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Case-When

Overview

case-when expression is introduced as a syntax like switch-case, but the followings are different.

  • switch-case is a statement, but case-when is an expression. case-when can be used inside an expression, and the result value can be assigned to a variable or be used with calculation.
  • case clause in switch allows a fall through, but when clause in case never do it. Always the only one of when clause is used.
  • switch-case will only check if the value is same, but case-when will check if the shape of an array or object is the same. This is like Ruby's pattern matching syntax of case-in.
  • In switch-case, the order of checking value is normally not guaranteed, but when it is in case-when, the order of checking is always the written order on the source code and starting it with the top.
  • When no condition is matched in switch-case, nothing is done. On the other hand, when it is in case-when, the exception of NoMatchingPatternException will be raised.
  • Putting a block to a when clause means an on the fly function call. Therefore return in a block of when clause never returns to a caller function, and just returns a value as a result of when clause.
  • case-when is an expression, so you need a block when you want to write a statement. As said above, a block is an automatically called function object, so you can write multiple statements in a block and do it.
  • case-when is an expression, so the end of statement requires a semicolon of ;.

Basic Usage

Normal Case

case-when example is below.

function example(y) {
    var x = case y
        when 1: 1
        when 2: 20
        when 3: 300
        when 4: 4000
        when 5: {
            return 50000;
                // this is an automatically called internal function,
                // so 50000 will be returned to the case expression itself
                // and it is just assigned to the variable `x`.
        }
        else: -1
    ;
    return x + 1;
}
7.times { => System.println(example(_)) };

This code shows the following result.

0
2
21
301
4001
50001
0

Without Colon

You do not have to use the colon :, and using it is only for an appearance.

function example(y) {
    var x = case y
        when 1 1
        when 2 20
        when 3 300
        when 4 4000
        when 5 {
            return 50000;
        }
        else -1
    ;
    return x + 1;
}
7.times { => System.println(example(_)) };

This will also show the same result above.

Inside Expression

If you want to use case-when inside an expression, use ( and ) to avoid a syntax confusion or syntax error.

For example, sometimes the result may be different from your expectation as below.

var y = 1;
var x = case y when 1: 1 when 2: 2 + 3;
// x means 4, or 1 ?
System.println(x);  // => 1, because it is `case y when 1: 1 when 2: (2 + 3)`.

If you want the result 4, you should write it below.

var y = 1;
var x = (case y when 1: 1 when 2: 2) + 3;
// x means 4, or 1 ?
System.println(x);  // => 4

No Fallthrough

No fallthrough mechanism in case-when. Just one condition is selected and do it. If not matching to any cases, it selects else clause. And if no else clause, the exception of NoMatchingPatternException will be raised.

function example(y) {
    return case y
        when 0 0
        when 1 1
        when 2 20
        when 3 300
        when 4 4000
        when 5 {
            return 50000;
        }
        // no else clause...
    ;
}
7.times { => System.println(example(_)) };

Here is the result.

0
1
20
300
4000
50000
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at function example(test.kx:12)
        at function __test_kx_14___anonymous_func228(test.kx:14)
        at <main-block>(test.kx:14)

Pattern Matching

As the same as an assignment, the when condition will check by a pattern matching. Therefore, you can use it below.

var obj = { x: 10, y: 20, z: { a: 100, b: 200 } };
case obj
when { x: vx, y: vy, z: { a: 100, b: 2000 } }: {
    System.println("Pattern 1 - %d %d" % vx % vy);
}
when { x: vx, y: vy, z: { a: 100, b: 200 } }: {
    System.println("Pattern 2 - %d %d" % vx % vy);
};

Here is a result.

Pattern 2 - 10 20

If Modifier

As case-when uses a pattern matching, you can use if-modifier for the value check.

var y = 20;
case y
when 1..10: System.println(y)
when v: System.println(v*2)
    // the above `when` condition does NOT mean `y equals v`.
    // it means `y is assigned to the variable v`.
    // as a result, this condition will match to any y value.
;

That is why, you can use if-modifier to check if y equals v.

var v = 15;
var y = 20;
case y
when 1..10:         System.println(y)
when m if (m == v): System.println(m*2)     // m is always matched,
                                            // but failed because of m != v.
when m:             System.println(m*10)    // m is matched to any value.
;

Examples

Example 1. Normal Case

Code

function example(y) {
    var x = case y
        when 1: 1
        when 2: 20
        when 3: 300
        when 4: 4000
        when 5: {
            return 50000;
                // this is an automatically called internal function,
                // so 50000 will be returned to the case expression itself
                // and it is just assigned to the variable `x`.
        }
        else: -1
    ;
    return x + 1;
}
7.times { => System.println(example(_)) };

Result

0
2
21
301
4001
50001
0

Example 2. Without Colon

Code

function example(y) {
    var x = case y
        when 1 1
        when 2 20
        when 3 300
        when 4 4000
        when 5 {
            return 50000;
        }
        else -1
    ;
    return x + 1;
}
7.times { => System.println(example(_)) };

