If you take a look in the PHP-manual regarding operator precedence you will find the following quote:
Although !
has a higher precedence than =
, PHP will still allow expressions similar to the following: if (!$a = foo())
, in which case the return value of foo()
is put into $a
.
This is not very informative, which expressions are similar to this expression?
The answer to this can be found in
this (huge!) file. This file is made with a tool called 'generate-prec-rules', which was created this week. It does exactly what the name implies, it generates the precedence rules given a set of common-precedence rules. The file above is created with the precedence rules from the yacc-definition of PHP version 5.
But how can we interpret these rules? Let's take a look at an example rule:
context-free priorities
"@" Expr -> Expr
<1> >
Expr "/" Expr -> Expr
This rule disallows a "/" on the second position of the '@'. Please note that this notation also counts the string-literal.
However, the expression
@ 4 / 5 is not invalid, it can still be parsed and evaluated. The rule above just explains how PHP places the parenthesizes in the expression, in this case like this:
(@ 4 ) / 5.
Take a minute to think about the expression and how it is influenced by the above rule, it took me a while to figure it out.
It is nice to have all of the rules, they describe the operator precedence very precisely, but the size of the file is a bit problematic. Every operator, there are about 32, gets a rule for each other operator for each expression it has. The '*' alone already occurs in 82 rules.
So the next step that we will have to take is to combine rules without changing the semantics of the precedence relations. An example of this is the combination of these two rules:
context-free priorities
Expr "*" Expr -> Expr
<2> >
Expr "+" Expr -> Expr
context-free priorities
Expr "*" Expr -> Expr
<0> >
Expr "+" Expr -> Expr
into this rule:
Expr "*" Expr -> Expr
> Expr "+" Expr -> Expr
Which means the same, but is a bit more compact and probably more understandable.
So when this tool is created, and some things are changed within SDF, we can incorporate the newly generated precedence rules into PHP-Front. This will solve some parsing problems and will give us an even better SDF-grammar for PHP. We will also try to generate a more understandable, one page list of the operator precedence within PHP to replace the quote in the current PHP-documentation.
A nice example of 'research meets real life' don't you think?
