Precedence (Mailfromd Manual)

4.17.9 Operator Precedence

Operator precedence is an abstract value associated with each language operator, that determines the order in which operators are executed when they appear together within a single expression. Operators with higher precedence are executed first. For example, ‘*’ has a higher precedence than ‘+’, therefore the expression a + b * c is evaluated in the following order: first b is multiplied by c, then a is added to the product.

When operators of equal precedence are used together they are evaluated from left to right (i.e., they are left-associative), except for comparison operators, which are non-associative (these are explicitly marked as such in the table below). This means that you cannot write:

if 5 <= x <= 10

Instead, you should write:

if 5 <= x and x <= 10

The precedence of the mailfromd operators where selected so as to match that used in most programming languages.¹⁵

The following table lists all operators in order of decreasing precedence:

(...): Grouping
$ %: Sendmail macros and mailfromd variables
* /: Multiplication, division
+ -: Addition, subtraction
<< >>: Bitwise shift left and right
< <= >= >: Relational operators (non-associative)
= != matches fnmatches: Equality and special comparison (non-associative)
&: Logical (bitwise) AND
^: Logical (bitwise) XOR
|: Logical (bitwise) OR
not: Boolean negation
and: Logical ‘and’.
or: Logical ‘or’
.: String concatenation

Footnotes

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The only exception is ‘not’, whose precedence in MFL is much lower than usual (in most programming languages it has the same precedence as unary ‘-’). This allows to write conditional expressions in more understandable manner. Consider the following condition:

if not x < 2 and y = 3

It is understood as “if x is not less than 2 and y equals 3”, whereas with the usual precedence for ‘not’ it would have meant “if negated x is less than 2 and y equals 3”.