Conditions and condition lists

Ceylon allows one or more conditions, which together form a condition list, in the if, while, and assert statements and in if comprehension clauses.

Note: This page is not describing the is, exists, and nonempty operators, which are considered a part of the expression grammar.

Usage

A condition list may occur in an if, while, or assert statements, or in an if clause of a comprehension.

Description

A condition list is satisfied if and only if every condition in the list is satisfied. The difference between a comma-separated condition list and a single Boolean condition constructed using the && operator is that a condition list may narrow the type of a reference, or introduce a new reference.

Integer m(Object x) {
    if (is Integer x, x < 10) {
        // x is of type Integer here
        return x + 1;
    }
    return 0;
}

Note: In the following discussion we will use if in the code examples. Keep in mind that condition lists may also occur in a while or assert statement.

Boolean conditions

A boolean condition is an expression of type Boolean. It is satisfied if the expression evaluates to true.

is conditions

A condition of form is Y x is satisfied if the reference x evaluates to an instance of the type Y. Within the associated block, x will have the type X&Y, where X is the previous type of x.

In other words, if (is Y x) similar to Java's if (x instanceof Y), immediately followed by a typecast, (Y) x.

Integer plusOneIfInteger(Object x) {
    if (is Integer x) {
        // x is of type Integer here
        return = x+1;
    }
    return 0;
} 

It's even possible to introduce a new local reference in the condition, in which case it is the new local reference which has the narrowed type:

Integer plusOneIfInteger(Object x) {
    if (is Integer i=x) {
        // x still of type Object here
        // but i is an Integer
        return i+1;
    }
    return 0;
}

For a variable reference or getter you are required to use this form. On a platform that supports concurrency this protects you from another thread changing the value while the block protected by the if is being executed (and potentially violating the guarantees of the type system).

It is acceptable to specify a generic type in an is condition, for example, if (is List<Integer> list) to distinguish a List<Integer> from a List<String>.

Note: Do not confuse the is condition of form is Type t with an is operator of form t is Type.

exists conditions

A condition of form exists x means is Object x, and is satisfied if the reference x refers to a non-null value. Within the associated block, x will have the non-optional type X&Object, where X is the previous type of x.

In other words, if (exists x) is similar to Java's if (x != null).

Integer plusOneIfExists(Integer? x) {
    if (exists x) {
        // x is of type Integer here
        return x+1;
    }
    return 0;
} 

It's possible to introduce a new local reference in the condition, in which case it is the new local reference that has the narrowed type:

Integer plusOneIfExists(Integer? x) {
    if (exists i=x) {
        // x still of type Integer? here
        return i+1;
    }
    return 0;
} 

Note: Do not confuse the exists condition of form exists t with an exists operator of form t exists.

nonempty conditions

A condition of form nonempty xs means is [X+] xs, where X is the element type of the sequence reference x, and is satisfied if the x refers to a nonempty sequence. Within the associated block, x will have the nonempty type [X+].

Integer firstPlusOne(Integer[] xs) {
    if (nonempty xs) {
        // xs of type [Integer+] here
        return xs.first+1;
    }
    return 0;
} 

It's possible to introduce a new local reference in the condition, in which case it is the new local reference which has the narrowed type:

Integer firstPlusOne(Integer[] xs) {
    if (nonempty ys=xs) {
        // xs still of type Integer[] here
        return ys.first+1;
    }
    return 0;
} 

Note: Do not confuse the nonempty condition of form nonempty xs with a nonempty operator of form xs nonempty.

See also