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Date: Wed, 15 Jul 2020 18:43:18 +0200
Message-ID: <CADyq6s+_GoMcWWQqoCi9w=v336cVLYiCjojXNrnf_PbGFFkTUw@mail.gmail.com>
To: Larry Garfield <larry@garfieldtech.com>
Cc: php internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC][Discussion] Objects can be declared falsifiable
From: ocramius@gmail.com (Marco Pivetta)

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Hey Larry,

On Wed, Jul 15, 2020 at 5:32 PM Larry Garfield <larry@garfieldtech.com>
wrote:

> 1) return null, which is a non-type, and thus you need to make the return
> type ?User or User|null, which means the caller *must* always check it's
> nullness.
>
> Allowing an object to falsify itself is a 4th,  more type-safe option.  It
> lets you return an object that both fails a boolean check (like null) but
> also has default values in it for the base case.  A user object likely
> wouldn't use that, but a value object like an Address very well could.
>
> Until we can support for-reals monads (which would require enums and
> generics to do properly; the former is possible the latter is very hard),
> it's the best option we have for more type-safe returns.
>

Adding a "falsey" state for code that expects `is_object($foo) === (bool)
$foo` seems to be a massive BC break to me, and reduces type safety of
boolean operators (and checks) by expanding the `object` type and all
operators and expressions that can interact with it.

In general, `?T` is very much equivalent to `Maybe T`

Where `instance Monad Maybe` is implemented in type-safe langs as (quoting
https://en.wikibooks.org/wiki/Haskell/Understanding_monads/Maybe - I
omitted `return` because the lifting bit is not really used in PHP):

 ```hs
    (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b
    (>>=) m g = case m of
                   Nothing -> Nothing
                   Just x  -> g x
```

`?T` is very much the same in PHP (pseudo-code, since nullability is not
represented the same way in Haskell):

```hs
    (>>=) :: ?T -> (T -> Maybe T2) -> ?T2
    (>>=) m g = case m of
                   null -> null
                   x  -> g x
```

Let's not confuse Java nullable issues with PHP nullability, which is quite
healthy :-)

I see adding a "falsey" evaluation to something that has been assumed (for
a looooooong time) as universally "truthy" is a very major issue.

We need code examples where this is a clear advantage over previous logic:
as it stands, I don't see why I would ever prefer `(bool) $object` over
`$object !== null`, or `$object->valid()` (which I believe to be an
anti-pattern)

Marco Pivetta

http://twitter.com/Ocramius

http://ocramius.github.com/

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