Composite generators in bind cause type error when shrinking

[Vinai]

This is the simplest example I could come up with that reproduces the error:

$this->forAll(
    Generator\bind(
        Generator\choose(1, 10),
        function ($x) {
            return Generator\tuple($x);
        } 
    )
)->then(function ($tuple) {
    $this->assertTrue(false, var_export($tuple, true));
});

The error is:

Error: Argument 1 passed to Eris\Generator\TupleGenerator::shrink()
must be an instance of Eris\Generator\GeneratedValueSingle,
instance of Eris\Generator\GeneratedValueOptions given,
called in /home/me/project/vendor/giorgiosironi/eris/src/Generator/BindGenerator.php on line 42

It seems to happen with composite generators being returned from inside of a bind generator function.
I've tried with tuple and associative.

In practice a GeneratedValueOptions should work in shrink, too, since the GeneratedValueOptions instance also has a input method, but I'm not sure how the type signature should be fixed.
The \Eris\Generator\TupleGenerator::shrink(GeneratedValueSingle $element) signature can't be changed because it has to match \Eris\Generator::shrink(GeneratedValueSingle $element).

Maybe if the argument type where changed to \Eris\Generator\GeneratedValue instead, and that interface then implemented the input method?

That would be a backward compatibility breaking change, but at the moment I'm not sure how to build generators that are related to other generated values without using composite generators with bind.

[Vinai]

As a workaround (a.k.a. hacky patch), I've added the following to the \Eris\Generator\BindGenerator::shrink method:

        while ($outerGeneratorValue instanceof GeneratedValueOptions) {
            $outerGeneratorValue = $outerGeneratorValue->last();
        }

Now the bind generator work as expected.
This is the complete shrink method:

    public function shrink(GeneratedValueSingle $element)
    {
        list($outerGeneratorValue, $innerGeneratorValue) = $element->input();
        // TODO: shrink also the second generator
        $outerGenerator = call_user_func($this->outerGeneratorFactory, $innerGeneratorValue->unbox());
        while ($outerGeneratorValue instanceof GeneratedValueOptions) {
            $outerGeneratorValue = $outerGeneratorValue->last();
        }
        $shrinkedOuterGeneratorValue = $outerGenerator->shrink($outerGeneratorValue);
        return $this->packageGeneratedValueSingle(
            $shrinkedOuterGeneratorValue,
            $innerGeneratorValue
        );
    }

I have no idea if I broke something else by adding this line... It doesn't seem like the right fix. Just thought I'd add the info here because it might be helpful.

[Vinai]

I've narrowed it down a bit. The error happens on the second shrink. It can be reproduced with the following test (in \Eris\Generator\BindGeneratorTest):

    public function testShrinkBindGeneratorWithCompositeValue()
    {
        $bindGenerator     = new BindGenerator(
            new ChooseGenerator(0, 5),
            function ($n) {
                return new TupleGenerator([$n]);
            }
        );
        $generatedValue    = $bindGenerator->__invoke(10, new RandomRange(new RandSource()));
        $firstShrunkValue  = $bindGenerator->shrink($generatedValue);
        $secondShrunkValue = $bindGenerator->shrink($firstShrunkValue);
        $this->assertInstanceOf('\Eris\Generator\GeneratedValue', $secondShrunkValue);
    }

From what I can see the problem is that according to the return type hint \Eris\Generator::shrink() returns either a GeneratedValueSingle<T> or a GeneratedValueOptions<T>, but the method argument allows onlyGeneratedValueSingle instances.
This breaks shrinking for generators that indeed return GeneratedValueOptions.

[ilario-pierbattista]

I assume that #127 fixes this. Feel free to reopen otherwise.