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digitalmars.D - Immutable separator to join() doesn't work

reply Mehrdad <wfunction hotmail.com> writes:
I noticed that the code below doesn't work, and I was wondering if it's 
by design (hopefully not):

     immutable SEP = ", ";
     ["a", "b"].join(SEP);

The fact that SEP is immutable(char[]) instead of immutable(char)[] 
shouldn't break the function.
Jul 10 2011
next sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
 I noticed that the code below doesn't work, and I was wondering if it's
 by design (hopefully not):
 
      immutable SEP = ", ";
      ["a", "b"].join(SEP);
 
 The fact that SEP is immutable(char[]) instead of immutable(char)[]
 shouldn't break the function.

It most definitely breaks the function, and it's a limitation of templates. Templates are instantiated with the exact type that they're given, so the compiler tries to instantiate join with immutable(char[]), but join _can't_ work with immutable(char[]), because it needs a mutable range. immutable ranges are worthless. If the compiler were smart enough to realize that it could instantiate join with immutable(char)[] and it would work, then you could use immutable(char[]), but since it isn't that smart, it doesn't work. The same problem happens with static arrays. They can't be used as ranges, so even though they'd work if the compiler picked a dynamic range as the type for the function, they don't work, because the compiler isn't that smart. The problem may be fixed at some point, but as it stands, it just doesn't work to use immutable arrays with range-based functions. - Jonathan M Davis
Jul 10 2011
next sibling parent Mehrdad <wfunction hotmail.com> writes:
On 7/10/2011 9:17 PM, Jonathan M Davis wrote:
 On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
 I noticed that the code below doesn't work, and I was wondering if it's
 by design (hopefully not):

       immutable SEP = ", ";
       ["a", "b"].join(SEP);

 The fact that SEP is immutable(char[]) instead of immutable(char)[]
 shouldn't break the function.

Templates are instantiated with the exact type that they're given, so the compiler tries to instantiate join with immutable(char[]), but join _can't_ work with immutable(char[]), because it needs a mutable range. immutable ranges are worthless. If the compiler were smart enough to realize that it could instantiate join with immutable(char)[] and it would work, then you could use immutable(char[]), but since it isn't that smart, it doesn't work. The same problem happens with static arrays. They can't be used as ranges, so even though they'd work if the compiler picked a dynamic range as the type for the function, they don't work, because the compiler isn't that smart. The problem may be fixed at some point, but as it stands, it just doesn't work to use immutable arrays with range-based functions. - Jonathan M Davis

Thanks for the info!
Jul 10 2011
prev sibling parent reply Timon Gehr <timon.gehr gmx.ch> writes:
Jonathan M Davis wrote:
 On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
 I noticed that the code below doesn't work, and I was wondering if it's
 by design (hopefully not):

      immutable SEP = ", ";
      ["a", "b"].join(SEP);

 The fact that SEP is immutable(char[]) instead of immutable(char)[]
 shouldn't break the function.

It most definitely breaks the function, and it's a limitation of templates. Templates are instantiated with the exact type that they're given, so the compiler tries to instantiate join with immutable(char[]), but join _can't_ work with immutable(char[]), because it needs a mutable range. immutable ranges are worthless. If the compiler were smart enough to realize that it could instantiate join with immutable(char)[] and it would work, then you could use immutable(char[]), but since it isn't that smart, it doesn't work. The same problem happens with static arrays. They can't be used as ranges, so even though they'd work if the compiler picked a dynamic range as the type for the function, they don't work, because the compiler isn't that smart. The problem may be fixed at some point, but as it stands, it just doesn't work to use immutable arrays with range-based functions. - Jonathan M Davis

