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Assertion Failed

rascal-0.41.2

Synopsis​

An assertion in the Rascal code is false.

Function​

data RuntimeException = AssertionFailed(str label);

Description​

An Assert statement can be used to check assumptions during the execution of a Rascal program. This error is generated if an assertion is not true.

Remedies:

  • Modify your code to make the assertion true.
  • Modify your assertion to reflect the current behaviour of your code.
  • Catch the AssertionFailed yourself, see try catch.

Examples​

A false assertion gives an error:

rascal>assert 3 > 4;
|prompt:///|(0,13,<1,0>,<1,13>): AssertionFailed()
    at $(|main:///_dollar_|)

Define a function that only increments positive integers:

rascal>int incrPositive(int n) { 
|1 >>>>  assert n > 0: "n should be greater than 0"; return n + 1; 
|2 >>>>}
int (int): function(|prompt:///|(0,89,<1,0>,<3,1>))

Calling it with a positive integer is fine:

rascal>incrPositive(3);
int: 4

But a negative argument gives an error:

rascal>incrPositive(-3);
|prompt:///|(43,28,<2,16>,<2,44>): AssertionFailed("n should be greater than 0")
    at incrPositive(|prompt:///|(0,89,<1,0>,<3,1>))
    at $(|prompt:///|(0,17,<1,0>,<1,17>))

We can also catch the AssertionFailed error. First import the Rascal exceptions (which are also included in Prelude) and IO:

rascal>import Exception;
ok
rascal>import IO;
ok
rascal>try 
|1 >>>>  println(incrPositive(-3)); 
|2 >>>>catch AssertionFailed(str msg): 
|3 >>>>  println("incrPositive: <msg>");
incrPositive: n should be greater than 0
ok

Benefits​

  • Enables the precise expression of assumptions in your code.
  • Asserts are actually executed when the compiler option enableAsserts is set to true (by default set to false).
  • In the RascalShell, enableAsserts is always true.