Re: Re: val initialization semantic is counterintuitive
martin odersky <[email protected]> Sun, 11 Sep 2016 11:42:45 +0200
| Newsgroups | gmane.comp.lang.scala |
|---|---|
| Message-ID | <CAENVNkaDn4X6FZx0+4VUf2m8d9ebq4rT7OZV_GckuUC0P=RCEw@mail.gmail.com> |
--001a11469152dcbffe053c38348d Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Sun, Sep 11, 2016 at 10:53 AM, S=C3=A9bastien Doeraene <sjrdoeraene@gmai= l.com> wrote: > Hello, > > Fun fact: we even have a test in Scala.js that makes sure that it behaves > like that even for Scala.js-defined JS classes ^^ > https://github.com/scala-js/scala-js/blob/v0.6.12/test- > suite/js/src/test/scala/org/scalajs/testsuite/jsinterop/ > ScalaJSDefinedTest.scala#L359-L367 > > Btw, indeed, this is one of the 5 warnings that we get when compiling our > test suite (not counting deprecation warnings): > > [warn] .../ScalaJSDefinedTest.scala:361: Reference to uninitialized value > y > [warn] val x =3D y > [warn] ^ > > Was that a real failure, or just a test to exercise this specifically? Cheers - Martin > Cheers, > S=C3=A9bastien > > On Sun, Sep 11, 2016 at 10:37 AM, Adriaan Moors <[email protected]> > wrote: > >> Val initialization is confusing, I agree, but note that you do get a >> warning. I'm a bit concerned about making this an error as you could >> override `val x` in a subclass (it's still weird, but should it be an >> error?). >> >> Welcome to Scala 2.12.0-RC1 (Java HotSpot(TM) 64-Bit Server VM, Java >> 1.8.0_102). >> Type in expressions for evaluation. Or try :help. >> >> scala> class T { >> | val x =3D y // compiles OK, x =3D 0 >> | val y =3D 5 >> | } >> *<console>:12: warning: Reference to uninitialized value y* >> val x =3D y // compiles OK, x =3D 0 >> ^ >> >> On Sat, Sep 10, 2016 at 10:44 PM martin odersky <[email protected]> >> wrote: >> >>> I think that's a suggestion worth considering! In general it is >>> extremely hard to detect uninitialized fields statically. But a scheme = of >>> disallowing direct forward references is easy to do and catches the mos= t >>> obvious bugs. >>> >>> Cheers >>> >>> - Martin >>> >>> >>> >>> On Sat, Sep 10, 2016 at 1:42 AM, Sofoklis Papasofokli < >>> [email protected]> wrote: >>> >>>> Hi, >>>> >>>> Strange that nobody commented on this one for so long. >>>> >>>> I know its an old thread but i keep getting issues from this, I also >>>> believe there should be a compiler error in this situation, its totall= y >>>> unexpected behavior. >>>> >>>> Best Regards, >>>> Sofoklis >>>> >>>> On Tuesday, March 22, 2011 at 5:14:16 PM UTC+2, Eugen Labun wrote: >>>>> >>>>> Hi all, >>>>> >>>>> eventually (or even surely) this is an old question but I haven't >>>>> found an explanation for this >>>>> simple situation (nor in Language Specification, nor in Scala-Book). >>>>> >>>>> (More complicated cases are described in this excellent faq from Paul= : >>>>> https://github.com/paulp/scala-faq/wiki/Initialization-Order) >>>>> >>>>> >>>>> Constructions like >>>>> >>>>> def m { >>>>> val x =3D y // compile error "forward reference extends over >>>>> definition of value x" >>>>> val y =3D 5 >>>>> } >>>>> >>>>> cause a compile error if used in methods, >>>>> but are accepted in constructors/initializers: >>>>> >>>>> class/object T { >>>>> val x =3D y // compiles OK, x =3D 0 >>>>> val y =3D 5 >>>>> } >>>>> >>>>> The 'x' in the code above gets initialized to 0 (!), not to 5. >>>>> >>>>> >>>>> Both -- acceptance by the compiler and initialization to 0 -- are >>>>> counterintuitive to me. >>>>> Would a compile error for the second case ("illegal forward >>>>> reference") not be a more preferable? >>>>> >>>>> >>>>> I'm aware of '-Xcheckinit' compiler option. But this introduces only >>>>> *runtime* check and doesn't >>>>> prevent the code to compile. >>>>> >>>>> >>>>> There was a ticket https://lampsvn.epfl.ch/trac/scala/ticket/399, but >>>>> it's closed. I cannot >>>>> understand the reason of closing (it cites one more example of such >>>>> counterintuitive behavior but >>>>> doesn't explain why this behavior can't be changed). >>>>> >>>>> >>>>> Java handles a semantically analogous situation as expected: >>>>> >>>>> class C { >>>>> final int a =3D 3; >>>>> { >>>>> System.out.println("a: " + a); // OK >>>>> System.out.println("y: " + y); // Error: illegal forward >>>>> reference >>>>> } >>>>> final