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Fail

rascal-0.33.7

Synopsis​

Let the current alternative of a pattern match fail.

Syntax​

fail;

fail Label;

Description​

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

  • The Patterns in a Pattern With Action 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, unless it has a label. For, While and If can have a label that fail can jump to, and function names can be used to fail the entire application of a function match. A failed function would backtrack to the next overloaded candidate.

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.
    • 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. 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 [*int nums1, int p, int q, *int nums2]:
>>>>>>>       if(p > q){
>>>>>>>          return sort(nums1 + [q, p] + nums2);
>>>>>>>       } else {
>>>>>>>           fail;
>>>>>>>       }
>>>>>>>     default: return numbers;
>>>>>>>   }
>>>>>>>}
list[int] (list[int]): function(|prompt:///|(0,252,<1,0>,<11,1>))
rascal>sort([10, 1, 5, 3]);
list[int]: [1,3,5,10]

Benefits​

  • fail offers direct programmeable influence on backtracking behavior

Pitfalls​

  • fail breaks stack-trace-based debugging because the failed call may have been in the past.