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digitalmars.D.bugs - [Issue 2832] New: pure function too pure

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2832

           Summary: pure function too pure
           Product: D
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: andrei metalanguage.com


pure int fun(int d, int divisor)
{
    if (d < 0)
        d -= divisor - 1;
    return d / divisor;
}

This doesn't compile because d is assumed to be constant. It doesn't have to
because it's a private copy of the function. Beware when fixing this - any
indirectly-referenced data is not part of function's private state.


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Apr 11 2009
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=2832


Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au





Comment: Allowing this would mean that nested pure functions would no longer be
able to use the parameters from the enclosing function.

pure int fun(int d, int divisor)
{
   int gun() pure { return d+1; } // would be illegal, since 'fun' could modify
d.
    return gun() + d / divisor;
}

Which in turn would mean that nested pure functions would become quite useless.
So there's a price to pay.

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Jul 03 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Max Samukha <samukha voliacable.com> changed:

           What    |Removed                     |Added
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                 CC|                            |samukha voliacable.com



PDT ---
Instead of introducing another inconsistency into the language for the
not-so-common case, you could take the opposite route:

pure int fun(int d, int divisor)
{
   immutable c = d;
   int gun() pure { return c + 1; }

   return gun() + d / divisor;
}

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Oct 13 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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 Instead of introducing another inconsistency into the language for the
 not-so-common case, you could take the opposite route:
 
 pure int fun(int d, int divisor)
 {
    immutable c = d;
    int gun() pure { return c + 1; }
 
    return gun() + d / divisor;
 }


I think the existing behaviour -- that you cannot change any of the parameters
in a pure function -- is simple and intuitive: pure functions can only modify
variables which they created themselves. A rule that pure nested functions can
use indirectly-referenced data, but cannot use parameters which are passed by
value, just seems complicated. 
Especially, in the case where a parameter contains a reference to other data,
it seems folly to be allowed to change part of the parameter, but not all of
it.

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Oct 13 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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PDT ---

 I think the existing behaviour -- that you cannot change any of the parameters
 in a pure function -- is simple and intuitive: pure functions can only modify
 variables which they created themselves.


But the passed-by-value parts of the arguments are copied and consequently can
be qualified as "variables which they created themselves".


 A rule that pure nested functions can
 use indirectly-referenced data, but cannot use parameters which are passed by
 value, just seems complicated.


I don't think it is too complicated. It can be trivially done like this:

pure int foo(in int d, ...)
{
 // now we should be able to use d in pure nested functions because it is 
 // guaranteed to not change during the function call.
}


 Especially, in the case where a parameter contains a reference to other data,
 it seems folly to be allowed to change part of the parameter, but not all of
 it.


I am not sure. For example, it seems to be fairly intuitive to be able to
rebind a string parameter, though changing the referenced part of it is not
allowed. I would agree if D's function parameters behaved like aliases to the
arguments, but they are more like the function's local variables, which
arguments are assigned to.

Now that I am trying to purify some functions (most of which have no nested
functions) I need to add the useless temporaries to make them compile :(

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Oct 13 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com
           Severity|normal                      |enhancement



23:30:21 PST ---
This would reintroduce "tail-const" for the parameters, something that we tried
hard to make work and failed. I'd rather just leave the parameters const. If
you have to make a mutable copy, as in:

  pure int fun(int d, int divisor)
  { auto x = d;
    if (x < 0)
        x -= divisor - 1;
    return x / divisor;
  }

that isn't much of a burden, and the compiler will optimize the extra copy away
anyway.

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Dec 11 2009
prev sibling next sibling parent d-bugmail puremagic.com writes:
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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
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                 CC|                            |dsimcha yahoo.com



*** Issue 4630 has been marked as a duplicate of this issue. ***

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Aug 15 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Tomash Brechko <tomash.brechko gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |tomash.brechko gmail.com



07:39:43 PDT ---
Note that (with dmd 2.049)

pure int f(int i)
{
  return ++i;
}

doesn't compile, yet

pure auto f(int i)
{
  return ++i;
}

does work (int return type is replaced with auto).

Also http://www.digitalmars.com/d/2.0/function.html section "Pure Functions"
gives an example of assignment to parameter,

  i = y;    // ok, reading immutable global state

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Sep 22 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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08:05:37 PDT ---
Created separate issue http://d.puremagic.com/issues/show_bug.cgi?id=4915 for
"int -> auto", as auto return type escapes pure completely.

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Sep 22 2010
prev sibling parent d-bugmail puremagic.com writes:
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Jesse Phillips <Jesse.K.Phillips+D gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |Jesse.K.Phillips+D gmail.co
                   |                            |m
         Resolution|                            |FIXED



13:19:39 PST ---
In the recent update of relaxed purity, this code now compiles. However I
believe this is a weakly pure function as a strongly pure function requires its
parameters to be immutable.

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Nov 16 2010