Thank you, Gianni (and Laurent). I really appreciate you both taking the time to explain the intricacies of Raku/Perl6. Sincerely, Bill.
On Wed, May 6, 2020 at 3:46 AM Gianni Ceccarelli <dak...@thenautilus.net> wrote: > > On 2020-05-06 William Michels via perl6-users <perl6-us...@perl.org> > wrote: > > So if the following code does useless work: > > > > perl6 -pe '.chop' demo1.txt > > > > why doesn't it fail with an error, "Useless use of ... in sink context > > (line 1)"? > > That's a very good question! > > My best attempt at an answer: > > * subroutines that are "pure functions" (i.e. have no side effects) > can be marked with the ``pure`` trait (e.g. ``sub sqrt($) is pure`` > is in the standard library) > * the parser or the optimiser (whoever notices first) will indeed emit > the warning "useless use of ..." when they see that the result of > calling a pure function is sunk > * but we're not calling a function, we're calling a method! > * and (currently) Rakudo can't infer the type of the object we're > using (it could, since the return value of ``IO::CatHandle::lines`` > can be declared, and thus the type of ``$_`` in that loop could be > inferred, but it's hard and hasn't been done yet) > * so Rakudo can't currently know what code that ``.chop`` will end up > calling, and if it has side-effects or not > * so it can't safely warn (it would be like warning that you're not > checking the return value of ``say``) > * yes, there is a sub version of ``chop`` (you can write ``chop($_)`` > instead of ``$_.chop``), but it just delegates to the method, so > without full type inference and data-flow analysis we still can't > know if the method "is pure" > > Hope this helps! > > -- > Dakkar - <Mobilis in mobile> > GPG public key fingerprint = A071 E618 DD2C 5901 9574 > 6FE2 40EA 9883 7519 3F88 > key id = 0x75193F88
