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digitalmars.D - Should this be flagged as a warning?

reply "Bernard Helyer" <b.helyer gmail.com> writes:
I tried to post this last night, but the NG wasn't having any of 
it.

I found myself writing a bug that looked like this

     match(ts, TokenType.Is);
     match(ts, TokenType.OpenParen);
     isExp.type == parseType(ts);

The bug being of course, that a type is parsed and ts is modified,
so the expression has side effects so it's not flagged as a 
useless
expression. But the comparison still has no effect, so should 
this be
flagged by DMD?

-Bernard.
Sep 25 2012
next sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Bernard Helyer:


     match(ts, TokenType.Is);
     match(ts, TokenType.OpenParen);
     isExp.type == parseType(ts);

 The bug being of course, that a type is parsed and ts is 
 modified, so the expression has side effects so it's not
 flagged as a useless expression. But the comparison still
 has no effect, so should this be flagged by DMD?



The top level operation is the ==, that is a pure expression, so 
maybe dmd should warn on this.

Bye,
bearophile
Sep 25 2012
prev sibling next sibling parent "Bernard Helyer" <b.helyer gmail.com> writes:
On Tuesday, 25 September 2012 at 19:29:26 UTC, Bernard Helyer 
wrote:

 I tried to post this last night, but the NG wasn't having any 
 of it.

 I found myself writing a bug that looked like this

     match(ts, TokenType.Is);
     match(ts, TokenType.OpenParen);
     isExp.type == parseType(ts);

 The bug being of course, that a type is parsed and ts is 
 modified,
 so the expression has side effects so it's not flagged as a 
 useless
 expression.



Err, the bug is that I wrote '==' instead of '='. That's what
I get for posting first thing in the morning, I guess.
Sep 25 2012
prev sibling next sibling parent "Bernard Helyer" <b.helyer gmail.com> writes:
On Tuesday, 25 September 2012 at 21:15:24 UTC, bearophile wrote:

 The top level operation is the ==, that is a pure expression, 
 so maybe dmd should warn on this.



Yeah, I thought so but I'm not sure. Hence me asking.
Sep 25 2012
prev sibling next sibling parent reply Don Clugston <dac nospam.com> writes:
On 25/09/12 21:30, Bernard Helyer wrote:

 I tried to post this last night, but the NG wasn't having any of it.

 I found myself writing a bug that looked like this

      match(ts, TokenType.Is);
      match(ts, TokenType.OpenParen);
      isExp.type == parseType(ts);

 The bug being of course, that a type is parsed and ts is modified,
 so the expression has side effects so it's not flagged as a useless
 expression. But the comparison still has no effect, so should this be
 flagged by DMD?

 -Bernard.




The "must have an effect" rule only applies to statements, not 
expressions, so this is according to the spec. It's not a bug.

This is a bit like the more extreme case I recently posted about:

int x, y, z;
x == y, ++z;

doesn't generate an error message even though x == y has no 
side-effects, because comma is an expression, not a statement.

IMHO, every expression should be required to have an effect. For example
foo() + foo();
shouldn't compile, unless + is an overloaded operator with side-effects.

It would be really interesting to add this check, and then compile 
existing code (such as Phobos) to see if it breaks valid code, or 
reveals bugs.
Sep 26 2012
next sibling parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 09/26/2012 11:45 AM, Don Clugston wrote:

 On 25/09/12 21:30, Bernard Helyer wrote:

 I tried to post this last night, but the NG wasn't having any of it.

 I found myself writing a bug that looked like this

      match(ts, TokenType.Is);
      match(ts, TokenType.OpenParen);
      isExp.type == parseType(ts);

 The bug being of course, that a type is parsed and ts is modified,
 so the expression has side effects so it's not flagged as a useless
 expression. But the comparison still has no effect, so should this be
 flagged by DMD?

 -Bernard.




 The "must have an effect" rule only applies to statements, not
 expressions, so this is according to the spec. It's not a bug.