There is no such thing as an immutable range because the range abstraction is based on mutation. This does not make sense, because intuitively, immutable(char[]) is a range type just as immutable(char)[] or char[] is a range type. It is quite odd that a data structure can only be a D range if it may be modified... It is not really a compiler issue, but one of library design. Maybe adding a wrapper range to Phobos to iterate over immutable 'ranges' would be an improvement to the current situation. Cheers, -Timon
Jul 11 2011
parent reply Timon Gehr <timon.gehr gmx.ch> writes:
Jonathan M Davis wrote:
 On 2011-07-11 09:06, Timon Gehr wrote:
 Jonathan M Davis wrote:
 On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
 I noticed that the code below doesn't work, and I was wondering if it's

 by design (hopefully not):
 immutable SEP = ", ";
 ["a", "b"].join(SEP);

 The fact that SEP is immutable(char[]) instead of immutable(char)[]
 shouldn't break the function.

It most definitely breaks the function, and it's a limitation of templates. Templates are instantiated with the exact type that they're given, so the compiler tries to instantiate join with immutable(char[]), but join _can't_ work with immutable(char[]), because it needs a mutable range. immutable ranges are worthless. If the compiler were smart enough to realize that it could instantiate join with immutable(char)[] and it would work, then you could use immutable(char[]), but since it isn't that smart, it doesn't work. The same problem happens with static arrays. They can't be used as ranges, so even though they'd work if the compiler picked a dynamic range as the type for the function, they don't work, because the compiler isn't that smart. The problem may be fixed at some point, but as it stands, it just doesn't work to use immutable arrays with range-based functions. - Jonathan M Davis

There is no such thing as an immutable range because the range abstraction is based on mutation. This does not make sense, because intuitively, immutable(char[]) is a range type just as immutable(char)[] or char[] is a range type. It is quite odd that a data structure can only be a D range if it may be modified... It is not really a compiler issue, but one of library design. Maybe adding a wrapper range to Phobos to iterate over immutable 'ranges' would be an improvement to the current situation.

??? A range is only of any value if you can process it. That generally requires calling popFront on it. That mutates it. So, sure, you can an immutable range, but it's useless, because you can't process it.

import std.range; void main(){ int[] arr; auto a=cast(immutable)map!"a"(arr); assert(!isInputRange!(typeof(a))); // this passes } So no, you cannot have an immutable range because it cannot fulfill the range interface.
 And why should that be surprising?

This is surprising: immutable SEP = ", "; ["a","b"].join(SEP); // -- nope! Guess what? That is because your array is not a range! Why? Easy! It cannot be changed! See? // -- I don't care, but this should work... I'll implement the function myself! immutable SEP = ", "; string sep=SEP; ["a","b"].join(sep); //sure thing
 There are plenty of things that are essentially
 useless if they're immutable. Take a stream for instance. Like a range, it's
 altered as you use. So, you can't have an immutable stream - or if you did, it
 would be useless.

You can. You can even have immutable IO if you choose the right abstractions ;).
 Immutability can be very useful, but not being able to
 mutate something can really get in the way of doing anything with it. The same
 goes for const.

You can always use it's value to create others.
 There has been some discussion in the past of trying to find a way to convert
 an immutable or const range to a tail-immutable or tail-const range (similar
 to how you can pass immutable(char[]) to a function which takes
 immutable(char)[] and have it work),
 but the language doesn't currently
 provide any way to do that, and it wouldn't necessarily make sense for all
 range types anyway. It's highly dependent on how their implemented and what
 they're actually iterating over.

I completely agree. It would be unnecessarily complicated (impossible?) to make all ranges work as immutable that would meaningfully be able to. Also it would give almost no benefit. But immutable(T[]) should clearly be accepted where immutable(T)[] would be. I've run across that problem myself It is just annoying, even more so if you do not actually need the generosity provided by those functions at all.
 It's definitely true that the template situation could use some improvement to
 better deal with immutable arrays, and an improvement to the type system to
 better handle tail-const for ranges where applicable would be desirable, but
 the fact remains that not everything can be fully const or immutable and be
 useful. It's just a fact of how const and immutable work. There's stuff that
 needs to be mutable, and when you make it so that you can't mutate them,
 they're useless.

