VCL Expressions

I have been working on VCL expressions recently, and we are approaching the home stretch now.

The data types in VCL are “sort of weird” seen with normal programming language eyes, in that they are not “general purpose” types, but rather tailored types for the task at hand.

For instance, we have both a TIME and a DURATION type, a quite unusual constellation for a programming language.

But in HTTP context, it makes a lot of sense, you really have to keep track of what is a relative time (age) and what is absolute time (Expires).

Obviously, you can add a TIME and DURATION, the result is a TIME.

Equally obviously, you can not add TIME to TIME, but you can subtract TIME from TIME, resulting in a DURATION.

VCL do also have “naked” numbers, like INT and REAL, but what you can do with them is very limited. For instance you can multiply a duration by a REAL, but you can not multiply a TIME by anything.

Given that we have our own types, the next question is what precedence operators have.

The C programming language is famous for having a couple of gottchas in its precedence rules and given our limited and narrow type repertoire, blindly importing a set of precedence rules may confuse a lot more than it may help.

Here are the precedence rules I have settled on, from highest to lowest precedence:

Atomic

‘true’, ‘false’, constants

function calls

variables

‘(’ expression ‘)’

Multiply/Divide

INT * INT

INT / INT

DURATION * REAL

Add/Subtract

STRING + STRING

INT +/- INT

TIME +/- DURATION

TIME - TIME

DURATION +/- DURATION

Comparisons

‘==’, ‘!=’, ‘<’, ‘>’, ‘~’ and ‘!~’

string existence check (-> BOOL)

Boolean not

‘!’

Boolean and

‘&&’

Boolean or

‘||’

Input and feedback most welcome!

Until next time,

Poul-Henning, 2010-09-21