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When statement

In Rell, when statement is similar to switch in C++ or Java, but uses the syntax of when in Kotlin. The when expression is a versatile construct used for multi-way branching based on the value of an expression. It allows you to define different cases and actions to be taken depending on the value of the given expression. Here's an example:

when(x) {
    1 -> return 'One';
    2, 3 -> return 'Few';
    else -> {
        val res = 'Many: ' + x;
        return res;
    }
}

In this example:

  1. x is the expression that is being evaluated in the when statement. The value of x will determine which branch of the when expression will be executed.
  2. 1 -> return 'One';: This line specifies the first case. If x is equal to 1, the expression 'One' will be returned.
  3. 2, 3 -> return 'Few';: This line specifies the second case. If x is either 2 or 3, the expression 'Few' will be returned.
  4. else -> { ... }: This is the default branch, executed when none of the previous cases match. It allows for more complex code to be executed. In this case:
    • A local variable res is declared to store the result.
    • The value of x is concatenated to the string 'Many: ' using the + operator.
    • The entire constructed string is stored in res.
    • Finally, res is returned.

Features

  • It can use both constants as well as arbitrary expressions.
  • When using constant values, the compiler checks that all values are unique.
  • When used with an enum type, values use simple names, not full names.

A form of when without an argument is equivalent to a chain of if ... else if:

when {
    x == 1 -> return 'One';
    x >= 2 and x <= 7 -> return 'Several';
    x == 11, x == 111 -> return 'Magic number';
    some_value > 1000 -> return 'Special case';
    else -> return 'Unknown';
}
  • Can use arbitrary boolean expressions.
  • When you specify multiple comma-separated expressions, any triggers the block (combined via OR).

You can use both forms of when (with and without an argument) as an expression:

return when(x) {
    1 -> 'One';
    2, 3 -> 'Few';
    else -> 'Many';
}
  • If you are using when as an expression, it must be exhaustive, the else statement can make it exhaustive.
  • You can use it in at-expression, in which case it's translated to SQL CASE WHEN ... THEN expression.