 - Jonathan M Davis

Nothing needs to be mutable. It is possible to work with immutable values only, but it is no always efficient. Having an immutable range interface would be quite easy. popFront would just have to return the next range instead of changing the current one. Cheers, -Timon
Jul 11 2011
next sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
Jonathan M Davis wrote:
 The solution is to fix template instantiation so that it's smarter when
 dealing with static arrays and const or immutable arrays:
 http://d.puremagic.com/issues/show_bug.cgi?id=6148

Unless I am missing something essential, supporting this would turn the complexity of failing template function instantiations to Omega((time for matching)*2^(number of const/immutable arrays in the input)). I don't think this is too great.
 It's a language issue, not a design issue. Making ranges function like slists
 (with head and tail or car and cdr) would be almost certainly be too
 inefficient (particularly for ranges where save is not super cheap, though
 it's at least supposed to be fairly cheap).

That Eg. join cannot take an immutable(char[]) is certainly a design issue. I think what you find to be a language issue are limitations inherent to templates that are very hard (NP hard in the general case) to overcome.
 Immutability might be nice, but it
 does have its costs, and in this case, D's templates aren't currenly smart
 enough to use immutable(E)[] instead of immutable(E[]).

I claim they cannot get that smart. If I'm right, this turns it into a complete design issue.
 And it's not like
 immutable ranges are going to work with non-array ranges anyway, so it's
 arguably a good idea to just expect immutable and const ranges to not work
 anyway.

 - Jonathan M Davis

immutable ranges don't exist in D. immutable arrays do. The issue is that many Phobos functions can only take ranges, what excludes immutable arrays. That is bad as immutable arrays share many properties with ranges and could be used as input to similar algorithms. This does not have top priority (at least for me), as it is just a mild annoyance that can be worked around. But it needs some thinking. Cheers, -Timon
Jul 11 2011
prev sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
so wrote:
 There is a simple workaround for this type of ranges that are like
 iterators, which we know the beginning and the end.
 We can improve isForwardRange!R by adding a line hasForwardRange!R. If it
 does have, we return an adaptor which gives us a mutable range.
 Good thing is because the original range is mutable we don't need to worry
 about anything else.

Wouldn't that be quite invasive? I imagine every function that would want to work on ranges would then have to provide two versions, one that does the work and one that calls the other version after having applied the adaptor(s)? Cheers, -Timon
Jul 11 2011
prev sibling next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On 2011-07-11 09:06, Timon Gehr wrote:
 Jonathan M Davis wrote:
 On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
 I noticed that the code below doesn't work, and I was wondering if it's
 
 by design (hopefully not):
 immutable SEP = ", ";
 ["a", "b"].join(SEP);
 
 The fact that SEP is immutable(char[]) instead of immutable(char)[]
 shouldn't break the function.

It most definitely breaks the function, and it's a limitation of templates. Templates are instantiated with the exact type that they're given, so the compiler tries to instantiate join with immutable(char[]), but join _can't_ work with immutable(char[]), because it needs a mutable range. immutable ranges are worthless. If the compiler were smart enough to realize that it could instantiate join with immutable(char)[] and it would work, then you could use immutable(char[]), but since it isn't that smart, it doesn't work. The same problem happens with static arrays. They can't be used as ranges, so even though they'd work if the compiler picked a dynamic range as the type for the function, they don't work, because the compiler isn't that smart. The problem may be fixed at some point, but as it stands, it just doesn't work to use immutable arrays with range-based functions. - Jonathan M Davis

There is no such thing as an immutable range because the range abstraction is based on mutation. This does not make sense, because intuitively, immutable(char[]) is a range type just as immutable(char)[] or char[] is a range type. It is quite odd that a data structure can only be a D range if it may be modified... It is not really a compiler issue, but one of library design. Maybe adding a wrapper range to Phobos to iterate over immutable 'ranges' would be an improvement to the current situation.