int x =3D y; // Error: illegal forward reference >>>>> final int y =3D 5; >>>>> >>>>> public static void main(String[] args) { >>>>> // final int x =3D y; // Error: cannot find symbol variable y >>>>> final int y =3D 5; >>>>> } >>>>> } >>>>> >>>>> >>>>> I understand that Scala vals do not map 1:1 to Java final variables, >>>>> and that generating bytecode >>>>> from Scala code is much more complicated (having in mind such things >>>>> as unified access principle and >>>>> therefore introducing methods for vals/vars, mapping of Scala's >>>>> primary constructor to constructors >>>>> and initializers in Java, inheritance and overriding val->def->var, >>>>> ...) >>>>> >>>>> But, despite how complicated can be the generated code, hopefully, >>>>> might it be possible to detect >>>>> such forward references in Scala parser and generate an error? >>>>> >>>>> >>>>> -- >>>>> Regards, >>>>> Eugen >>>>> >>>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "scala-language" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> For more options, visit https://groups.google.com/d/optout. >>>> >>> >>> >>> >>> -- >>> >>> Martin Odersky >>> EPFL and Lightbend >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "scala-language" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >>> >> -- >> You received this message because you are subscribed to the Google Group= s >> "scala-language" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n >> email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. >> > > -- > You received this message because you are subscribed to the Google Groups > "scala-language" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > --=20 Martin Odersky EPFL and Lightbend --=20 You received this message because you are subscribed to the Google Groups "= scala-language" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to [email protected]. For more options, visit https://groups.google.com/d/optout. --001a11469152dcbffe053c38348d Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable <div dir=3D"ltr"><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">= On Sun, Sep 11, 2016 at 10:53 AM, S=C3=A9bastien Doeraene <span dir=3D"ltr"= ><<a href=3D"mailto:[email protected]" target=3D"_blank">sjrdoeraene= @gmail.com</a>></span> wrote:<br><blockquote class=3D"gmail_quote" style= =3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir= =3D"ltr"><div><div><div><div>Hello,<br><br></div>Fun fact: we even have a t= est in Scala.js that makes sure that it behaves like that even for Scala.js= -defined JS classes ^^<br><a href=3D"https://github.com/scala-js/scala-js/b= lob/v0.6.12/test-suite/js/src/test/scala/org/scalajs/testsuite/jsinterop/Sc= alaJSDefinedTest.scala#L359-L367" target=3D"_blank">https://github.com/scal= a-js/<wbr>scala-js/blob/v0.6.12/test-<wbr>suite/js/src/test/scala/org/<wbr>= scalajs/testsuite/jsinterop/<wbr>ScalaJSDefinedTest.scala#L359-<wbr>L367</a= ><br></div><br>Btw, indeed, this is one of the 5 warnings that we get when = compiling our test suite (not counting deprecation warnings):<br><br><span = style=3D"font-family:monospace,monospace">[warn] .../ScalaJSDefinedTest.sca= la:<wbr>361: Reference to uninitialized value y<br>[warn]=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0 val x =3D y<br>[warn]=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ^</span><br><br></div><= /div></div></blockquote><div>Was that a real failure, or just a test to exe= rcise this specifically?=C2=A0</div><div><br></div><div>Cheers</div><div><b= r></div><div>=C2=A0- Martin</div><div><br></div><div><br></div><div>=C2=A0<= /div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-le= ft:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div><div></div>Cheers= ,<br></div>S=C3=A9bastien<br></div><div class=3D"HOEnZb"><div class=3D"h5">= <div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On Sun, Sep 11, 2= 016 at 10:37 AM, Adriaan Moors <span dir=3D"ltr"><<a href=3D"mailto:adri= [email protected]" target=3D"_blank">[email protected]</a>></span> w= rote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;borde= r-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div>Val initializ= ation is confusing, I agree, but note that you do get a warning. I'm a = bit concerned about making this an error as you could override `val x` in a= subclass (it's still weird, but should it be an error?).