The membership checking operator

The in binary operator is offered by Freya to perform membership tests against several data structures.

Membership tests

The in operator is a binary operator with same precedence as comparisons and the is operator:

expression in container

Freya supports this syntactic variant for negating the result of the test:

expression not in container

There are two different scenarios where in can be used:

• The right side evaluates to a type or interface that implements or inherits from the ICollection interface

In this case, in is translated like this:

ICollection[X](container).Contains(expression)

Please note that this includes the case where container is an array, since all arrays are assumed to implement the ICollection[X] generic interface.

• The right side evaluates to a type that offers an accesible Contains method with only one input parameter compatible with the left operand, and returning a boolean value.

In this case, there's no need for a typecast:

container.Contains(expression)

Membership tests against literal arrays

When the second operand is a literal array, the compiler can optimize the generated code. When the literal array has few enough elements, the in expression is translated as a concatenation of equality checks:

// Original test
expression in [item1, item2, item3]

// Generated code
var temp = expression;
temp = item1 or temp = item2 or temp = item3

When the literal array is long enough and when all of its items are constant expressions, the in test may be implemented with a switch IL operation code, according to the type of the left operand.

See also

The Freya Programming Language
Expressions
Nullable types
Type declarations
Interface types
Literal arrays