Inconsistency with CoffeeScript

[Artazor]

CoffeeScript's decision to infer generatorness from yield yields to orthogonality violation.
The following js snippet is semantically homogenous and correct:

var so = require('so');
var fs = require('then-fs');

var case1 = so(function*(){
    var value = 'Immediate';
    return value;
});

var case2 = so(function*(){
    var value = yield fs.readFile('somefile.txt', 'utf8');
    return value;
});

var combined = so(function*(){
    var a = yield case1();
    var b = yield case2();
    return {a:a, b:b};
});

var main = so(function*(){
    console.log(yield combined());
});

main().done();

This is achieved because so has signature described as follows:

type PromiseGen a = a -> [Promise] -- consider generators that yields Promises
so::PromiseGen a -> (a -> Promise) -- convert generator to the function that returns a Promise
-- Here monadic nature of Promise is omitted for simplicity

In contrast, the following CoffeScript counterpart will fail without changing the semantics of so()

so = require 'so'
fs = require 'then-fs'

case1 = so -> 'Immediate' # fails since argument is not a GeneratorFunction
case2 = so -> yield fs.readFile 'somefile.txt', 'utf8'

combined = so -> 
    a = yield case1()
    b = yield case2()
    {a, b}

main = so -> console.log yield combined()

main().done();

Generators without yield are strange in the case of generating sequences (i.e. using generators as intended), but are pretty valid corner case for asynchronous flow control based on generators.

So if CoffeeScript will release it's proposed semantics we should consider implementing so() argument check to be a GeneratorFunction and change it's semantics if it is not.

Would appreciate any thoughts from @ForbesLindesay, @jashkenas, @Nami-Doc

[vendethiel]

Repeating what has been said -- yield return is a way to have a yield without it being effective

[Artazor]

Thank you, for the reply! Yes yield return is the solution of the problem.

[Artazor]

So the final solution would look like:

so = require 'so'
fs = require 'then-fs'

case1 = so -> yield return 'Immediate' # solution!
case2 = so -> yield fs.readFile 'somefile.txt', 'utf8'

combined = so -> 
    a = yield case1()
    b = yield case2()
    {a, b}

main = so -> console.log yield combined()

main().done();