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[Bug 29586] [XQ31] 2.5.6.2 The judgement subtype-itemtype(Ai, Bi)

From: <bugzilla@jessica.w3.org>
Date: Tue, 03 May 2016 10:03:45 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-29586-523-LHgsQRgZkn@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29586

--- Comment #7 from Tim Mills <tim@cbcl.co.uk> ---
(In reply to John Snelson from comment #5)
> I agree with Michael Kay - this is a deliberate design decision. In fact,
> things break if a map type can be a subtype of a function type - as I
> described in the email referenced.

The current specification says

"The judgement subtype-itemtype(Ai, Bi) determines if the ItemType Ai is a
subtype of the ItemType Bi. Ai is a subtype of Bi if and only if at least one
of the following conditions applies:

Ai is map(*) (or, because of the transitivity rules, any other map type), and
Bi is function(*)."

and the F&O diagram shows map(*) and array(*) as being derived from
function(*).

In the referenced message, you say

"This leads to the inescapable conclusion that even though a map value is 
a function value, a map ItemType is not a function ItemType - they are 
fundamentally different types of test."

I'm missing something here, because this seems contradicatory.  Could you also
please elaborate on what breaks?

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