Lambdas do not support complex return types

[vshih]

<?php
require 'mustache.php-2.14.2/src/Mustache/Autoloader.php';
Mustache_Autoloader::register();
$m = new Mustache_Engine;

echo $m->render('Hi, {{abc.def}}', ['abc' => function () { return ['def' => 12345]; }]), "\n";

Outputs: Hi,

But I expected: Hi, 12345

[bobthecow]

This is on purpose.

Lambdas don't make sense in the middle of dot notation. If you check the section for dot notation in the Variable Resolution wiki page, you'll see that a lambda in the first part of a dot notation will never render as anything but an empty string. Once the abc part of your tag is resolved, it doesn't invoke the lambda, it checks the lambda itself for a property or method called def.

Given this, the only "valid" things you could have in dot notation after abc are bindTo or call (the two methods on a Closure instance) and those wouldn't even be valid because they wouldn't be called with valid arguments. In fact, checking for methods on Closures is explicitly skipped because it doesn't make sense to resolve variables inside them.

I think the issue might be that you misunderstand what lambdas are, in a Mustache context. They're not a getter, or a way of lazily evaluating code, they're a way of customizing the render of a block (see the docs on Lambdas).

Here's the non-dot-notation equivalent of your example:

$data = [
  'abc' => function () {
    return ['def' => 12345];
  },
];

echo $m->render('Hi, {{# abc }}{{ def }}{{/ abc }}', $data), "\n";

If you run that, it'll give you a hint as to what's going wrong:

PHP Warning:  Array to string conversion in .../NoopCache.php(45) : eval()'d code on line 25
Hi, Array

In other words, it's not traversing into the value you returned, it's trying to echo it. This is because the non-dot-notation form of your template is equivalent to something like this:

echo 'Hi, ', $data['abc']('{{ def }}'), "\n";

… which does the exact same thing, with the same warning.

Knowing what it's doing, we could modify your example to render the right thing:

$data = [
  'abc' => function ($tpl) use ($m) {
    return $m->render($tpl, ['def' => 12345]);
  },
];

echo $m->render('Hi, {{# abc }}{{ def }}{{/ abc }}', $data), "\n";

// or echo 'Hi, ', $data['abc']('{{ def }}'), "\n";

Hi, 12345

… but that's probably not very helpful :)

[vshih]

Thanks for the response.

Well I'm not dreaming up this case, it's mentioned in https://mustache.github.io/mustache.5.html:

Template:

* {{time.hour}}
* {{today}}

Hash:

{
  "year": 1970,
  "month": 1,
  "day": 1,
  "time": function() {
    return {
      "hour": 0,
      "minute": 0,
      "second": 0
    }
  },
  ...

Output:

* 0
* 1970-1-1

[bobthecow]

okay ignore my big long answer. i was thinking just about the section lambda spec, not the interpolation spec. lemme poke at this and get back to you :)

[bobthecow]

Okay so. This is a bug in mustache.php's dotted name resolution. It works as intended with lambdas and non-dotted names, but that doesn't really help when you're trying to access a property returned from the lambda.

And despite clearly being mentioned in the description of lambdas in documentation, dotted name resolution is not actually in the lambdas spec 🙃 I'll get a pull request to add it to the lambdas spec, but in the meantime I can fix Mustache.php's implementation.

[vshih]

Excellent!

[bobthecow]

Okay, this should be fixed in the fix/dotted-lambdas branch. Do you mind confirming with your use case?

[vshih]

Works great so far! I'll continue to test.

[vshih]

Okay, here's another case.

<?php
require 'src/Mustache/Autoloader.php';
Mustache_Autoloader::register();
$m = new Mustache_Engine([
    'pragmas' => [Mustache_Engine::PRAGMA_FILTERS],
]);

$m->addHelper('first_name', function ($array) {
    return $array[0]->name;
});

$names = [
    (object) ['name' => 'Albert'],
    (object) ['name' => 'Betty'],
    (object) ['name' => 'Charles'],
];

echo $m->render(
    "Non-lambda:  {{non_lambda | first_name}}\n"
    . "Lambda:  {{lambda | first_name}}\n",
    [
        'non_lambda' => $names,
        'lambda' => function () use ($names) {
            return $names;
        },
    ]
);

Output:

PHP Warning:  Array to string conversion in src/Mustache/Template.php on line 174

Warning: Array to string conversion in src/Mustache/Template.php on line 174
PHP Warning:  Attempt to read property "name" on string in test.php on line 9

Warning: Attempt to read property "name" on string in test.php on line 9
Non-lambda:  Albert
Lambda:  

The non-Lambda case seems to work fine, but the same data returned via lambda fails.

[vshih]

Maybe this? https://github.com/bobthecow/mustache.php/pull/416

[vshih]

Here's another similar one:

<?php
require 'src/Mustache/Autoloader.php';
Mustache_Autoloader::register();
$m = new Mustache_Engine([
    'pragmas' => [Mustache_Engine::PRAGMA_FILTERS],
]);

$m->addHelper('first_name', function ($array) {
    return $array[0]->name;
});

$names = [
    (object) ['name' => 'Albert'],
    (object) ['name' => 'Betty'],
    (object) ['name' => 'Charles'],
];

echo $m->render(
    "Non-lambda names:\n"
    . "{{#non_lambda}} {{name}}\n{{/non_lambda}}"
    . "Non-lambda names:\n"
    . "{{#lambda}} {{name}}\n{{/lambda}}",
    [
        'non_lambda' => $names,
        'lambda' => function () use ($names) {
            return $names;
        },
    ]
);

Output:

PHP Warning:  Array to string conversion in src/Mustache/Cache/NoopCache.php(45) : eval()'d code on line 31

Warning: Array to string conversion in src/Mustache/Cache/NoopCache.php(45) : eval()'d code on line 31
Non-lambda names:
 Albert
 Betty
 Charles
Non-lambda names:
Array%

[bobthecow]

I've got a fix for that one in https://github.com/bobthecow/mustache.php/tree/fix/dotted-lambdas

[vshih]

Great!

Okay I think I have one more case for you. It's this test case in testLambdaFilters:

            array('{{% FILTERS }}{{# people | first_person }}{{ name }}{{/ people }}', $data, 'Albert'),

But where {{people}} is a lambda which returns $people. If I modify the test data like that, I get:

1) Mustache_Test_FiveThree_Functional_FiltersTest::testLambdaFilters with data set #3 ('{{% FILTERS }}{{# people | fi...ple }}', array(Closure Object (...), Closure Object (...), Closure Object (...), Closure Object (...), Closure Object (...)), 'Albert')
Error: Cannot use object of type Closure as array

[bobthecow]

Thanks! This should be resolved in the branch now.

[bobthecow]

opened a PR to make it a bit easier to reason about :)

[vshih]

Latest works for me. Looks great!

[vshih]

You think this will make it to the next release, and when that might be?

[bobthecow]

It'll be in 2.15.0. As to when that'll be … the last minor version release was December 2021 :P

[bobthecow]

I wanted to also get the bump to spec v1.4.x in that release, but I haven't had the chance to implement that yet.

[vshih]

All righty, thanks for the work.