Item10932: SEARCH's topic param visits each topic only once, even if a topic occurs multiple times

The following SEARCH has a really useful undocumented ... bug? feature?

%SEARCH{"1"
  type="query"
  topic="Item10000,Item10000,Item10000,Item10001"
  format="   * [[$web.$topic][$topic]]"
}%

Test:

• Item10000
• Item10001

As you can see, it visits each topic in the topic param only once, even if it's listed multiple times. This doesn't align with the FORMAT equivalent:

%FORMAT{
  "Item10000,Item10000,Item10000,Item10001"
  type="topic"
  format="   * [[$web.$topic][$topic]]"
}%

Test:

• Item10000
• Item10000
• Item10000
• Item10001

PaulAlexander has therefore started exploiting this behaviour for cases where we need to process each member of a list only once, even if a member occurs more than once in that list.

The example use-case is a SEARCH that generates this list of topics into another SEARCH's topic param (thus unique-ifying it). The inner SEARCH is finding a list of all slvalues[property='Cite'].value in a set of topics - the result is we can build a list of bibliographic references in use across a set of topics (and have each reference occur only once in that list).

So the question is:

• Document the behaviour, and fix FORMAT to behave the same
• Fix the behaviour, make SEARCH behave the same as FORMAT
  • In this case, what mechanism can we use to reliably unique a list? CALC's rendering behaviour is too crazy for me to bother with I'm afraid.

-- PaulHarvey - 29 Jun 2011

Sorry to set this WaitingFor the SEARCH gurus again, but I guess this affects ResultSets or something complicated like that...

-- PaulHarvey - 30 Jun 2011

On IRC Sven seemed indifferent, but favoured making the behaviour align with whatever an older T/Foswiki does.

-- PaulHarvey - 30 Jun 2011

Our very own Foswiki 1.0.9 on foswiki.org (which I'd forgotten about) shows us that SEARCH's treatment of the topic param is consistent across Foswiki 1.0, Foswiki 1.1, and trunk

-- PaulHarvey - 30 Jun 2011

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