It also strikes me that rewriting
A AND (B OR C)
as
B C +A
is pretty neat - it eliminates the parentheses OR becomes whitespace and
AND becomes +
It's a Reverse Polish Monad Filter (RPMF)
/Joe
On Thursday, 20 December 2018 10:38:37 UTC+1, joearms wrote:
>
>
>
> On Wednesday, 19 December 2018 23:21:15 UTC+1, Jeremy Ruston wrote:
>>
>> Hi Joe
>>
>> Then a filter that finds A is "[tag[A]]"
>>
>> B OR C is "[tag[B]] [tag[C]]"
>>
>> A AND B is "[tag[A]tag[B]]"
>>
>> A and (B or C) has to be rewritten as
>> (A and B) or (A and C) and is
>>
>> "[tag[A]tag[B]] [tag[A]tag[C]]”
>>
>>
>> You can directly do A and (B or C) as:
>>
>> [tag[B]] [tab[C]] +[tag[A]]
>>
>> There are three runs here:
>>
>> * The first two are processed in the same way: they start with an
>> implicit [all[tiddlers]] to select all “real” tiddlers (ie, non-shadow
>> tiddlers), whether or not they are system tiddelrs. The [tag[x]] filter
>> operator selects those tiddlers that carry the indicated tag. At the end of
>> processing each run, the results of the run are merged into the
>> accumulator, the temporary holding storage for intermediate results.
>>
>> * The third run starts with a “+” which means that instead of kicking off
>> with all the real tiddlers, it instead uses the accumulator as the input to
>> the run. The [tag[A]] then selects those tiddlers from the accumulator that
>> carry the tag “A”.
>>
>> The results of the third run replace the accumulator, and become the
>> results of the entire filter.
>>
>
> Great - IMHO the above explanation is far better than the definitive
> explanation
> https://tiddlywiki.com/#Filter%20Expression
>
> So the algorithm is
>
> 1) create an empty output set (call this accumulator)
> 2) A <whitespace> B means
> starting from the set of all tiddlers add those that match A to
> accumulator
> starting from the set of all tiddlers add those that match B to
> accumulator
> 3) +X means starting from the accumulator
> match all X's and create a NEW accumulator which is passed onto
> the next step
>
> So A B +C where A B and C are "runs" means (in pseudo code)
>
> accumulator = {} // empty set
> accumulator += matchalltags oftype A from all tiddlers
> accumulator += matchalltags oftype B from all tiddlers
> accumulator1 = matchalltags oftype C from accumulator
> return accumulator1
>
>
>> The other filter prefixes that are available are “-“ and “~”. The action
>> of minus is that the results of the run are removed from the accumulator
>> (the input to the run is still the list of all “real” tiddlers). The action
>> of tilde is to only evaluate the run if the accumulator is empty; the
>> results of the run then become the accumulator.
>>
>> Which in a strange way reminds me of horn clauses in Prolog where I'd
>> write:
>>
>> ```
>> filter(a, b) :- ...
>> filter(a, c) :- ..
>> ```
>>
>> So really these filter expressions are equivalent to queries in predicate
>> logic.
>>
>> (which is why they are so powerful :-)
>>
>>
>> Thank you! It’s slightly embarrassing to have to invent something rather
>> than being able to adopt something tried and trusted, so it’s good to hear
>> that it’s not too terrible.
>>
>>
> You've invented the monad :-) - monads just pass an invisible state
> through a pipeline of operations
> that manipulate the state (you call it an accumulator)
>
> Cheers
>
> /Joe
>
>
>
>> Best wishes
>>
>> Jeremy
>>
>>
>> Cheers
>>
>> /Joe
>>
>>
>>
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