??? A range is only of any value if you can process it. That generally requires calling popFront on it. That mutates it. So, sure, you can an immutable range, but it's useless, because you can't process it. And why should that be surprising? There are plenty of things that are essentially useless if they're immutable. Take a stream for instance. Like a range, it's altered as you use. So, you can't have an immutable stream - or if you did, it would be useless. Immutability can be very useful, but not being able to mutate something can really get in the way of doing anything with it. The same goes for const. There has been some discussion in the past of trying to find a way to convert an immutable or const range to a tail-immutable or tail-const range (similar to how you can pass immutable(char[]) to a function which takes immutable(char)[] and have it work), but the language doesn't currently provide any way to do that, and it wouldn't necessarily make sense for all range types anyway. It's highly dependent on how their implemented and what they're actually iterating over. It's definitely true that the template situation could use some improvement to better deal with immutable arrays, and an improvement to the type system to better handle tail-const for ranges where applicable would be desirable, but the fact remains that not everything can be fully const or immutable and be useful. It's just a fact of how const and immutable work. There's stuff that needs to be mutable, and when you make it so that you can't mutate them, they're useless. - Jonathan M Davis
Jul 11 2011
prev sibling next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On 2011-07-11 13:30, Timon Gehr wrote:
 Jonathan M Davis wrote:
 On 2011-07-11 09:06, Timon Gehr wrote:
 Jonathan M Davis wrote:
 On Sunday 10 July 2011 21:09:27 Mehrdad wrote:
 I noticed that the code below doesn't work, and I was wondering if
 it's
 
 by design (hopefully not):
 immutable SEP = ", ";
 ["a", "b"].join(SEP);
 
 The fact that SEP is immutable(char[]) instead of immutable(char)[]
 shouldn't break the function.

It most definitely breaks the function, and it's a limitation of templates. Templates are instantiated with the exact type that they're given, so the compiler tries to instantiate join with immutable(char[]), but join _can't_ work with immutable(char[]), because it needs a mutable range. immutable ranges are worthless. If the compiler were smart enough to realize that it could instantiate join with immutable(char)[] and it would work, then you could use immutable(char[]), but since it isn't that smart, it doesn't work. The same problem happens with static arrays. They can't be used as ranges, so even though they'd work if the compiler picked a dynamic range as the type for the function, they don't work, because the compiler isn't that smart. The problem may be fixed at some point, but as it stands, it just doesn't work to use immutable arrays with range-based functions. - Jonathan M Davis

There is no such thing as an immutable range because the range abstraction is based on mutation. This does not make sense, because intuitively, immutable(char[]) is a range type just as immutable(char)[] or char[] is a range type. It is quite odd that a data structure can only be a D range if it may be modified... It is not really a compiler issue, but one of library design. Maybe adding a wrapper range to Phobos to iterate over immutable 'ranges' would be an improvement to the current situation.

??? A range is only of any value if you can process it. That generally requires calling popFront on it. That mutates it. So, sure, you can an immutable range, but it's useless, because you can't process it.

import std.range; void main(){ int[] arr; auto a=cast(immutable)map!"a"(arr); assert(!isInputRange!(typeof(a))); // this passes } So no, you cannot have an immutable range because it cannot fulfill the range interface.
 And why should that be surprising?

This is surprising: immutable SEP = ", "; ["a","b"].join(SEP); // -- nope! Guess what? That is because your array is not a range! Why? Easy! It cannot be changed! See? // -- I don't care, but this should work... I'll implement the function myself! immutable SEP = ", "; string sep=SEP; ["a","b"].join(sep); //sure thing
 There are plenty of things that are essentially
 useless if they're immutable. Take a stream for instance. Like a range,
 it's altered as you use. So, you can't have an immutable stream - or if
 you did, it would be useless.

You can. You can even have immutable IO if you choose the right abstractions ;).
 Immutability can be very useful, but not being able to
 mutate something can really get in the way of doing anything with it. The
 same goes for const.

