Bug #131094 for future: then() seems incorrectly documented

Wed Nov 27 06:28:37 2019 felipe [...] felipegasper.com - Ticket created

Subject:	then() seems incorrectly documented
Date:	Wed, 27 Nov 2019 05:43:50 -0500
To:	bug-future [...] rt.cpan.org
From:	Felipe Gasper <felipe [...] felipegasper.com>

Currently the POD says: ----- then (2 arguments) $future = $f1->then( \&done_code, \&fail_code ) The then method can also be passed the $fail_code block as well, giving a combination of then and else behaviour. This operation is designed to be compatible with the semantics of other future systems, such as Javascript's Q or Promises/A libraries. ----- … yet this isn’t how then() works, as a quick example shows: ----- perl -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123 } )->then( sub { print 456 } )' 123 ----- It should print “123456”, but instead it only prints “123”. A quick comparison with node.js’s implementation: ----- Show quoted text

> Promise.resolve(123).then( () => console.log(123) ).then( () => console.log(234) );

Promise { <pending> } Show quoted text

> 123

234 ----- If Future::then() is described as “compatible” with Promise/A, I would think it would obey the same semantics and immediately resolve the returned Future. Given, though, that Future is already widely used, I assume that its current functionality is as intended; hence, the documentation should indicate what Future::then() does differently from then() on promises. Thank you! -FG

Wed Nov 27 08:11:12 2019 leonerd-cpan [...] leonerd.org.uk - Correspondence added

On Wed Nov 27 06:28:37 2019, felipe@felipegasper.com wrote: Show quoted text

> perl -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { > print 123 } )->then( sub { print 456 } )' > 123

Oh this is awkward. That's supposed to print an error that the first `then` block didn't return a Future. Instead nothing happened - the error has gone missing :( In any case, as every chaining code block is supposed to yield a Future, your example didn't do that so it doesn't run properly. Instead: $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123; Future->done } )->then( sub { print 456 } )' 123456 I'll investigate why the error went missing -- Paul Evans

Wed Nov 27 08:11:13 2019 The RT System itself - Status changed from 'new' to 'open'

Wed Nov 27 08:14:00 2019 leonerd-cpan [...] leonerd.org.uk - Correspondence added

On Wed Nov 27 08:11:12 2019, PEVANS wrote: Show quoted text

> On Wed Nov 27 06:28:37 2019, felipe@felipegasper.com wrote:

> > perl -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { > > print 123 } )->then( sub { print 456 } )' > > 123

> > Oh this is awkward. That's supposed to print an error that the first > `then` block didn't return a Future. Instead nothing happened - the > error has gone missing :(

Ohright; it's not thrown as an inline error, but instead becomes the failure state of the returned Future. Since you just discarded that, it doesn't appear. Try ->get'ing it: $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123 } )->then( sub { print 456 } )->get' Expected __ANON__(-e line 1) to return a Future at -e line 1. 123 $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123; Future->done } )->then( sub { print 456 } )->get' Expected __ANON__(-e line 1) to return a Future at -e line 1. 123456 $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123; Future->done } )->then( sub { print 456; Future->done } )->get' 123456 -- Paul Evans

Wed Nov 27 08:49:16 2019 felipe [...] felipegasper.com - Correspondence added

Subject:	Re: [rt.cpan.org #131094] then() seems incorrectly documented
Date:	Wed, 27 Nov 2019 08:49:04 -0500
To:	bug-future [...] rt.cpan.org
From:	Felipe Gasper <felipe [...] felipegasper.com>

Show quoted text

> Le 27 nov. 2019 à 08:14, Paul Evans via RT <bug-future@rt.cpan.org> a écrit : > > Ohright; it's not thrown as an inline error, but instead becomes the failure state of the returned Future. Since you just discarded that, it doesn't appear. Try ->get'ing it: > > $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123 } )->then( sub { print 456 } )->get' > Expected __ANON__(-e line 1) to return a Future at -e line 1. > 123 > > $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123; Future->done } )->then( sub { print 456 } )->get' > Expected __ANON__(-e line 1) to return a Future at -e line 1. > 123456 > > $ perl -Mwarnings -MFuture -e'my $f = Future->done(123); my $f2 = $f->then( sub { print 123; Future->done } )->then( sub { print 456; Future->done } )->get' > 123456

Ah, thank you for clarifying! Mostly what I’m getting at, though, is that this interface appears to work quite differently from then() as promise objects implement it. With promises, the return from the then() callback is given to the next promise down the chain, and only if the then() callback throws/dies is the returned promise a failure. That being the case, should the documentation be updated to describe a bit more fully what Future::then() does? Thank you!

