The word "predicate" was used here not in logical terms, but vaguely refers to the term used in (functional) programming. The origin of the term was though largely stemmed from perl. Perl functions map, grep, and sort, for example, receive their first argument a special function, and a list of arguments. In functional programming, these mapping functions (callbacks, not map/grep themselves) are sometimes called predicates.
In Io::Lambda, functions like sleep() and tail() are declared very similarly. They also accept a function and a list of arguments. The only semantic difference is that the label "predicate" has moved away from the callback to the function itself. That possibly questions if the choice of the word itself was appropriate, however, I couldn't find a better one. | [reply] |
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).
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Yes, you're right, even though callback to grep is indeed a predicate in logical sense, the others are not. That's why I'm saying that IO::Lambda use of the word does not correlate with its use neither in logical/mathematical, nor in programming domains. Also, in IO::Lambda not callbacks, but functions accepting these callbacks are called predicates, which furthers the semantic distance between the word and its expected meaning even more. I wish there would be an established term describing such a functionality, but I don't know any. I was forced for devise a word for it, but as much as I dislike making up new words, and I know that it's sometimes acceptable to redefine an existing word, I chose "predicate", which I think gets closest to the definition of "function with perl signature (&), that accepts a callback on perl stack and parameters on the context stack, executes the callback when the event associated with function, and defined by parameter passed on context, is completed". The same concerns are valid for "lambda": there are no one-word terms for "lightweight FSM generator object tied with anonymous subroutine", except possibly "a monad", but people run away screaming from that word :)
In common use, if I'm not mistaken, I'm not a native speaker, predicates can be thought as constructs (words, sentences) answering questions "if?" and sometimes "when?", f.ex. "we'll talk when you come back", also assuming that we won't talk if you won't come back. In this sense, one can think of, say, predicate "read" in the same fashion: the callback will be called when a socket becomes readable, but it won't be called if the socket never becomes readable.
Nevertheless, I'm open to renaming "predicate" into something more appropriate.
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