I do not understand why (1) iterators are specified using a self-updating
API when a functional API would seem preferable, or why (2) generators
are linked to functions, when block-level generators would seem to be
sufficient and less complex. In some more detail:
1. Why do iterators have an imperative API?
As currently specified, an Iterator's next returns a {done,value} pair
(ItrResult), updating the Iterator in place. A functional alternative
would have next return a {done,value,next} triple, leaving the original
Iterator unmodified.
It seems a lot easier to implement the imperative API in terms of
the functional one than the other way round, if one needed both.
In practice, I do not see any advantage to forcing an imperative API.
2. Why are generators linked to functions, and not to blocks?
It is not difficult to implement a form of generators in ES5. Here
is one using TypeScript for classes and arrow functions (it uses
a functional Iterator API, and .then-chained expressions instead
of ;-chained statements):
definition:
https://gist.github.com/clausreinke/5984869#file-monadic-ts-L91-L125
usage examples:
https://gist.github.com/clausreinke/5984869#file-monadic-ts-L492-L529
The main source-level disadvantages of these user-defined
generators wrt built-in generators are:
(a) lack of syntax sugar for .then-chaining
(b) no coverage of statement blocks and their built-in control
structures
(a) would be cheap to fix (monad comprehensions have been
suggested here multiple times), (b) could be fixed by introducing
block-level generators as built-ins.
Block-level generators are shallower than function-level ones
(continuation reaches to the end of the current block),
expression-level generators are even shallower (continuation
reaches to the end of the current .then-chained expression).
This would allow us to introduce shallow generators without
messing with "function" - generators would simply be another
class of object that "function" allows us to write abstraction
over. Composing shallow continuations could be left to user-
level functions, not built-ins, so everything but the block-level
generators would remain (re-)programmable.
Claus
http://clausreinke.github.com/
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