Typestate-oriented programming

[olleharstedt]

Did you guys discuss typestate-oriented programming anything?

Building on article [1], it would allow you to "tag" types and resources, like so:

/** @var File<Open> */
$handle = fopen("/tmp/inputfile.txt", "r");
/** @var string */
$buffer = fgets($handle, 4096));
// This changes $handle to @var File<Closed>
fclose($handle);
// Type error: $handle must have type File<Open>
fgets($handle, 1);

See [2] for more details about this paradigm. It corresponds loosely to what in functional programming is done with linear (or affine) types.

[1] http://marcosh.github.io/post/2020/05/26/phantom-types-in-php.html

[2] https://www.cs.cmu.edu/%7Ejssunshi/pubs/onward2009-state.pdf

[andyg0808]

@olleharstedt I'm wondering if [1] was the intended link, because the article there seems to have more to do with marking interpretations of primitive types, rather than marking states.

[olleharstedt]

@andyg0808 It was intended, since phantom types can simulate typestate in some settings.

[olleharstedt]

Psalm seems to already have typestate related features, as shown here: #1255

Related to array_shift:

/** @var array{string, int} */
$arr = ["foo", 1];
array_shift($arr);  // Modifies the type

Seems like this happens in method handleByRefArrayAdjustment in Psalm? Maybe there's a way to make it possible to define your own functions like this, instead of the hard-coded list?

NB: array_shift is not type-safe if there's a reference to the argument. For that, you'd need borrow pointer logic and all that jazz that Rust uses, or, maybe more inline with Psalm philosophy, just give up if there's a reference detected.

Basically the method AssignmentAnalyzer::assignByRefParam should be programmable by annotations and not depend on any hard-coding nor reference operator (since all objects always are passed as references (no value semantics like arrays)).

[muglug]

Psalm already handles closing file handlers via the resource-closed type: https://psalm.dev/r/2737e2dc70

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/2737e2dc70

<?php

$fp = tmpfile();

if ($fp) {
    fclose($fp);
	fwrite($fp, "hello");
}

Psalm output (using commit e1cc27f):

ERROR: InvalidArgument - 7:9 - Argument 1 of fwrite expects resource, closed-resource provided

[muglug]

I don't plan to add any annotations to change state beyond existing @param-out annotations

[olleharstedt]

I don't plan to add any annotations to change state beyond existing @param-out annotations

@muglug Do you plan to do something with alias tracking? E.g. to fix array_shift in the case where argument has an alias in the same scope.