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digitalmars.D.bugs - [Issue 11046] New: [enh] Pure functions with default and out parameters are allowed

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11046

           Summary: [enh] Pure functions with default and out parameters
                    are allowed
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: maxim maxim-fomin.ru



---
extern(C) int printf(const char*, ...);

pure foo(out int arg = x) { }

int x = 1;

void main()
{
   foo();
   printf("%d\n", x); // 0
}

Strictly speaking, spec does not prohibit pure functions from modifying its
arguments. On the other hand, there is no much sense in supporting such code.

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Sep 15 2013
next sibling parent d-bugmail puremagic.com writes:
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Maxim Fomin <maxim maxim-fomin.ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[enh] Pure functions with   |Pure functions with default
                   |default and out parameters  |and out parameters are
                   |are allowed                 |allowed



---
Sorry, it is definitely a bug because global variable is mutable.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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---
Another test-case:

int a;

pure foo(ref int a = a) { ++a; }

void main() pure
{
   foo();
}

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Jonathan M Davis <jmdavisProg gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |jmdavisProg gmx.com
         Resolution|                            |WONTFIX
            Summary|Pure functions with default |[enh] Pure functions with
                   |and out parameters are      |default and out parameters
                   |allowed                     |are allowed



PDT ---
Of course, it makes sense to support such code. In such a case, it's a weakly
pure function rather than a strongly pure one. It can then be used by strongly
pure functions.

Weakly pure functions are functions which which cannot access global mutable
state (e.g. they can't access mutable global variables or have mutable static
variables in their bodies). By itself, that is _all_ that pure indicates. It
means _nothing_ about altering its arguments or returning the same value every
time or anything of the sort.

Strongly pure functions are pure functions whose parameters are all either
immutable or implicitly convertible to immutable. In such a case, the compiler
can guarantee that the function's arguments aren't mutated, and the compiler
can take advantage of that for optimization - e.g. calling foo only once in the
expression foo(2) * foo(2). Weakly pure functions cannot be optimized like
that, but they can be called from strongly pure functions (since they do not
affect anything outside of the strongly pure function and therefore do not
affect its guarantees), so they become very useful in writing strongly pure
functions.

So, pure in D really is not functional purity. Rather, it provides the building
blocks for function purity (and strongly pure functions are then functionally
pure).

In fact, the primary benefit of pure is arguably the fact that it guarantees
that the function does not access global mutable state rather than anything to
do with functional purity or optimizations.

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

 Sorry, it is definitely a bug because global variable is mutable.


It mutates global mutable state via one of its arguments. pure functions cannot
access global mutable state _directly_, but they can do so if passed a
reference or pointer to global data. This is intended behavior and does not
present a problem. It's just fine for weakly pure functions and violates
nothing for strongly pure functions (because they could never pass a reference
or pointer to  a weakly pure function, because they can't access them).

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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safety0ff.bugz gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |safety0ff.bugz gmail.com



I think the problem is this:

import std.stdio;

pure foo1() { bar();}   // Compiles (accepts-invalid)
//pure foo2() { bar(x);} // Equivalent but doesn't compile (correct behaviour)

pure bar(out int arg = x) { }

int x = 1;

void main()
{
        foo1();
        writeln(x); // 0
}

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PDT ---

 I think the problem is this:


_That_ is a bug and should be reported as such, but it's different from arguing
that pure functions should not be allowed to have out parameters with default
arguments, which is what this enhancement request was asking for. It also has
nothing to do with out parameters, unlike this enhancement request.



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---


 I think the problem is this:


 
 _That_ is a bug and should be reported as such, but it's different from arguing
 that pure functions should not be allowed to have out parameters with default
 arguments, which is what this enhancement request was asking for. It also has
 nothing to do with out parameters, unlike this enhancement request.
 



The issue was _not_ about banning all pure functions with out parameters for
which there are default values, but about not taking into account module scope
of default argument. This is essentially the same issue (dmd does not check
status) you reported but with a better test case.

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prev sibling parent d-bugmail puremagic.com writes:
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PDT ---

 The issue was _not_ about banning all pure functions with out parameters for
 which there are default values, but about not taking into account module scope
 of default argument. This is essentially the same issue (dmd does not check
 status) you reported but with a better test case.


Well, then you needed to have been clearer in the title and description of the
issue. "Pure functions with default and out parameters are allowed!" says
nothing about global variables, and while the default argument in the example
happens to use a global variable, the text that goes with it talks about
modifying the arguments, not about global variables, and the fact that a pure
function can modify its arguments is completely acceptable. It's just a weakly
pure function rather than a strongly pure one in that case.

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Sep 16 2013