Mon Jan 27 21:32:44 2020 leonerd-cpan [...] leonerd.org.uk - Correspondence added

On Wed Nov 27 08:49:16 2019, felipe@felipegasper.com wrote: Show quoted text

> Mostly what I’m getting at, though, is that this interface appears to > work quite differently from then() as promise objects implement it. > With promises, the return from the then() callback is given to the > next promise down the chain, and only if the then() callback > throws/dies is the returned promise a failure.

But that is what is happening here. $ perl -MFuture -E 'say Future->done("First") ->then(sub { say "Previous result was $_[0]"; Future->done("second") }) ->then(sub { say "The next result was $_[0]"; Future->done }) ->get' Previous result was First The next result was second -- Paul Evans

Tue Jan 28 19:40:34 2020 felipe [...] felipegasper.com - Correspondence added

Subject:	Re: [rt.cpan.org #131094] then() seems incorrectly documented
Date:	Tue, 28 Jan 2020 19:40:24 -0500
To:	bug-future [...] rt.cpan.org
From:	Felipe Gasper <felipe [...] felipegasper.com>

Show quoted text

> On Jan 27, 2020, at 9:32 PM, Paul Evans via RT <bug-future@rt.cpan.org> wrote: > > <URL: https://rt.cpan.org/Ticket/Display.html?id=131094 > > > On Wed Nov 27 08:49:16 2019, felipe@felipegasper.com wrote:

>> Mostly what I’m getting at, though, is that this interface appears to >> work quite differently from then() as promise objects implement it. >> With promises, the return from the then() callback is given to the >> next promise down the chain, and only if the then() callback >> throws/dies is the returned promise a failure.

> > But that is what is happening here. > > $ perl -MFuture -E 'say Future->done("First") > ->then(sub { say "Previous result was $_[0]"; Future->done("second") }) > ->then(sub { say "The next result was $_[0]"; Future->done }) > ->get' > Previous result was First > The next result was second

The *return* from the callback is what promises propagate, though. Future appears to require another Future in order to propagate and discard the callback’s return, which diverges from the usual promise interface: ===== Show quoted text

> node

Welcome to Node.js v12.13.1. Type ".help" for more information. Show quoted text

> Promise.resolve("first").then( v => { console.log(v); return "second" } ).then( v => { console.log(v); } )

Promise { <pending> } Show quoted text

> first

second Show quoted text

> perl -MPromises::Deferred -e'my $d = Promises::Deferred->new(); $d->resolve("first"); $d->promise()->then( sub { print shift; return "second" } )->then( sub { print shift } )'

firstsecond Show quoted text

> perl -MPromise::ES6 -e'Promise::ES6->resolve("first")->then( sub { print shift; return "second" } )->then( sub { print shift } )'

firstsecond Show quoted text

> perl -MFuture -e'Future->done("first")->then( sub { print shift; return "second" } )->then( sub { print shift } )'

Calling ->then in void context at -e line 1. first ===== I’m not proposing that Future be changed, but merely that the documentation clarify this divergence. -FG

Mon Oct 19 13:18:13 2020 leonerd-cpan [...] leonerd.org.uk - Correspondence added

Behaviour was changed in 0.45 to accept non-Future return values which are silently upgraded. -- Paul Evans

Mon Oct 19 13:18:13 2020 leonerd-cpan [...] leonerd.org.uk - Status changed from 'open' to 'resolved'

Mon Oct 19 13:18:13 2020 leonerd-cpan [...] leonerd.org.uk - Fixed in 0.45 added