| Type | Bug | Status | reviewed | Date | 14-Aug-2015 01:30 |
|---|---|---|---|---|---|
| Version | r3 master | Category | Native | Submitted by | fork |
| Platform | All | Severity | minor | Priority | normal |
| Summary | CASE statements don't evaluate "body" code for false "conditions" |
|---|---|
| Description |
Where an IF statement is laid out as: if condition body CASE statements are laid out roughly as: case [ condition1 body1 condition2 body2 condition3 body3 ] The rule for CASE is that the bodies don't not need to be blocks, or evaluate to one...it can be any expression that produces a value. Moreover, if the expression evaluates to a block, it will be executed in a second evaluation step. (Behavior-wise, the updated behavior of Rebol3's IF brings it into parity.) >> code: [print "Hello"] >> case [true code] Hello There is however a problem in that Rebol's CASE (in both Rebol2 and Rebol3) only uses a DO/NEXT evaluation on the bodies inside a case if the condition is "TRUE?". This leads to a problem that the truth or falsehood of an expression can change the "shape" of the case's execution: >> condition: true >> case [condition 10 + 20] == 30 >> condition: false >> case [condition 10 + 20] >> case [condition 10 + 20] ** Script error: cannot use add on none! value ** Where: + case ** Near: + 20 This is because false conditions skip one item. It needs to skip however many items it takes, which is what would have happened if this had been written as IF CONDITION 10 + 20. |
| Example code |
| Assigned to | n/a | Fixed in | - | Last Update | 5-Jan-2016 23:45 |
|---|
| Comments | |
|---|---|
|
(0004662)
Ladislav 11-Dec-2015 14:26 |
The described problem exists also in Rebol 2. |
|
(0004663)
Ladislav 11-Dec-2015 14:40 |
Observation:
a: 1 if false (a: 2 [a: 3]) a ; == 2 Does that mean that a: 1 case [ false (a: 2 [a: 3]) ] a should yield 2 too? |
|
(0004675)
Ladislav 3-Jan-2016 21:33 |
This needs a more complete specification of the desired behaviour. |
| Date | User | Field | Action | Change |
|---|---|---|---|---|
| 5-Jan-2016 23:45 | ladislav | Description | Modified | - |
| 5-Jan-2016 23:45 | ladislav | Status | Modified | submitted => reviewed |
| 3-Jan-2016 21:33 | ladislav | Comment : 0004675 | Added | - |
| 14-Dec-2015 17:17 | ladislav | Comment : 0004664 | Removed | - |
| 13-Dec-2015 16:18 | Ladislav | Comment : 0004664 | Added | - |
| 13-Dec-2015 16:16 | Ladislav | Comment : 0004663 | Modified | - |
| 11-Dec-2015 14:41 | ladislav | Comment : 0004663 | Modified | - |
| 11-Dec-2015 14:40 | ladislav | Comment : 0004663 | Added | - |
| 11-Dec-2015 14:29 | ladislav | Comment : 0004662 | Modified | - |
| 11-Dec-2015 14:28 | ladislav | Comment : 0004662 | Modified | - |
| 11-Dec-2015 14:26 | ladislav | Comment : 0004662 | Added | - |
| 14-Aug-2015 01:30 | Fork | Ticket | Added | - |