 This is a bit like the more extreme case I recently posted about:

 int x, y, z;
 x == y, ++z;

 doesn't generate an error message even though x == y has no
 side-effects, because comma is an expression, not a statement.

 IMHO, every expression should be required to have an effect.



I'd rather restrict this to expressions whose value is unused.
Otherwise 'a = b is c;' will be illegal, because 'b is c' does not have
an effect.


 For example foo() + foo();
 shouldn't compile, unless + is an overloaded operator



Yes.


 with side-effects.



Not sure about that. In generic code, it is possible and likely (as pure 
is inferred for lambdas and template functions) that a pure
function ends up being called for potential side effects.


 It would be really interesting to add this check, and then compile
 existing code (such as Phobos) to see if it breaks valid code, or
 reveals bugs.
Sep 26 2012
parent Don Clugston <dac nospam.com> writes:
On 26/09/12 14:19, Timon Gehr wrote:

 On 09/26/2012 11:45 AM, Don Clugston wrote:

 On 25/09/12 21:30, Bernard Helyer wrote:

 I tried to post this last night, but the NG wasn't having any of it.

 I found myself writing a bug that looked like this

      match(ts, TokenType.Is);
      match(ts, TokenType.OpenParen);
      isExp.type == parseType(ts);

 The bug being of course, that a type is parsed and ts is modified,
 so the expression has side effects so it's not flagged as a useless
 expression. But the comparison still has no effect, so should this be
 flagged by DMD?

 -Bernard.




 The "must have an effect" rule only applies to statements, not
 expressions, so this is according to the spec. It's not a bug.

 This is a bit like the more extreme case I recently posted about:

 int x, y, z;
 x == y, ++z;

 doesn't generate an error message even though x == y has no
 side-effects, because comma is an expression, not a statement.

 IMHO, every expression should be required to have an effect.



 I'd rather restrict this to expressions whose value is unused.



If the value is used, it has an effect.


 Otherwise 'a = b is c;' will be illegal, because 'b is c' does not have
 an effect.


 For example foo() + foo();
 shouldn't compile, unless + is an overloaded operator



 Yes.


 with side-effects.



 Not sure about that. In generic code, it is possible and likely (as pure
 is inferred for lambdas and template functions) that a pure
 function ends up being called for potential side effects.



I wouldn't give it special treatment. I think it should be allowed if 
and only if

plus(foo(), foo());

compiles, when plus is pure nothrow.
Sep 26 2012
prev sibling parent Ben Davis <entheh cantab.net> writes:
On 26/09/2012 10:45, Don Clugston wrote:

 On 25/09/12 21:30, Bernard Helyer wrote:

 I tried to post this last night, but the NG wasn't having any of it.

 I found myself writing a bug that looked like this

      match(ts, TokenType.Is);
      match(ts, TokenType.OpenParen);
      isExp.type == parseType(ts);

 The bug being of course, that a type is parsed and ts is modified,
 so the expression has side effects so it's not flagged as a useless
 expression. But the comparison still has no effect, so should this be
 flagged by DMD?

 -Bernard.




 The "must have an effect" rule only applies to statements, not
 expressions, so this is according to the spec. It's not a bug.

 This is a bit like the more extreme case I recently posted about:

 int x, y, z;
 x == y, ++z;

 doesn't generate an error message even though x == y has no
 side-effects, because comma is an expression, not a statement.

 IMHO, every expression should be required to have an effect. For example
 foo() + foo();
 shouldn't compile, unless + is an overloaded operator with side-effects.



What's a realistic use case for allowing foo() + foo(), even if it is 
overloaded to have side-effects? It looks like an expression with no 
side-effects. Surely if anyone actually intends it to have side-effects, 
then they've made a fundamental design mistake in their API?

I know C++ has << used this way, but that's a bit horrible.
Sep 26 2012
prev sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
Timon Gehr:


 In generic code, it is possible and likely (as pure is inferred 
 for lambdas and template functions) that a pure function ends 
 up being called for potential side effects.



Already tried that :-(

Bye,
bearophile
Sep 26 2012