntactic style does not have to be excessively verbose. I like it enough to consider its style as a primary source of inspiration.

Note: When will I actually post anything related to HelenOS?

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