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Exceptions: Difference between revisions
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custom_error =
if 'oops' print
=={{header|Ruby}}==
===Defining an exception===
<ruby>class SillyError < Exception
def initialize(args = nil)
@args = args
end
end</ruby>
Note: Often new exceptions are defined simply with no body. For example:
<ruby>class MyInvalidArgument < ArgumentError
end</ruby>
This example makes "MyInvalidArgument" an type of ArgumentError (one of the built-in exceptions). It's simply declared as a subclass of the existing exception and no over-riding is necessary. (A rescue clause for ArgumentError would catch MyInvalidArgument exceptions ... but one's code could insert a more specific exception handler for the more specific type of exception).
===Throwing an exception===
<ruby>def spam
raise SillyError, 'egg'
end</ruby>
===Handling an exception===
rescue/else/ensure
<ruby>begin
foo
rescue SillyError, se
print se.args
bar()
else
# no exception occurred
quux
ensure
baz
end</ruby>
The "rescue" clause is like the "catch" clause in other languages. The "ensure" clause is like the "finally" clause in other languages.
===Catch/Throw===
Ruby has a separate exception-like system that is meant to be used to exit out of deep executions that are not errors.
<python>def foo
throw :done
end
catch :done do
foo
end</python>
You can only "throw" and "catch" symbols. Like exceptions, the throw can be made from a function defined elsewhere from the catch block.
=={{header|Standard ML}}==
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