You can always use it's value to create others.
 There has been some discussion in the past of trying to find a way to
 convert an immutable or const range to a tail-immutable or tail-const
 range (similar to how you can pass immutable(char[]) to a function which
 takes
 immutable(char)[] and have it work),
 but the language doesn't currently
 provide any way to do that, and it wouldn't necessarily make sense for
 all range types anyway. It's highly dependent on how their implemented
 and what they're actually iterating over.

I completely agree. It would be unnecessarily complicated (impossible?) to make all ranges work as immutable that would meaningfully be able to. Also it would give almost no benefit. But immutable(T[]) should clearly be accepted where immutable(T)[] would be. I've run across that problem myself It is just annoying, even more so if you do not actually need the generosity provided by those functions at all.
 It's definitely true that the template situation could use some
 improvement to better deal with immutable arrays, and an improvement to
 the type system to better handle tail-const for ranges where applicable
 would be desirable, but the fact remains that not everything can be
 fully const or immutable and be useful. It's just a fact of how const
 and immutable work. There's stuff that needs to be mutable, and when you
 make it so that you can't mutate them, they're useless.
 
 - Jonathan M Davis

Nothing needs to be mutable. It is possible to work with immutable values only, but it is no always efficient. Having an immutable range interface would be quite easy. popFront would just have to return the next range instead of changing the current one.

The solution is to fix template instantiation so that it's smarter when dealing with static arrays and const or immutable arrays: http://d.puremagic.com/issues/show_bug.cgi?id=6148 It's a language issue, not a design issue. Making ranges function like slists (with head and tail or car and cdr) would be almost certainly be too inefficient (particularly for ranges where save is not super cheap, though it's at least supposed to be fairly cheap). Immutability might be nice, but it does have its costs, and in this case, D's templates aren't currenly smart enough to use immutable(E)[] instead of immutable(E[]). And it's not like immutable ranges are going to work with non-array ranges anyway, so it's arguably a good idea to just expect immutable and const ranges to not work anyway. - Jonathan M Davis
Jul 11 2011
prev sibling next sibling parent so <so so.so> writes:
On Tue, 12 Jul 2011 00:01:55 +0300, Jonathan M Davis <jmdavisProg gmx.com>  
wrote:

 The solution is to fix template instantiation so that it's smarter when
 dealing with static arrays and const or immutable arrays:
 http://d.puremagic.com/issues/show_bug.cgi?id=6148

 It's a language issue, not a design issue. Making ranges function like  
 slists
 (with head and tail or car and cdr) would be almost certainly be too
 inefficient (particularly for ranges where save is not super cheap,  
 though
 it's at least supposed to be fairly cheap). Immutability might be nice,  
 but it
 does have its costs, and in this case, D's templates aren't currenly  
 smart
 enough to use immutable(E)[] instead of immutable(E[]). And it's not like
 immutable ranges are going to work with non-array ranges anyway, so it's
 arguably a good idea to just expect immutable and const ranges to not  
 work
 anyway.

There is a simple workaround for this type of ranges that are like iterators, which we know the beginning and the end. We can improve isForwardRange!R by adding a line hasForwardRange!R. If it does have, we return an adaptor which gives us a mutable range. Good thing is because the original range is mutable we don't need to worry about anything else.
Jul 11 2011
prev sibling next sibling parent so <so so.so> writes:
On Tue, 12 Jul 2011 00:37:04 +0300, so <so so.so> wrote:

 Good thing is because the original range is mutable we don't need to  
 worry about anything else.

Typo, "original range is immutable"
Jul 11 2011
prev sibling next sibling parent so <so so.so> writes:
On Tue, 12 Jul 2011 00:58:04 +0300, Timon Gehr <timon.gehr gmx.ch> wrote:

 so wrote:
 There is a simple workaround for this type of ranges that are like
 iterators, which we know the beginning and the end.
 We can improve isForwardRange!R by adding a line hasForwardRange!R. If  
 it
 does have, we return an adaptor which gives us a mutable range.
 Good thing is because the original range is mutable we don't need to  
 worry
 about anything else.

