Re: D UFCS anti-pattern
On 4/25/14, Steven Schveighoffer via Digitalmars-d
digitalmars-d@puremagic.com wrote:
Recently, I observed a conversation happening on the github pull request
system.
Another case of hijacking is the std.conv.text function. I've seen two
people so far that have accidentally called the global text function
by mistake, e.g.:
-
import std.conv;
import std.stdio;
class C
{
this(string user, string text) { _user = user; _text = text; }
string _user;
string _text;
}
void main()
{
auto c = new C(John, this is my message);
string input = c.text;
writeln(input); // test.C, oops!
}
-
UFCS has its benefits but it also has its drawbacks. And yet I
wouldn't want to ban the above code, but maybe we could figure out
some system for protecting against UFCS calls in some contexts. The
way I worked around the common `.text` pattern was to define a
@disable'd field in the class, which would override the global text
function and trigger a compilation error.
Re: D UFCS anti-pattern
On Friday, 25 April 2014 at 07:52:20 UTC, Andrej Mitrovic via Digitalmars-d wrote: Another case of hijacking is the std.conv.text function. I've seen two people so far that have accidentally called the global text function by mistake, e.g.: Make that three. The ability to call non-@property functions without parenthesis is another of my favorite anti-features.
Re: D UFCS anti-pattern
On Fri, 25 Apr 2014 00:58:51 -0400, Jonathan M Davis via Digitalmars-d
digitalmars-d@puremagic.com wrote:
If it doesn't work to override a free function with a member
function, I honestly don't see much point to UFCS. The whole idea
behind it is to make it so that you don't have to care whether a
function is a free function or a member function. The current situation
essentially forces you to not use UFCS except in cases where you're
trying to add member functions to built-in types. And as such,
calling functions on user-defined types using UFCS runs a high risk of
not compiling, because all it takes is for the user-defined type to
define a function with the same name - even if it takes completely
different arguments - and now the compiler won't even try to use the
free function anymore.
Right but the pattern is:
someGlobal(T t, U u)
{
static if(is(typeof(t.someGlobal(u
t.someGlobal(u);
else
// use default implementation.
}
Essentially, someGlobal is encouraging not only overriding itself for a
type, but for only a PORTION of the implementation.
If you override put completely, allowing all the mechanisms put uses, that
is absolutely fine. But for only implementing a single part, the hook put
uses to interface with the type should NOT be named put. This means the
member put is far less functional than the global function put.
What I'm saying is that the hook and the global function should not be
named the same. Not that we should change the override rules.
I really think that we should fix it so that stuff like
outputRange.put(foo) works - including when types define put
themselves. AFAIK, that means changing the overload rules so that
member functions conflict with free functions only when they take the
same arguments - in which case the member function would be called, as
it is now, except that the cases where a free function matches the
arguments would also work, allowing us to override free functions with
member functions where appropriate and prevent simple name collisions
from making UFCS not work (i.e. when the member function takes
completely different arguments, UFCS would still use the free
function). Without a change along those lines, I'd be strongly inclined
to argue against using UFCS in any situation except in those where you
need to add member functions to the built-in types. And the only
common case for that that I'm aware of is making it so that arrays can
function as ranges.
I don't think this is a good idea. A type first and foremost is in charge
of its API.
Another possible option is to make sure put(R, ...) is as limited as the
member R.put(...). This means, if R.put doesn't support the parameters,
put(R, ...) should also reject them.
This at least is consistent, but I think code will break for not much of a
good reason.
-Steve
Re: D UFCS anti-pattern
On Fri, 25 Apr 2014 03:52:09 -0400, Andrej Mitrovic via Digitalmars-d
digitalmars-d@puremagic.com wrote:
On 4/25/14, Steven Schveighoffer via Digitalmars-d
digitalmars-d@puremagic.com wrote:
Recently, I observed a conversation happening on the github pull request
system.
