picrin/contrib/20.for/docs/doc.rst

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(picrin control list)
---------------------
Monadic list operators.
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.