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Fastly VCL provides various arithmetic and conditional operators.

Operator precedence

Operator precedence defines the order of operations when evaluating an expression. Higher precedence operators are evaluated before those with lower precedence. Operators are listed in the following table, highest precedence first. For example, a || b && c reads as a || (b && c) because && has higher precedence than ||.

Operator associativity determines which side binds first for multiple instances of the same operator at equal precedence. For example, a && b && c reads as (a && b) && c because && has left to right associativity.

String concatenation

Adjacent strings are concatenated implicitly, or explicitly by using the + operator. All of the following lines of code achieve the same result:

set req.http.some-header = "helloworld";
set req.http.some-header = "hello" "world";
set req.http.some-header = "hello" + "world";

Concatenation works with literal strings, as well as headers and variables (including non-string types):

set var.string_variable = "Hello " + req.http.cookie:user_name + " you are number " + var.int_variable;

Conditional operators

Conditional operators produce BOOL values, suitable for use in if (...) statements:

if (!(beresp.status >= 500 && beresp.status < 600)) {
  # ... the response is not an HTTP 5XX status
}

Conditional expressions may be inverted by prefixing the ! operator, and combined using AND and OR operators (&& and || respectively).

The following operators are valid for comparisons:

FLOAT comparisons have special cases for operands which are NaN: The != operator always evaluates to true when either operand is NaN. All other conditional operators always evaluate to false when either operand is NaN.

For example, if a given variable is NaN, that variable will compare unequal to itself: both var.nan == var.nan and var.nan >= var.nan will be false.

STRING comparisons have special cases for operands which are not set (as opposed to empty): The != and !~ operators always evaluate to true when either operand is not set. All other conditional operators always evaluate to false when either operand is not set. For example, if a variable is not set, that variable will compare unequal to itself: Both req.http.unset == req.http.unset and req.http.unset ~ ".?" will be false.

Floating point infinities are signed, and compare as beyond the maximum and minimum values for FLOAT types, such that for any finite value: −∞ < n < +∞.

Note that as there are currently no numeric expressions in general; these operators are limited to use with specific operands. For example, var.i < 5 is permitted but 2 < 5 is not.

Assignment operators

These operators allow values or the results of expressions to be assigned to a variable using the set statement.

If both operands (left-hand side and right-hand-side) are INTEGER, an INTEGER variant of the operator, where available, is used. The advantage of this is that the operation is faster and more precise (compared with FLOAT variants), but because the range of INTEGER is more limited, overflows can happen. For example, the integer variants of the +=, -=, and *= operators wrap around as if they were unsigned 64-bit integers.

FLOAT arithmetic has special cases for operands which are NaN: Arithmetic operators evaluate to NaN when either operand is NaN.

FLOAT arithmetic has special cases for operands which are floating point infinities: In general all arithmetic operations evaluate to positive or negative infinity when either operand is infinity. However, some situations evaluate to NaN. Some of these situations are domain errors, in which case fastly.error is set to EDOM accordingly. Others situations are not domain errors: ∞ − ∞ and 0 × ∞. These evaluate to NaN but do not set fastly.error.

The left-shift <<= is equivalent to multiplying by powers of two, but the high-order bits disappearing. For example, <<= 3 is equivalent to multiplying by 8, but the highest three bits will disappear.

The right-shift >>= is the arithmetic shift: that is, for both positive and negative integers it is equivalent to dividing by powers of two. For example, >>= 3 is equivalent to dividing by 8, but the lowest three bits will disappear. For negative integers this is also known as sign-extension: at the binary level, the sign bit (the highest bit) is copied to all the vacant bits.

The left-rotate rol= moves the bits towards the high end but the bits dropping off from the high end are rotated back to the low end. For example, rol= 1 moves the highest bit to the low end, and moves all the other bits up.

The right-rotate ror= moves the bits towards the low end but the bits dropping off the low end are rotated back to the high end. For example, ror= 1 moves the lowest bit to the high end, and moves all the other bits down.

Shift and rotate operations with negative amounts perform the operation in the opposite direction. For example, <<= -3 is equivalent to >>= 3, and rol= >> -3 is equivalent to ror= 3.

The remainder operator %= performs the same function as a modulus operator for positive numbers.

For right operands larger than the bit width of INTEGER (64), shifts will yield zero for non-negative numbers, or -1 for negative numbers, and rotates will use the operand modulo the bit width of INTEGER. For example, >>= 99 will yield zero or -1, and ror= 99 is equivalent to ror= 35 because 99 % 64 == 35.

Reserved punctuation

Punctuation appears in various syntactic roles which are not operators (that is, they do not produce a value).

The following lexical tokens are reserved, but not used: * & | >> << ++ -- %