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From: Bart Schaefer <schaefer@brasslantern.com>
Message-id: <120222092827.ZM24425@torch.brasslantern.com>
Date: Wed, 22 Feb 2012 09:28:27 -0800
In-reply-to: <20120222095248.1ea8140b@pwslap01u.europe.root.pri>
Comments: In reply to Peter Stephenson <Peter.Stephenson@csr.com>
 "Re: let unset array element remove compatible with bash" (Feb 22,  9:52am)
References:
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	<120221210106.ZM13374@torch.brasslantern.com>
	<20120222095248.1ea8140b@pwslap01u.europe.root.pri>
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Subject: Re: let unset array element remove compatible with bash
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On Feb 22,  9:52am, Peter Stephenson wrote:
}
} So it's not simply a case of unconditionally deleting an element.  I
} think you need to check the element is present in the first place.

Is there a straightforward way to do that?  The code in subst.c that
handles ${+var[x]} is rather convoluted, and every combination of
getvalue()/fetchvalue() that I've tried always returns non-NULL for
subscripted expressions.

Which probably explains this comment in subst.c:

    /*
     * Indicates we have tried to get a value in v but that was
     * unset.  I don't quite understand why (v == NULL) isn't
     * good enough, but there are places where we seem to need
     * to second guess whether a value is a real value or not.
     */
    int vunset = 0;

Then there's this horror, which appears to be fetching the entire
array to call arrlen() on it, just to discover whether one of the
elements is not set:

	    /* Value retrieved from parameter/subexpression is scalar */
	    if (v->pm->node.flags & PM_ARRAY) {
		/*
		 * Although the value is a scalar, the parameter
		 * itself is an array.  Presumably this is due to
		 * being quoted, or doing single substitution or something,
		 * TODO: we're about to do some definitely stringy
		 * stuff, so something like this bit is probably
		 * necessary.  However, I'd like to leave any
		 * necessary joining of arrays until this point
		 * to avoid the multsub() horror.
		 */
		int tmplen = arrlen(v->pm->gsu.a->getfn(v->pm));

		if (v->start < 0)
		    v->start += tmplen + ((v->flags & VALFLAG_INV) ? 1 : 0);
		if (!(v->flags & VALFLAG_INV) &&
		    (v->start >= tmplen || v->start < 0))
		    vunset = 1;
	    }

Gaah.