Name Rascal/Statements/Fail

Synopsis Let the current alternative of a pattern match fail.

Syntax fail;

Description A fail statement is only allowed in statements that are controlled by the outcome of a pattern match:

• The Patterns in a PatternWithAction in Switch or Visit statement controls the statements in the action part.
• The test (expression) of a While or Do statement controls the statements in the body part.
• The test (expressions) of a For statement control the statements in the body part.
• The formal parameter declaration of a Function.

The fail statement is associated with the innermost pattern match by which it is controlled.

When fail is executed:

• If the associated pattern has more alternatives, the next alternative is explored,
• otherwise the pattern as a whole fails.
  • In the case of switch or visit this means that the next case will be tried. Any bindings caused by the pattern or side-effects caused by the action are undone.
  • For while, do and for, this implies that any bindings caused by the pattern are undone and that the next alternative in the test is tried; otherwise the loop is terminated.
  • For a function call it means that the next function declaration (or the default one) is tried.

Examples Here is an example taken from Recipes:Bubble. It uses a fail for the case that no unsorted element can be found in the list of numbers. As a result, the whole case fails and the default case is used.

rascal>import IO;
ok
rascal>public list[int] sort(list[int] Numbers){
>>>>>>>  switch(Numbers){
>>>>>>>    case [list[int] Nums1, int P, int Q, list[int] Nums2]:
>>>>>>>       if(P > Q){
>>>>>>>          return sort(Nums1 + [Q, P] + Nums2);
>>>>>>>       } else {
>>>>>>>       	  fail;
>>>>>>>       }
>>>>>>>     default: return Numbers;
>>>>>>>   }
>>>>>>>}
list[int] (list[int]): list[int] sort(list[int]);
rascal>sort([10, 1, 5, 3]);
list[int]: [10,1,5,3]

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