From: Matthew Kerwin <lists@...>
Date: 2013-02-12T06:28:59+09:00
Subject: Re: elegant way to determine if something is defined

sto.mar wrote in post #1096319:
> Hmmm... very dangerous.
>
> That would catch all kinds of exceptions, not only the NameError:
>
>    a = []
>    b = a + 1
>    # => TypeError: can't convert Fixnum into Array
>
> vs.
>
>    a = []
>    b = a + 1 rescue 99
>    b  # => 99

Back in the olden days, MOO code had the following:

    `x + 1 ! E_TYPE';
... returns (x+1) or E_TYPE, or raises E_VARNF.

    `x.y ! E_PROPNF, E_PERM => 17'
... returns x.y if x is an object and x.y is a readable property;
returns 17 if x is an object but x.y is not defined or
not readable; or raises something else (like E_VARNF or E_INVIND)

    `1 / 0 ! ANY';
... returns E_DIV

I liked being able to explicitly trap a certain class of exception, and
either return the exception itself or a default value.

Additionally it had try/catch blocks.

[1] http://files.moo.ca/1/1/7/ProgrammersManual_26.html#SEC26

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