</div><div><fo= nt face=3D"monospace"><br></font></div><div><font face=3D"monospace">Welcom= e to Scala 2.12.0-RC1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).<= /font></div><div><font face=3D"monospace">Type in expressions for evaluatio= n. Or try :help.</font></div><div><font face=3D"monospace"><br></font></div= ><div><font face=3D"monospace">scala> class T {</font></div><div><font f= ace=3D"monospace">=C2=A0 =C2=A0 =C2=A0| =C2=A0 val x =3D y // compiles OK, = x =3D 0</font></div><div><font face=3D"monospace">=C2=A0 =C2=A0 =C2=A0| =C2= =A0 val y =3D 5</font></div><div><font face=3D"monospace">=C2=A0 =C2=A0 =C2= =A0| }</font></div><div><font face=3D"monospace"><b><console>:12: war= ning: Reference to uninitialized value y</b></font></div><div><font face=3D= "monospace">=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0val x =3D y // compiles OK, x= =3D 0</font></div><div><font face=3D"monospace">=C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0^</font></div></div><br><div class=3D= "gmail_quote"><div dir=3D"ltr">On Sat, Sep 10, 2016 at 10:44 PM martin oder= sky <<a href=3D"mailto:[email protected]" target=3D"_blank">odersky@gmai= l.com</a>> wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"ma= rgin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"lt= r">I think that's a suggestion worth considering! In general it is extr= emely hard to detect uninitialized fields statically. But a scheme of disal= lowing direct forward references is easy to do and catches the most obvious= bugs.=C2=A0<div><br></div><div>Cheers</div><div><br></div><div>=C2=A0- Mar= tin<br><div><br></div><div><br></div></div></div><div class=3D"gmail_extra"= ></div><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">On Sat, Se= p 10, 2016 at 1:42 AM, Sofoklis Papasofokli <span dir=3D"ltr"><<a href= =3D"mailto:[email protected]" target=3D"_blank">[email protected]</a>= ></span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0= 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir=3D"ltr">Hi,<b= r><br>Strange that nobody commented on this one for so long. <br><br>I know= its an old thread but i keep getting issues from this, I also believe ther= e should be a compiler error in this situation, its totally unexpected beha= vior.<br><br>Best Regards,<br>Sofoklis<br><br>On Tuesday, March 22, 2011 at= 5:14:16 PM UTC+2, Eugen Labun wrote:<blockquote class=3D"gmail_quote" styl= e=3D"margin:0;margin-left:0.8ex;border-left:1px #ccc solid;padding-left:1ex= ">Hi all,<p>eventually (or even surely) this is an old question but I haven= 't found an explanation for this<br>simple situation (nor in Language S= pecification, nor in Scala-Book).</p><p>(More complicated cases are describ= ed in this excellent faq from Paul:<br><a href=3D"https://github.com/paulp/= scala-faq/wiki/Initialization-Order" rel=3D"nofollow" target=3D"_blank">htt= ps://github.com/paulp/scala<wbr>-faq/wiki/Initialization-Order</a><wbr>)</p= ><p><br>Constructions like</p><p>=C2=A0 def m {<br>=C2=A0 =C2=A0 val x =3D = y // compile error "forward reference extends over definition of value= x"<br>=C2=A0 =C2=A0 val y =3D 5<br>=C2=A0 }</p><p>cause a compile err= or if used in methods,<br>but are accepted in constructors/initializers:</p= ><p>=C2=A0 class/object T {<br>=C2=A0 =C2=A0 val x =3D y // compiles OK, x = =3D 0<br>=C2=A0 =C2=A0 val y =3D 5<br>=C2=A0 }</p><p>The 'x' in the= code above gets initialized to 0 (!), not to 5.</p><p><br>Both -- acceptan= ce by the compiler and initialization to 0 -- are counterintuitive to me.<b= r>Would a compile error for the second case ("illegal forward referenc= e") not be a more preferable?</p><p><br>I'm aware of '-Xchecki= nit' compiler option. But this introduces only *runtime* check and does= n't<br>prevent the code to compile.</p><p><br>There was a ticket <a hre= f=3D"https://lampsvn.epfl.ch/trac/scala/ticket/399" rel=3D"nofollow" target= =3D"_blank">https://lampsvn.epfl.ch/trac/s<wbr>cala/ticket/399</a>, but it&= #39;s closed. I cannot<br>understand the reason of closing (it cites one mo= re example of such counterintuitive behavior but<br>doesn't explain why= this behavior can't be changed).