Wouldn't that be quite invasive? I imagine every function that would want to work on ranges would then have to provide two versions, one that does the work and one that calls the other version after having applied the adaptor(s)? Cheers, -Timon

In this case, not necessarily. hasForwardRange could just be a global function that checks if its argument is an array and this i think would solve all our current problems.
Jul 11 2011
prev sibling next sibling parent so <so so.so> writes:
On Tue, 12 Jul 2011 00:58:04 +0300, Timon Gehr <timon.gehr gmx.ch> wrote:

 Wouldn't that be quite invasive? I imagine every function that would  
 want to work
 on ranges would then have to provide two versions, one that does the  
 work and one
 that calls the other version after having applied the adaptor(s)?

 Cheers,
 -Timon

Right, we probably need to change functions but no we don't need to overload any of them.
Jul 11 2011
prev sibling next sibling parent reply "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On 2011-07-11 14:45, Timon Gehr wrote:
 Jonathan M Davis wrote:
 The solution is to fix template instantiation so that it's smarter when
 dealing with static arrays and const or immutable arrays:
 http://d.puremagic.com/issues/show_bug.cgi?id=6148

Unless I am missing something essential, supporting this would turn the complexity of failing template function instantiations to Omega((time for matching)*2^(number of const/immutable arrays in the input)). I don't think this is too great.
 It's a language issue, not a design issue. Making ranges function like
 slists (with head and tail or car and cdr) would be almost certainly be
 too inefficient (particularly for ranges where save is not super cheap,
 though it's at least supposed to be fairly cheap).

That Eg. join cannot take an immutable(char[]) is certainly a design issue. I think what you find to be a language issue are limitations inherent to templates that are very hard (NP hard in the general case) to overcome.
 Immutability might be nice, but it
 does have its costs, and in this case, D's templates aren't currenly
 smart enough to use immutable(E)[] instead of immutable(E[]).

I claim they cannot get that smart. If I'm right, this turns it into a complete design issue.

I think that they can, but regardless, I don't think that it makes sense to redesign ranges at this point. The loss of immutable and const arrays is annoying but not all that big a deal. Worst case, casting immutable(E[]) to immutable(E)[] solves the problem.
 And it's not like
 immutable ranges are going to work with non-array ranges anyway, so it's
 arguably a good idea to just expect immutable and const ranges to not
 work anyway.
 
 - Jonathan M Davis

immutable ranges don't exist in D. immutable arrays do. The issue is that many Phobos functions can only take ranges, what excludes immutable arrays. That is bad as immutable arrays share many properties with ranges and could be used as input to similar algorithms. This does not have top priority (at least for me), as it is just a mild annoyance that can be worked around. But it needs some thinking.

This enhancement request would make the situation with immutable and const arrays so that they're much more in line with mutable container types and static arrays: http://d.puremagic.com/issues/show_bug.cgi?id=6289 - Jonathan M Davis
Jul 11 2011
parent "Daniel Murphy" <yebblies nospamgmail.com> writes:
"Jonathan M Davis" <jmdavisProg gmx.com> wrote in message 
news:mailman.1552.1310429761.14074.digitalmars-d puremagic.com...
 This enhancement request would make the situation with immutable and const
 arrays so that they're much more in line with mutable container types and
 static arrays:

 http://d.puremagic.com/issues/show_bug.cgi?id=6289

 - Jonathan M Davis

WAIT WHAT? That doesn't work?!?
Jul 11 2011
prev sibling next sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Tuesday 12 July 2011 15:46:41 Daniel Murphy wrote:
 "Jonathan M Davis" <jmdavisProg gmx.com> wrote in message
 news:mailman.1552.1310429761.14074.digitalmars-d puremagic.com...
 
