The difference between "errors / exceptions" and "throw / trick"?

Question

I am a bit confused about error handling in Rebol. It has THROW and CATCH constructs:

>> repeat x 10 [if x = 9 [throw "Nine!"]]
** Throw error: no catch for throw: make error! 2

>> catch [repeat x 10 [if x = 9 [throw "Nine!"]]]
== "Nine!"

But this cast and trap are not related to the handler passed to the TRY refinement / PICTURE:

>> try/except [throw "Nine!"] [print "Exception handled!"]
** Throw error: no catch for throw: make error! 2

This is very specific to the Rebol error type:

>> try/except [print 1 / 0] [print "Error handled!"]
Error handled!

If you want, you can initiate your own errors, but do not use THROW, as in other languages. Discard errors will simply lead to a complaint that they were not caught, like any other type of value:

>> try/except [throw make error! "Error string"] [print "Error handled!"]
** Throw error: no catch for throw: make error! 2

You must do this in order to make the evaluator try and execute something like an error! call what he considers an “exception”:

>> try/except [do make error! "Error string"] [print "Error handled!"]
Error handled!

(Note. You can use pre-made errors, for example cause-error 'Script 'invalid-type function!- see system/catalog/errorsfor more).

//, . , :

>> probe try [do make error! "Some error"]
make error! [
    code: 800
    type: 'User
    id: 'message
    arg1: "some error"
    arg2: none
    arg3: none
    near: none
    where: none
]
** User error: "Some error"

>> probe try [make error! "Some error"]
make error! [
    code: 800
    type: 'User
    id: 'message
    arg1: "Some error"
    arg2: none
    arg3: none
    near: none
    where: none
]
** User error: "Some error"

, CATCH . , "named throws":

>> code: [repeat x 10 [if x = 9 [throw/name "Nine!" 'number]]]

>> catch/name [do code] 'number
== "Nine!"

>> catch/name [do code] 'somethingelse
** Throw error: no catch for throw: make error! 2

, :

• ? , THROW CATCH DO TRY/EXCEPT?

• /EXCEPT /ON -ERROR - ?

• "no catch for throw: make error! 2" - ? make error! 2?

Answer 1

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Answer 2

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Answer 3

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Sorry, I may not have answered your question directly, but I thought I might turn to your second question: should it be called ON-ERROR. I would say no, because an exception does not necessarily mean an error; as a means of controlling the flow, this is just a more general instruction about the occurrence of a condition that should not allow further execution along a certain path.