Assertions in design by contract: Difference between revisions
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Subroutine signature allow the programmer to make the parameters supplied to a routine be checked to make sure they are the of correct quantity, type, and value and can constrain the returned value(s) to a particular type. See the below snippet for a brief demonstration. Perl 6 does some static analysis to trap errors at compile time, but traps many errors at run time since it is often impossible to tell if a variable is of the correct type before the program is run.
When Perl 6 encounters a design-by-contract violation, it will fail with an error message telling where the problem occurred, what it expected and what it actually got. Some failures are trappable and resumeable. Failures may be trapped by adding a CATCH { } block in the current scope. Failures will transfer execution to the CATCH block where the failure may be analysed and further action taken. Depending on the failure type, execution may be resumable. Some
In this snippet, the routine repeat takes one Integer that must be greater than 1, a String and returns a String. (Note that as written, it is incorrect since it actually returns a boolean.)
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