</p><p><br>Java handles a semantically= analogous situation as expected:</p><p>=C2=A0 class C {<br>=C2=A0 =C2=A0 f= inal int a =3D 3;<br>=C2=A0 =C2=A0 {<br>=C2=A0 =C2=A0 =C2=A0 System.out.pri= ntln("a: " + a); // OK<br>=C2=A0 =C2=A0 =C2=A0 System.out.println= ("y: " + y); // Error: illegal forward reference<br>=C2=A0 =C2=A0= }<br>=C2=A0 =C2=A0 final int x =3D y; =C2=A0// Error: illegal forward refe= rence<br>=C2=A0 =C2=A0 final int y =3D 5;</p><p>=C2=A0 =C2=A0 public static= void main(String[] args) {<br>=C2=A0 =C2=A0 =C2=A0 // final int x =3D y; /= / Error: cannot find symbol variable y<br>=C2=A0 =C2=A0 =C2=A0 final int y = =3D 5;<br>=C2=A0 =C2=A0 }<br>=C2=A0 }</p><p><br>I understand that Scala val= s do not map 1:1 to Java final variables, and that generating bytecode<br>f= rom Scala code is much more complicated (having in mind such things as unif= ied access principle and<br>therefore introducing methods for vals/vars, ma= pping of Scala's primary constructor to constructors<br>and initializer= s in Java, inheritance and overriding val->def->var, ...)</p><p>But, = despite how complicated can be the generated code, hopefully, might it be p= ossible to detect<br>such forward references in Scala parser and generate a= n error?</p><p><br><span><font color=3D"#888888">--<br>Regards,<br>Eugen<br= ></font></span></p><span><font color=3D"#888888"><p></p><p></p><p></p><p></= p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p></p><p>= </p></font></span></blockquote></div><span><font color=3D"#888888"> <p></p> -- <br> You received this message because you are subscribed to the Google Groups &= quot;scala-language" group.<br> To unsubscribe from this group and stop receiving emails from it, send an e= mail to <a href=3D"mailto:[email protected]" targ= et=3D"_blank">scala-language+unsubscribe@goo<wbr>glegroups.com</a>.<br> For more options, visit <a href=3D"https://groups.google.com/d/optout" targ= et=3D"_blank">https://groups.google.com/d/op<wbr>tout</a>.<br> </font></span></blockquote></div><br><br clear=3D"all"><div><br></div></div= ><div class=3D"gmail_extra">-- <br><div data-smartmail=3D"gmail_signature">= <br>Martin Odersky<br>EPFL and Lightbend</div><span><font color=3D"#888888"= > </font></span></div><span><font color=3D"#888888"> <p></p> -- <br> You received this message because you are subscribed to the Google Groups &= quot;scala-language" group.<br> To unsubscribe from this group and stop receiving emails from it, send an e= mail to <a href=3D"mailto:[email protected]" targ= et=3D"_blank">scala-language+unsubscribe@goo<wbr>glegroups.com</a>.<br> For more options, visit <a href=3D"https://groups.google.com/d/optout" targ= et=3D"_blank">https://groups.google.com/d/op<wbr>tout</a>.<br> </font></span></blockquote></div><span><font color=3D"#888888"> <p></p> -- <br> You received this message because you are subscribed to the Google Groups &= quot;scala-language" group.<br> To unsubscribe from this group and stop receiving emails from it, send an e= mail to <a href=3D"mailto:[email protected]" targ= et=3D"_blank">scala-language+unsubscribe@goo<wbr>glegroups.com</a>.<br> For more options, visit <a href=3D"https://groups.google.com/d/optout" targ= et=3D"_blank">https://groups.google.com/d/op<wbr>tout</a>.<br> </font></span></blockquote></div><br></div> <p></p> -- <br> You received this message because you are subscribed to the Google Groups &= quot;scala-language" group.<br> To unsubscribe from this group and stop receiving emails from it, send an e= mail to <a href=3D"mailto:[email protected]" targ= et=3D"_blank">scala-language+unsubscribe@<wbr>googlegroups.com</a>.<br> For more options, visit <a href=3D"https://groups.google.com/d/optout" targ= et=3D"_blank">https://groups.google.com/d/<wbr>optout</a>.<br> </div></div></blockquote></div><br><br clear=3D"all"><div><br></div>-- <br>= <div class=3D"gmail_signature" data-smartmail=3D"gmail_signature"><br>Marti= n Odersky<br>EPFL and Lightbend</div> </div></div> <p></p> -- <br /> You received this message because you are subscribed to the Google Groups &= quot;scala-language" group.<br /> To unsubscribe from this group and stop receiving emails from it, send an e= mail to <a href=3D"mailto:[email protected]">scal= [email protected]</a>.<br /> For more options, visit <a href=3D"https://groups.google.com/d/optout">http= s://groups.google.com/d/optout</a>.<br /> --001a11469152dcbffe053c38348d--