Another case of hijacking is the std.conv.text function. I've seen two
people so far that have accidentally called the global text function
by mistake, e.g.:
-
import std.conv;
import std.stdio;
class C
{
this(string user, string text) { _user = user; _text = text; }
string _user;
string _text;
}
void main()
{
auto c = new C(John, this is my message);
string input = c.text;
writeln(input); // test.C, oops!
}
-
UFCS has its benefits but it also has its drawbacks. And yet I
wouldn't want to ban the above code, but maybe we could figure out
some system for protecting against UFCS calls in some contexts. The
way I worked around the common `.text` pattern was to define a
@disable'd field in the class, which would override the global text
function and trigger a compilation error.
I think this really comes down to a poorly named function. textOf, toText,
textify even, are better names that wouldn't cause this problem.
-Steve
Re: D UFCS anti-pattern
On Friday, 25 April 2014 at 12:21:04 UTC, Steven Schveighoffer wrote: I think this really comes down to a poorly named function. textOf, toText, textify even, are better names that wouldn't cause this problem. -Steve Well, in this case, yes, because it's a noun. For all functions that are verbs the issue still stands.
D UFCS anti-pattern
Recently, I observed a conversation happening on the github pull request
system.
In phobos, we have the notion of output ranges. One is allowed to output
to an output range by calling the function 'put'.
Here is the implementation of put:
void put(R, E)(ref R r, E e)
{
static if(is(PointerTarget!R == struct))
enum usingPut = hasMember!(PointerTarget!R, put);
else
enum usingPut = hasMember!(R, put);
enum usingFront = !usingPut isInputRange!R;
enum usingCall = !usingPut !usingFront;
static if (usingPut is(typeof(r.put(e
{
r.put(e);
}
else static if (usingPut is(typeof(r.put((E[]).init
{
r.put((e)[0..1]);
}
else static if (usingFront is(typeof(r.front = e, r.popFront(
{
r.front = e;
r.popFront();
}
else static if ((usingPut || usingFront) isInputRange!E
is(typeof(put(r, e.front
{
for (; !e.empty; e.popFront()) put(r, e.front);
}
else static if (usingCall is(typeof(r(e
{
r(e);
}
else static if (usingCall is(typeof(r((E[]).init
{
r((e)[0..1]);
}
else
{
static assert(false,
Cannot put a ~E.stringof~ into a ~R.stringof);
}
}
There is an interesting issue here -- put can basically be overridden by a
member function of the output range, also named put. I will note that this
function was designed and written before UFCS came into existence. So most
of the machinery here is designed to detect whether a 'put' member
function exists.
One nice thing about UFCS, now any range that has a writable front(), can
put any other range whose elements can be put into front, via the
pseudo-method put.
In other words:
void foo(int[] arr)
{
int[] result = new int[arr.length];
result.put(arr); // put arr into result.
}
But there is an issue with this. If the destination range actually
implements the put member function, but doesn't implement all of the
global function's niceties,
r.put(...) is not as powerful/useful as put(r,...). Therefore, the odd
recommendation is to *always* call put(r,...)
I find this, at the very least, to be confusing. Here is a case where UFCS
ironically is not usable via a function call that so obviously should be
UFCS.
The anti-pattern here is using member functions to override or specialize
UFCS behavior. In this case, we even hook the UFCS call with the member
function, encouraging the name conflict!
As a possible solution, I would recommend simply change the name of the
hook, and have the UFCS function forward to the hook. This way, calling
put(r,...) and r.put(...) is always consistent.
Does this make sense? Anyone have any other possible solutions?
A relevant bug report (where I actually advocate for adding more of this
horrible behavior): https://issues.dlang.org/show_bug.cgi?id=12583
-Steve
Re: D UFCS anti-pattern
On Thu, 24 Apr 2014 22:21:32 -0400
Steven Schveighoffer via Digitalmars-d digitalmars-d@puremagic.com
wrote:
Recently, I observed a conversation happening on the github pull
request system.
In phobos, we have the notion of output ranges. One is allowed to
output to an output range by calling the function 'put'.
