The triple of for/in/yield enables you to write a list operation in a very easy and simple code. One of the best examples is list composition::
(for (let ((a (in '(1 2 3)))
(b (in '(2 3 4))))
(yield (+ a b))))
;=> (5 6 7 6 7 8 7 8 9)
All monadic operations are done in *for* macro. In this example, *in* operators choose an element from the given lists, a and b are bound here, then *yielding* the sum of them. Because a and b are values moving around in the list elements, the expression (+ a b) can become every possible result. *yield* operator is a operator that gathers the possibilities into a list, so *for* macro returns a list of 3 * 3 results in total. Since expression inside *for* macro is a normal expression, you can write everything that you can write elsewhere. The code below has perfectly the same effect to above one::
(for (yield (+ (in '(1 2 3))
(in '(4 5 6)))))
The second best exmaple is filtering. In the next case, we show that you can do something depending on the condition of chosen elements::
(for (let ((x (in (iota 10))))
(if (even? x)
(yield x)
(null))))
;=> (0 2 4 6 8)
This expression is equivalent to ``(filter even? (iota 10))`` but it is more procedual and non-magical.
-**(for expr)**
[Macro] Executes expr in a list monad context.
-**(in list)**
Choose a value from list. *in* function must only appear in *for* macro. The delimited continuation from the position of *in* function to the outside *for* macro is executed for each element in list. If list contains no values, that is ``(in '())``, the continuation is discarded.
-**(yield value)**
Yields value from the monad context. The result of *for* will be a list of yielded values.
-**(null . value)**
Returns ``()`` whatever value is given. The identity element of list composition. This operator corresponds to Haskell's fail method of Monad class.
Symbol to Object table. Internally it is implemented on hash-table.
Note that dictionary is not a weak map; if you are going to make a highly memory-consuming program with dictionaries, you should know that dictionaries keep their bound objects and never let them free until you explicitly deletes bindings.
If there is no value already associated with key, this function newly creates a binding of key with obj. Otherwise, updates the existing binding with given obj.
