[rules-dev] exploring functional programming

[Wolfgang Laun]

Well, meaningful examples sure would help :)

(1) On binding: Did I understand this correctly:
A legacy binding denoted by <var> ':' is restricted to the scope of 'for'.
The assignment <var> '=' creates a binding that can be used locally but is
exported (for use in other CEs or on the RHS).

But note that a legacy form binding may not be usable until the end of the
"for", only until the first contracting transformation (see function F
below).


(2) On functions: Examples appear to illustrate (at least) 2 kinds of
functions:
T : Coll<X> -> Coll<X>, a mapping of a collection of type X to the same
domain
F : Coll<X> -> Y, a folding of a collection of type X into a single instance
of type Y (which could be = X)
Possibly you are also thinking of simple functions that map between single
instances:
g : X -> Y

The tricky part here is the transformation type T (such as map()). How will
this work in the case
  map( $, $.price*2)
You can't change the objects in the domain collection. So, how will the
result be built? Will X have to implement Cloneable? What if the field
(here: price) does not have a setter? map applies a function to a list of
the function's domain values, which is very well for *values*, but we're
living in an object-based environment. [Other transformations such as
reverse() or tail() do not imply changes on the components.]

Another example, using a Collection<String> as input; the mapping should
produce the uppercased strings, concatenated:
   for( String( length > 10 ); map( $, $.toUpperCase() ); $c: concatenate( $
) ) # not sure about the last '$'
But what if I want to implement a transformation such as "every 'x_y' to
'xY'"? How would I specify this function so that map can call it? Java
currently does not have functions as objects (and I think the terrible
proposal for closures has been shunted into a very dead end track). Or would
I have to implement each non-trivial map-type function anew, from scratch?
Then map isn't a higher-order function at all!

Finally, permit me to say: If every currently implemented feature were well
documented, both in manuals and javadoc, and readily usable, downloaded
distributions without broken links (e.g. in javadoc) and missing libraries,
and if there were no serious open JIRAs - then, and only then, I'd say that
this is worth the time and the effort. I do beg your pardon if you think
this is coarse or boorish, but these are my sentiments, and they had to come
out, sooner or later.

Cheers
Wolfgang





On 1 November 2010 07:24, Mark Proctor <mproctor at codehaus.org> wrote:

