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To: internals@lists.php.net
Message-ID: <FA63B996-599D-4BF5-9921-8F4237C62058@gmail.com>
Subject: Re: [PHP-DEV] Analysis of property visibility, immutability, and cloning proposals
From: rowan.collins@gmail.com (Rowan Tommins)

On 2 January 2021 21:25:08 GMT+00:00, Larry Garfield <larry@garfieldtech=2E=
com> wrote:
>If a stream is not seekable, then it would have to consume and destroy
>$fp in the process (unset it)=2E  So:
>
>[$line1, $fp2] =3D read_line($fp);
>[$line2, $fp2] =3D read_line($fp);
>
>The second line would throw an error that $fp "has been consumed" or
>something like that=2E  But even that still creates potential for
>spooky-action-at-a-distance if $fp was passed into a function, gets
>read in that function, and then the parent call scope has a broken $fp
>lying around=2E


Yes, that is where "uniqueness attributes" come in: in Clean, that's basic=
ally how I/O looks, but either of those scenarios would produce an error *a=
t compile time*=2E The type system includes the constraint that the file ha=
ndle must not be reachable from anywhere else when passed to the read_line =
function, whether that's use of the same variable after the call, assignmen=
t to an extra variable, capture by some other function, or storage in an ar=
ray or record=2E

The same constraint can be added to custom functions, allowing the compile=
r to reuse the memory for, say, a large array that you're adding an item to=
=2E So you still write the code as though it was immutable, and can reason =
about it that way, but can also prove that it's safe to actually mutate it =
in place=2E

Similar things can be done, in a slightly different way, with Rust's owner=
ship/lifetime system: the "borrow checker" proves that the manipulations yo=
u're doing are free of "action at a distance" by prohibiting anything that =
would create ambiguous "ownership"=2E


Regards,

--=20
Rowan Tommins
[IMSoP]