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From: Herbert Kuchen <herbert.kuchen_at_wi.uni-muenster.de>

Date: Thu, 1 Oct 2015 13:54:44 +0200

I like the proposal,

Herbert

Am 01.10.2015 um 13:45 schrieb Michael Hanus:

*> Dear Colleagues,
*

*>
*

*> usually language definitions become more complex with
*

*> every new version (see the history of Java). In this sense,
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*> I want to make an unusual proposal:
*

*>
*

*> Simplify the definition of Curry
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*>
*

*> In short, I propose to remove the type "Success" from Curry.
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*>
*

*> Let me explain this proposal (which was initiated by Sergio Antoy)
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*> in more detail.
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*>
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*> Originally, the type "Success" was introduced in order to distinguish
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*> Boolean expressions, which are typicially used in if-then-else,
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*> from constraints, typically occurring in condition of rules.
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*> Constraints are only intended to be satisfied so that they should
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*> be never evaluated to "False". As a consequence, there are two
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*> kinds of equalities in Curry: Boolean equality ("==") which evaluates
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*> to True or False, and equational constraints ("=:=") that evaluate
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*> to "success" only (or their evaluation fails). Although there were
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*> good reasons for this distinction ("==" is similar to functional
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*> programming whereas "=:=" is unification like in logic programming),
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*> it also comes with some drawbacks:
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*>
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*> - The difference between Bool and Success is often not easy to grasp
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*> for beginners (my experience from lectures).
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*>
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*> - It is sometimes not obvious which kind of equality should be chosen
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*> in a particular application.
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*>
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*> - Standard combinators on Booleans are duplicated for the Success type
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*> (e.g., see the library "Constraint" in PAKCS/KiCS2).
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*>
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*> All these problems can be avoided if the type Success is removed.
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*> As a consequence, one can drop "=:=" and make "==" a flexible
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*> operation (currently, "==" suspends if one argument is a free
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*> variable), i.e., one can consider "==" as a standard function
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*> which is evaluated by narrowing. Actually, this is done in the
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*> Curry implementation KiCS2 and it works quite well.
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*>
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*> However, there is one argument to keep "=:=": it could be more
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*> efficient than "==". For instance, the evaluation of
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*>
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*> xs == ys && xs++ys == [True]
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*>
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*> results in an infinite search space (since the first equality
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*> is solved by binding xs and ys to Boolean lists and there are
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*> infinitely many possibilities), whereas the constraint
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*>
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*> xs =:= ys & xs++ys =:= [True]
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*>
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*> has a finite search space (since the first equational constraint
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*> binds xs and ys to the same variable). Fortunately, this operational
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*> aspect can be hidden: if it is known that a Boolean equality *must*
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*> be evaluated to True (and not to False), then one can replace it
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*> by an equational constraint which simply binds one argument,
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*> if it is a free variable, to the other. For instance, if an
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*> equality occurs in a condition of a rule, as in
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*>
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*> last xs | xs==_++[x] = x where x free
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*>
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*> it can be safely transformed into
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*>
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*> last xs | xs===_++[x] = x where x free
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*>
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*> where "===" is the same as "=:=" but returns True (instead
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*> of "success"), i.e., it can be considered as defined by
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*>
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*> (===) :: a -> a -> Bool
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*> x === y | x =:= y = True
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*>
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*> BTW, this transformation can be done at compile time.
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*> PAKCS and KiCS2 contain a transformation phase which
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*> automatically does this, see the paper
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*> http://www.informatik.uni-kiel.de/~mh/papers/LOPSTR15.html
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*> for more technical details.
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*>
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*> The introduction into KiCS2, which can be found at
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*> http://www-ps.informatik.uni-kiel.de/kics2/starting.html
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*> already explains the standard FLP features without a
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*> reference to the type Success. Moreover, this is
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*> implemented since some time in PAKCS and KiCS2
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*> and we didn't get any problems with it. Of course,
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*> for backward compatibility, the type Success and "=:="
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*> should not be immediately removed from the implementations.
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*> Hence, the concrete proposal is as follows:
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*>
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*> 1. Drop references to the type Success, use Bool instead.
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*>
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*> 2. Replace occurrences of "=:=" by "==" and define "=="
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*> as a flexible function based on narrowing.
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*>
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*> 3. Add "===" as above to the prelude but don't use it
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*> explicitly.
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*>
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*> I hope you agree to this simplification.
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*> Of course, I welcome any comments or objections.
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*>
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*> Best regards,
*

*>
*

*> Michael
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*>
*

*>
*

*>
*

*> _______________________________________________
*

*> curry mailing list
*

*> curry_at_lists.RWTH-Aachen.DE
*

*> http://MailMan.RWTH-Aachen.DE/mailman/listinfo/curry
*

Received on Thu Oct 15 2015 - 17:13:42 CEST

Date: Thu, 1 Oct 2015 13:54:44 +0200

I like the proposal,

Herbert

Am 01.10.2015 um 13:45 schrieb Michael Hanus:

Received on Thu Oct 15 2015 - 17:13:42 CEST

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