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From: Greg Colvin (gcolvin_at_[hidden])
Date: 20010525 23:03:51
From: joel de guzman <joel_at_[hidden]>
> Hi,
>
> I am having second thoughts with the + operator for
> sequencing. At first, it seems to be ok but further
> use of the operator gives me a feeling that something
> is wrong.
>
> Why I dislike the + for EBNF sequencing:
>
> 1) The same reason why I dislike string's + operator for
> concatenation. Concatenation and sequencing is *not*
> commutative. a + b should be equivalent to b + a. I
> feel that this is wrong and should not be propagated.
>
> Among all the other binary operators in Spirit, only this one
> violates the basic laws of commutativity as basically understood
> of the operator. I believe an accepted noncommutative operator
> is more appropriate.
>
> 2) EBNF operators [&^] are 'setlike' operators. It is
> quite common to see  to mean set union, & to mean set
> intersection,  to mean set difference and ^ to mean set
> exclusive or.
>
> Yet it is also common to see the + operator as a substitute
> for set union. Although this is arguable in the view point
> of the rule: "when in doubt, do as the ints do", since
> a  b != a + b (for integers), some set libraries and even
> languages do use the + as a synonym for . For some this
> closely resemble the mathematical set operator (+ enclosed
> in a circle).
>
> 3) It clashes with the meaning of + in EBNF and regular
> expressions which is solely used for iteration and means
> repeat one or more. One of the goals of Spirit is to
> mimic as closely as possible the syntax of EBNF.
None of the above bother me, but you are the designer. EBNF
is far enough away from algebra that I don't expect perfect
analogies, and infix + has long been used for concatenation.
What we really need is this:
http://www.research.att.com/~bs/whitespace98.pdf
;>
> Possible substitutes:
>
> a >> b >> c... The original Spirit syntax. >> arrows denote sequencing.
> Noncommutative
Ugly.
> a / b / c ... Noncommutative. Similar to unix file paths and URLs e.g.
> root/development/c++/myfile.cpp
Livable.
> a % b % c... Noncommutative. A bit of an eyesore though
Less of an eyesore than <<.
And don't forget
a*b*c
Commutative for numbers, but not matrices, and a common
substitute in computer languages for
a b c
in algebra. Nice high precedence too.
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