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authorColin Okay <cbeok@protonmail.com>2020-07-10 08:46:56 -0500
committerColin Okay <cbeok@protonmail.com>2020-07-10 08:46:56 -0500
commit66493a782127ce40ec099e1a822bf80c0d1c9f43 (patch)
tree280eaed737c73f71e520d9d2d55a78a5624cbb85
parenta0af6f14e7a2e0a1389ea872d7d7fc5fb85a4d38 (diff)
Tutorial note about one-time-use generators
-rw-r--r--README.md43
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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