Here is the implementation of put:
void put(R, E)(ref R r, E e)
{
static if(is(PointerTarget!R == struct))
enum usingPut = hasMember!(PointerTarget!R, put);
else
enum usingPut = hasMember!(R, put);
enum usingFront = !usingPut isInputRange!R;
enum usingCall = !usingPut !usingFront;
static if (usingPut is(typeof(r.put(e
{
r.put(e);
}
else static if (usingPut is(typeof(r.put((E[]).init
{
r.put((e)[0..1]);
}
else static if (usingFront is(typeof(r.front = e,
r.popFront( {
r.front = e;
r.popFront();
}
else static if ((usingPut || usingFront) isInputRange!E
is(typeof(put(r, e.front
{
for (; !e.empty; e.popFront()) put(r, e.front);
}
else static if (usingCall is(typeof(r(e
{
r(e);
}
else static if (usingCall is(typeof(r((E[]).init
{
r((e)[0..1]);
}
else
{
static assert(false,
Cannot put a ~E.stringof~ into a ~R.stringof);
}
}
There is an interesting issue here -- put can basically be overridden
by a member function of the output range, also named put. I will note
that this function was designed and written before UFCS came into
existence. So most of the machinery here is designed to detect
whether a 'put' member function exists.
One nice thing about UFCS, now any range that has a writable front(),
can put any other range whose elements can be put into front, via
the pseudo-method put.
In other words:
void foo(int[] arr)
{
int[] result = new int[arr.length];
result.put(arr); // put arr into result.
}
But there is an issue with this. If the destination range actually
implements the put member function, but doesn't implement all of the
global function's niceties,
r.put(...) is not as powerful/useful as put(r,...). Therefore, the
odd recommendation is to *always* call put(r,...)
I find this, at the very least, to be confusing. Here is a case where
UFCS ironically is not usable via a function call that so obviously
should be UFCS.
The anti-pattern here is using member functions to override or
specialize UFCS behavior. In this case, we even hook the UFCS call
with the member function, encouraging the name conflict!
As a possible solution, I would recommend simply change the name of
the hook, and have the UFCS function forward to the hook. This way,
calling put(r,...) and r.put(...) is always consistent.
Does this make sense? Anyone have any other possible solutions?
A relevant bug report (where I actually advocate for adding more of
this horrible behavior):
https://issues.dlang.org/show_bug.cgi?id=12583
If it doesn't work to override a free function with a member
function, I honestly don't see much point to UFCS. The whole idea
behind it is to make it so that you don't have to care whether a
function is a free function or a member function. The current situation
essentially forces you to not use UFCS except in cases where you're
trying to add member functions to built-in types. And as such,
calling functions on user-defined types using UFCS runs a high risk of
not compiling, because all it takes is for the user-defined type to
define a function with the same name - even if it takes completely
different arguments - and now the compiler won't even try to use the
free function anymore.
I really think that we should fix it so that stuff like
outputRange.put(foo) works - including when types define put
themselves. AFAIK, that means changing the overload rules so that
member functions conflict with free functions only when they take the
same arguments - in which case the member function would be called, as
it is now, except that the cases where a free function matches the
arguments would also work, allowing us to override free functions with
member functions where appropriate and prevent simple name collisions
from making UFCS not work (i.e. when the member function takes
completely different arguments, UFCS would still use the free
function). Without a change along those lines, I'd be strongly inclined
to argue against using UFCS in any situation except in those where you
need to add member functions to the built-in types. And the only
common case for that that I'm aware of is making it so that arrays can
function as ranges.
But this issue goes far beyond put.
- Jonathan M Davis
Re: D UFCS anti-pattern
On Friday, 25 April 2014 at 02:21:32 UTC, Steven Schveighoffer wrote: A relevant bug report (where I actually advocate for adding more of this horrible behavior): https://issues.dlang.org/show_bug.cgi?id=12583 -Steve See also: https://issues.dlang.org/show_bug.cgi?id=9074 Appender!string x; x.put(repeat( ).take(4)); //fails put(x, repeat( ).take(4)); //works
