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@@ -276,6 +276,8 @@ is where *consumers* come in. There is one fundamental consumer, a macro, called `for`. (*Triumphant Horns Play*) +Every other consumer in `GTWIWTG` uses `for` under the good. + Here is how it looks when you use it: ``` lisp @@ -340,16 +342,47 @@ Finally, the body is evaluated for each iteration. [Aside: `for` used to be called `iter`, but I didn't want to step on the toes of `series` and `iterate` users :P]. +### Generators are Consumed at Most Once + +Even if you don't think you're "using up" the whole generator, a +generator can only be passed to a single consumer. Once that consumer +finishes, the generator is consumed. Here is an example: + +``` lisp + +>(let ((foo (seq "foobar"))) + (print (take 2 foo)) + (print (collect foo))) + +(#\f #\o) +NIL + +``` + +Even though you only *seemed* to use the first two members of the +generator `foo`, the `take` form will mark the generator as having +been consumed in its entirety. + +That is, even when the whole sequence was not actually generated, a +consuming form leaves its generator in an unusable state. This +approach has been taken in order to automatically close streams for +stream-backed generators - i.e. it has been done in the spirit of +letting you not have to think about how generators work. + +You need only remember the rule: Generators Are Consumed At Most Once. + ### A Word Of Warning! (Or, there's a reason those forms all end in `!`.) -You must be cautious with that third approach in the example above, -i.e. incrementally building up generators. The reason for caution is -that generators cannot be "combined twice". I.e. they are not -functional objects and combining them with one other effectively -"destroys" them as independent objects. +You must be cautious when incrementally building up generators. The +reason for caution is that generators cannot be "combined twice", but +you may be tempted to try doing just that. + +I.e.These generators are not functional objects. Combining them with +one other effectively "destroys" them as independent objects, by +marking them as unfit for use in other combining forms. [Aside: This was done for efficiency reasons, and I might make a "purely functional" parallel universe for these generators in the |