Hmm - the callback to 'grep' is a predicate in the sense used in mathematical logic (a function that takes an element and returns true of false), the others (for map and sort) are not. This is your choice - but I would at least verify in your sources if indeed all the callbacks are called predicates, I googled for the term and I cannot find such a usage of it (for example the Merriam-Webster dictionary definition is very close to the mathematical one).

In reply to Re^8: IO::Lambda: call for participation by zby
in thread IO::Lambda: call for participation by dk

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