>  Look at your responses i would say ignore the content of the examples,
> they are there just to illustrate the syntax not to show a valid use case or
> scenario. What I'm doing is looking to other languages to see what works
> there and try to bring them across, the CEP type uses cases rely a lot more
> on functional programming capabilities.
>
>
> On 31/10/2010 13:01, Wolfgang Laun wrote:
>
> Hello Mark,
>
> I've been thinking about this, on and off, for 5 days now, and I still
> fail to see the benefits. Perhaps there are more convincing use cases
> than those in your original mail?
>
> Notice that the "for" scope now contains two kinds of bindings: one that
> is strictly local to for(...) (bi below) and another one that must be
> exported
> such as the binding $s in
>
>   for( bi : BasketItem( customer == $c ); $s = sum( $bi.price); )
>
> We can certainly make it:
> $s : sum(....)
>
> My thinking is we still need to create a sytnax to assign the results of
> the expression when it returns a collection, instead of iterating it. For
> this I was thinking of introducting:
> $var = someexpr;
>
> We can't do the following as there isn't enough of a differentiator in the
> syntax between normal patterns.
> $var : someexpr;
>
> But we can do = for assignment of expr, but keep : for bindings of a
> functional result. Just seeing how different things look.
>
>
> This will be confusing.
>
>
> I fail to see the usefulness of filter() in
>
>    for( $b2 : BasketItem( customer == $c ); filter( $b2, $b2.price > 100) )
>
> filter is just a function, like min, max and average, it's not special.
> Functional programming has a number of higher order functions, filter, map
> and fold. There is nothing extra to support that, so it's just added for
> completeness to show we could/can do it.
>
>
> where one can better (and perhaps even more efficiently) write
>
>    for( $b2 : BasketItem( customer == $c, price > 100) )
>
> The filter function was just an example for completeness, it could have
> been any function name.
>
>
> I don't understand what this construct should achieve:
>
>     $r = $bi | map( $, $.price * 2) | filter( $, $.price < 100);
>
> I wouldn't take the example as "best practice"or actually meaningful I was
> just trying to illustrate the syntax.
>
>
> Given that $bi iterates over BasketItem objects, the first map would create
> a numeric value by doubling $bi.price, so I suppose the next pipeline
> transmits
> numbers so that filtering applies to - what? java.lang.Number? Then it
> can't be
> $.price;
>
> The contents is automatically passed into the function. The first argument
> is what the higher order "map" function would store, the second one is the
> modifier. Typically "map" is single argument map( * 2 ), but I can't see how
> that would work for our syntax, especially when passing objects instead of
> values.
>
>
> it could be $.floatValue or similar. But again: the entire selection
> could be written as a constraint of the BaskerItem pattern. Creating a
> collection just for determining a count isn't straightforward.
>
> The idea of accumulate is to produce a scalar result from processing a
> collection of facts.
>
>  It would indeed be useful to extend this so that a result
> tuple can be constructed. But this could be achieved by a complex
> accumulate function that returns an object with multiple fields;
> in fact, min/max/count/avg/sum can be wrapped into a single function
> "stats" returning an object of class "Statistics" with appropriate fields.
> Any filtering on the result can be written as a constraint on that
> temporary fact pattern of type Statistics, e.g.:
>
> Statistics( $min: min, $max: max )
>   from accumulate( $bi: BasketItem( $p: price ) stats( $p ) )
>
> that's something that can be achieved now, but requires plumbing from the
> end user. Multiple functions support would avoid that and is what people
> from other products would expect. The last part of the acc provides the
> propagation constraint, it's sugar to avoid verbosity.
>
>
> All that remains is the syntactic sugar that's provided by
>    x in [0..4]
>    y in [0..4]
>    $c : Cell( row == y, col == x );
> as a on-the-fly pattern for selecting other patterns. Again, this could be
> written more concisely as
>
> Don't take the example as "best practice". It's showing that 'in', which is
> the same as 'from', can look more compact and is what people from a more
> functional world would expect.
>
> i'm thinking of allowing 'from' and 'in' to be used interchangable. Where
> 'from' would be used with "from entry-point" where as 'in' for iterationg
> the results of an expression.
>
>  CEP cases can also need specific ordering, the example below could find
> the cells in any order.
>
>    $c : Cell( row >= 0 && <= 4, col >= 0 && <= 4 )
> and a (minor) extension such as a set expression might simplify this:
>   $c : Cell( row in [0..4], col in [0,1,2,3,4] )
>
> Regards
> Wolfgang
>
> On 26 October 2010 01:36, Mark Proctor <mproctor at codehaus.org> wrote:
>
>> We are thinking of moving "accumulate" to a simple "for" keyword. We
>> might allow 'in' and 'from' to be used interchangably and allow ';' semi
>> colons to separate the sections. I'm also wondering ho we could allow
>> function pipelines for the function part of the element. We also need
>> type inference and "default" return values.
>>
>> So here are some code snippets to show what I'm thinking, as
>> improvements over what we do with accumulate now.
>>
>> // Simple 'or' very simlar to accumulate before. ; for section
>> separating. With a new boolean section at the end to decide whether to
>> propagate or not. Will probably use '=' for function assignments.
>> $c : Customer()
>> for( $bi : BasketItem( customer == $c );
>>         $s = sum( $bi.price);
>>         $s > 100 )
>>
>> // Multiple functions are ofcourse allowed
>> $c : Customer()
>> for( $bi : BasketItem( customer == $c );
>>         $mn =min( $bi.price),
>>         $mx = max( $bi.price); )
>>
>> // As are multiple patterns, and as before patterns can come 'from' an
>> expression and with type inference we can get some nice compact text:
>> for ( x in [0..4]
>>       y in [0..4]
>>       $c : Cell( row == y, col == x );
>>       $avg = avg( $cell.value );
>>       $avg > 100 )
>>
>> The above is possible now with the following:
>> Integer( this > 100) from
>>     accumulate( x : Integer() from [0, 1, 2, 3, 4]
>>                 y : Integer() from [0, 1, 2, 3, 4],
>>                 $c : Cell( row == y, col == x ),
>>                 avg( $c.value) )
>>
>> I think the proposed additions reall help with declarative readability.
>>
>> The normal chaining of elements is supported:
>> $c : Customer()
>> for( $b1 : BasketItem() in
>>           for( $b2 : BasketItem( customer == $c );
>>                  filter( $b2, $b2.price > 100); );
>>         $a = avg( $b1.price ); )
>>
>> 'for' will have a default return type for each bound function. In this
>> case it returns a single value $a, if there are multiple bound results
>> an array/map must be used for access.
>>
>> I also think we should allow pipelineing of functions, much as you would
>> do in a normal functional programming, possibly using haskell like
>> "stream fusion" capable functions.
>>
>> // '$' refers to the magic contents of the function which is "piped" in.
>> So $bi is piped to map, $ refers to each value evaluated in the
>> function, with type inference. 'map' results are piped to 'filter'. The
>> final results are assigned to $r.
>> $c : Customer()
>> for( $bi : BasketItem( customer == $c );
>>         $r = $bi | map( $, $.price * 2) |
>>                        filter( $, $.price < 100);
>>         $r.size > 100 )
>>
>> More ideas welcome :) But I think this opens up some very interesting
>> areas for DRL, with something that will hopefully feel more natural for
>> developers.
>>
>> Mark
>>
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>
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