Synopsis Illegal operation on an empty list.
Function data RunTimeException = EmptyList();
Usage import Exception;
Description Rascal provides many operations and functions on lists, see Rascal:Values/List and Rascal:Prelude/List. This error is generated when a function or operation cannot handle the empty list.

  • Guard the function or operation with a test on the empty list (Rascal:isEmpty) and take alternative action in that case.
  • Catch the EmptyList yourself, see Rascal:TryCatch.
Examples Import the List library and introduce L with an empty list as value:
rascal>import List;
rascal>L = [];
list[void]: []
Taking the head of an empty list gives an error:
|rascal://List|(3636,1037,<170,0>,<209,51>): EmptyList()
	at *** somewhere ***(|rascal://List|(3636,1037,<170,0>,<209,51>))
	at head(|stdin:///|(5,1,<1,5>,<1,6>))

This is the case when taking the tail as well:
|rascal://List|(24027,958,<1194,0>,<1238,57>): EmptyList()
	at *** somewhere ***(|rascal://List|(24027,958,<1194,0>,<1238,57>))
	at tail(|stdin:///|(5,1,<1,5>,<1,6>))

We can also catch the EmptyList error. First import the Rascal exceptions (which are also included in Prelude) and IO:
rascal>import Exception;
rascal>import IO;
rascal>try println(head(L)); catch EmptyList(): println("Cannot take head of empty list");
Cannot take head of empty list
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