From: Igor Pirnovar <lists@...>
Date: 2012-11-04T16:08:18+09:00
Subject: Re: bug?: local variable created in if modifier not available in modified expression

Mean L. wrote in post #1082083:

> (local1 if local1 = "created") is supposed to be equivalent to
> (if local1 = "created" then local1 end)
>
> but the later works as expected and the former does not.

Ignoring the confusing and to interpreter ambiguous syntax above, which
BTW can easily be fixed, we can see the problem in the code as written
is not that variables inside the conditional statement behave as block
local variable, or that in an assignment to a nonexistent variable
inside the conditional expression causes that variable to lose its
dynamic property, somehow turning it into a static variable in front of
the conditional expression!

Neither of these assumptions is correct. The reason, you did not get the
results as you expected is because you are not taking account, that the
conditional in this situation is executed last, since it has the lowest
precedence of all operators, the only operator with lower precedence is
the block expression (begin/end). The same is true for all conditional
keywords {{ if, until, while, unless }}, and you can expect they behave
in the same way as 'if'.

In the following code you can see that neither is it true that variables
before the conditional behave as static variables nor, are the variables
inside the conditional expression, local to some invisible block:

1. p "v1:#{v1||='Not created' if ((v1='created')!='')}"  #=> v1:created
2. p "v2:#{v2||='Not CREATED' if (v2='created') != ''}"  #=> v2:created
3. p "if:#{if (v3 = 'created') != ''; v3; "XYZ"; end}"   #=> if:XYZ
4. p "v3:#{v3}"                                          #=> v3:created
5. p "v4:#{if (v4 = 'created') != ''; v4; end}"          #=> v4:created
6. p "v5:#{v5 if ((v5='created')!='')}"                  #=> FAILS


Lines number 3 and 5 above show that a conditional expression 'if'
returns the value of the last expression in its block expression.
However, when a conditional expression is used as modifier, obviously,
there is no return value there, and line #6 fails with:

  `<main>': undefined local variable or method `v5' \
        for main:Object (NameError)

You can salvage this situation by delaying the access to 'v5' variable,
by using v5||='Not created' idiom, which proves, that variable 'v5'
comes from inside the 'if' expression, and is indeed, the dynamically
created 'v5' rather than local static variable that shadows the
temporary inner conditional-block's 'v5'.


Regards, igor

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