Result

0
2
21
301
4001
50001
0

Example 3. Not Matched

Code

function example(y) {
    return case y
        when 0 0
        when 1 1
        when 2 20
        when 3 300
        when 4 4000
        when 5 {
            return 50000;
        }
        // no else clause...
    ;
}
7.times { => System.println(example(_)) };

Result

0
1
20
300
4000
50000
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at function example(test.kx:12)
        at function __test_14___anonymous_func228(test.kx:14)
        at <main-block>(test.kx:14)

Example 4. Pattern Matching

Code

var obj = { x: 10, y: 20, z: { a: 100, b: 200 } };
case obj
when { x: vx, y: vy, z: { a: 100, b: 2000 } }: {
    System.println("Pattern 1 - %d %d" % vx % vy);
}
when { x: vx, y: vy, z: { a: 100, b: 200 } }: {
    System.println("Pattern 2 - %d %d" % vx % vy);
}
;

Result

Pattern 2 - 10 20

Example 5. If Modifier

Code

var v = 15;
var y = 20;
case y
when 1..10: System.println(y)
when m if (m == v): System.println(m*2)
when m: System.println(m*10)    // matched to any value.
;

Result

200

Example 6. Various Example (1)

Code

function test(y, m, n) {
    return case y
        when 1:
            1
        when 2..10:
            10
        when v if (v == m):
            v + 42
        when v if (v == n):
            v + 58
        else:
            -1
    ;
}
System.println(test(-1));
System.println(test(1));
System.println(test(2));
System.println(test(5));
System.println(test(10));
System.println(test(100, 100, 2));
System.println(test(200, 1, 200));

Result

-1
1
10
10
10
142
258

Example 7. Various Example (2)

Code

function test([y, z, i, j, k, x]) {
    var val = case [y, z, i, j, k]
        when [1, ...a]:
            a[2]
        when [2..10, a, b]:
            a + b + 10
        when [v, m, ...n] if (10 < v && v <= 20):
            v + m + n[0] + 42
        when [v, 50, ...a] if (v == x):
            v + 58
        else:
            -1
    ;
    return val;
}
System.println(test([1, 2, 3, 4, 5, 6]));
System.println(test([2, 2, 3, 4, 5, 6]));
System.println(test([10, 12, 13, 14, 15, 16]));
System.println(test([11, 12, 13, 14, 15, 16]));
System.println(test([50, 50, 13, 14, 15, 50]));
System.println(test([50, 50, 13, 14, 15, 51]));
System.println(test([50, 51, 13, 14, 15, 50]));

Result

4
15
35
78
108
-1
-1

Example 8. Various Example (3)

Code

function test(m, obj) {
    return case obj
        when { x: 5, y: y, sub: z } if (y == m):
            { => y }()
        when { x: 6, y: y, sub: { a: 1, b: b, c: c} }:
            y + b + c
        when { x: 10, y: y, sub: z }:
            z
    ;
}
System.println(test(20, { x: 5, y: 20, sub: { a: 1, b: 2, c: 3 } }));
System.println(test(20, { x: 6, y: 10, sub: { a: 1, b: 2, c: 3 } }));
System.println(test(20, { x: 10, y: 10, sub: { a: 1, b: 2, c: 3 } }));
System.println(test(10, { x: 5, y: 20, sub: { a: 1, b: 2, c: 3 } }));

Result

20
15
{"a":1,"b":2,"c":3}
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at function test(test.kx:9)
        at <main-block>(test.kx:14)

Example 9. DateTime

Code

using DateTime;

function func(n) {
    case n
    when (DateTime("2000/1/1")..DateTime("2000/1/5")) {
        System.print("range 1 - ");
        System.println(n);
    } when (DateTime("2000/1/6")...DateTime("2000/1/10")) {
        System.print("range 2 - ");
        System.println(n);
    } when (DateTime("2000/1/11")..) {
        System.print("range 3 - ");
        System.println(n);
    } else {
        System.print("not matched - ");
        System.println(n);
    };
}

(DateTime("1999/12/30")..DateTime("2000/1/20")).each { => func(_1) };

Result

not matched - 1999/12/30 00:00:00
not matched - 1999/12/31 00:00:00
range 1 - 2000/01/01 00:00:00
range 1 - 2000/01/02 00:00:00
range 1 - 2000/01/03 00:00:00
range 1 - 2000/01/04 00:00:00
range 1 - 2000/01/05 00:00:00
range 2 - 2000/01/06 00:00:00
range 2 - 2000/01/07 00:00:00
range 2 - 2000/01/08 00:00:00
range 2 - 2000/01/09 00:00:00
not matched - 2000/01/10 00:00:00
range 3 - 2000/01/11 00:00:00
range 3 - 2000/01/12 00:00:00
range 3 - 2000/01/13 00:00:00
range 3 - 2000/01/14 00:00:00
range 3 - 2000/01/15 00:00:00
range 3 - 2000/01/16 00:00:00
range 3 - 2000/01/17 00:00:00
range 3 - 2000/01/18 00:00:00
range 3 - 2000/01/19 00:00:00
range 3 - 2000/01/20 00:00:00