 This enhancement request would make the situation with immutable and
 const arrays so that they're much more in line with mutable container
 types and static arrays:
 
 http://d.puremagic.com/issues/show_bug.cgi?id=6289
 
 - Jonathan M Davis

WAIT WHAT? That doesn't work?!?

Nope. It works for static arrays but not for const or immutable arrays. Try it. It'll fail. I don't know _why_ it doesn't work, but it doesn't. If it did, this would be a much smaller issue. It would be nice if templates were improved such that they instantiated range-based functions in a manner which worked for static arrays and const or immutable arrays, but if you could solve the problem by slicing a const or immutable array, it would make the situation far less problematic. - Jonathan M Davis
Jul 11 2011
parent "Daniel Murphy" <yebblies nospamgmail.com> writes:
"Jonathan M Davis" <jmdavisProg gmx.com> wrote in message 
news:mailman.1554.1310450510.14074.digitalmars-d puremagic.com...
 Nope. It works for static arrays but not for const or immutable arrays. 
 Try
 it. It'll fail. I don't know _why_ it doesn't work, but it doesn't. If it 
 did,
 this would be a much smaller issue. It would be nice if templates were
 improved such that they instantiated range-based functions in a manner 
 which
 worked for static arrays and const or immutable arrays, but if you could 
 solve
 the problem by slicing a const or immutable array, it would make the 
 situation
 far less problematic.

 - Jonathan M Davis

Yeah, looking at the implementation and the test cases that rely on this, it seems to have been done to allow slicing typedefs to yeild the same type. I really doubt this is something we need to support any more. Every time this issue came up, I've always assumed this was how it worked! Honestly, template deduction with implicit conversions is very unlikely to ever happen. While it looks nice for one parameter, it quickly turns into a huge mess for multiple parameters. There is a fairly easy workaround that could be used throughout phobos: Accept T when isXXXRange!T || isXXXRange!(T[]), and use a static if to slice it when necessary. This would solve the problem for containers, static arrays, immutable arrays, and other immutable ranges.
Jul 11 2011
prev sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Tuesday 12 July 2011 16:16:59 Daniel Murphy wrote:
 There is a fairly easy workaround that could be used throughout phobos:
 Accept T when isXXXRange!T || isXXXRange!(T[]), and use a static if to slice
 it when necessary.  This would solve the problem for containers, static
 arrays, immutable arrays, and other immutable ranges.

I don't know whether that's a good idea for containers or const/immutable arrays, but it _definitely_ is a bad idea for static arrays. It would end up copying the entire array just because you forgot to slice it. Personally, I'm _far_ more inclined to say that you should just expect to have to slice something when you pass it to a range-based function. I think that the fact that the "container" that gets most used at this point is the dynamic array has gotten people used to not having to use slices much when proper containers would require them. Since arrays are really slices, it errodes the line between container and range, and I think that as proper containers are completed in std.container and enter mainstream use, it's going to throw a lot of people off, because they aren't going to function the quite same as arrays (primarily due to the fact that a container and a range are not the same thing with actual containers). But regardless, while your suggestion might be a good idea in some cases, it's definitely not a good solution for static arrays. And I'm skeptical that it's a good idea in any case, but it would allow for immutable arrays to be used with range-based functions. It would likely be better, however, to simply make it so that slices of them can be used with range-based functions such as is the case with static arrays. - Jonathan M Davis
Jul 12 2011
parent "Daniel Murphy" <yebblies nospamgmail.com> writes:
"Jonathan M Davis" <jmdavisProg gmx.com> wrote in message 
news:mailman.1557.1310461819.14074.digitalmars-d puremagic.com...
 Personally, I'm
 _far_ more inclined to say that you should just expect to have to slice
 something when you pass it to a range-based function.

That's my thinking too.
 It would likely be better, however, to simply make it
 so that slices of them can be used with range-based functions such as is 
 the
 case with static arrays.

I really think this was always supposed to work, but the compiler was modified to allow slicing typedefs to result in typedefs. Hopefully my patch for this will get pulled soon.
Jul 12 2011