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digitalmars.D - Why not all statement are expressions ?

reply deadalnix <deadalnix gmail.com> writes:
Hi,

Working on D I noticed that some statement, notably assert, are 
expression of type void. Why not all statement (that are not expression 
already) are expression ?
May 07 2012
next sibling parent reply Jacob Carlborg <doob me.com> writes:
On 2012-05-07 19:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void. Why not all statement (that are not expression
 already) are expression ?

I would like that as well. -- /Jacob Carlborg
May 07 2012
next sibling parent reply "Nick Sabalausky" <SeeWebsiteToContactMe semitwist.com> writes:
"Jacob Carlborg" <doob me.com> wrote in message 
news:jo98d1$frl$1 digitalmars.com...
 On 2012-05-07 19:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void. Why not all statement (that are not expression
 already) are expression ?

I would like that as well.

I'm usually fairly ambivalent about the idea of statements being expressions, but I would *love* for switch to be usable as an expression. For instance, in Haxe, you can do stuff like the following, which I get a ton of use out of and often wish D had: a = switch(b) { case 1: "foo"; case 2: "bar"; case 3: "baz"; case 4: "whee"; default: "blork"; } The D equivalents aren't terrible, but they aren't nearly as nice.
May 07 2012
next sibling parent reply deadalnix <deadalnix gmail.com> writes:
Le 07/05/2012 22:27, Nick Sabalausky a écrit :
 "Jacob Carlborg"<doob me.com>  wrote in message
 news:jo98d1$frl$1 digitalmars.com...
 On 2012-05-07 19:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void. Why not all statement (that are not expression
 already) are expression ?

I would like that as well.

I'm usually fairly ambivalent about the idea of statements being expressions, but I would *love* for switch to be usable as an expression. For instance, in Haxe, you can do stuff like the following, which I get a ton of use out of and often wish D had: a = switch(b) { case 1: "foo"; case 2: "bar"; case 3: "baz"; case 4: "whee"; default: "blork"; } The D equivalents aren't terrible, but they aren't nearly as nice.

This won't work anyway. We are talking about language grammar here. If made expression, statement would be of type void. Just like assert is. The question is why assert is an expression ? Why not other statement don't follow the same pattern ?
May 07 2012
next sibling parent "Nick Sabalausky" <SeeWebsiteToContactMe semitwist.com> writes:
"deadalnix" <deadalnix gmail.com> wrote in message 
news:jo9be0$mgh$1 digitalmars.com...
 This won't work anyway. We are talking about language grammar here. If 
 made expression, statement would be of type void. Just like assert is.

 The question is why assert is an expression ? Why not other statement 
 don't follow the same pattern ?

Ok, I see what you mean. Maybe it's just because assert works like a function and functions are always expressions? Just a guess.
May 07 2012
prev sibling parent reply Jacob Carlborg <doob me.com> writes:
On 2012-05-07 22:37, deadalnix wrote:

 This won't work anyway. We are talking about language grammar here. If
 made expression, statement would be of type void. Just like assert is.

Says who? :) -- /Jacob Carlborg
May 07 2012
parent reply deadalnix <deadalnix gmail.com> writes:
Le 08/05/2012 08:22, Jacob Carlborg a écrit :
 On 2012-05-07 22:37, deadalnix wrote:

 This won't work anyway. We are talking about language grammar here. If
 made expression, statement would be of type void. Just like assert is.

Says who? :)

OK, make statement expression could open some door. But here isn't the point of my question. I don't want to make any proposal without being sure of why the decision was made in a first place.
May 08 2012
parent Jacob Carlborg <doob me.com> writes:
On 2012-05-08 12:27, deadalnix wrote:
 Le 08/05/2012 08:22, Jacob Carlborg a écrit :
 On 2012-05-07 22:37, deadalnix wrote:

 This won't work anyway. We are talking about language grammar here. If
 made expression, statement would be of type void. Just like assert is.

Says who? :)

OK, make statement expression could open some door. But here isn't the point of my question. I don't want to make any proposal without being sure of why the decision was made in a first place.

Fair enough. -- /Jacob Carlborg
May 08 2012
prev sibling parent Stewart Gordon <smjg_1998 yahoo.com> writes:
On 07/05/2012 21:27, Nick Sabalausky wrote:
<snip>
 I'm usually fairly ambivalent about the idea of statements being
 expressions, but I would *love* for switch to be usable as an expression.

Switch cases are sequences of statements. Allowing them to alternatively be expressions would create ambiguity in the grammar. If you're designing a language and want switch expressions as well as switch statements, then don't try to make one look like the other. For example, C didn't try to make conditional expressions look like if statements - it created a new syntax for conditional expressions. That said, it's possible to design a language to have no distinction between expressions and statements. But you can't really conflate the two syntaxes in an existing language. Of course, D could have been designed as such a language. But it wasn't. It was designed to have the same overall look and feel as C. Stewart.
Jul 15 2012
prev sibling next sibling parent "David Piepgrass" <qwertie256 gmail.com> writes:
 I'm usually fairly ambivalent about the idea of statements 
 being
 expressions, but I would *love* for switch to be usable as an 
 expression.
 For instance, in Haxe, you can do stuff like the following, 
 which I get a
 ton of use out of and often wish D had:

 a = switch(b)
 {
     case 1: "foo";
     case 2: "bar";
     case 3: "baz";
     case 4: "whee";
     default: "blork";
 }

 The D equivalents aren't terrible, but they aren't nearly as 
 nice.

This won't work anyway. We are talking about language grammar here. If made expression, statement would be of type void. Just like assert is.

I see what you're saying, but this switch expression should really be of type string. I certainly wish more things were expressions. "a = if (x) y; else z;" isn't especially useful since we have "a = x ? y : z", but consider instead something that doesn't map so easily to an expression: // very loosely based on some Android code I wrote recently dpWidth = _lastKnownWidth = if (window.isVisible()) { auto m = context.getResources().getSystemMetrics(); // final statement as value of "if" expr window.getWidth() / m.pixelDensity(); } else if (_lastKnownWidth != 0) _lastKnownWidth; else screenInfo().getWidth(); Or how about: auto area = { auto tmp = foo.bar(baz); tmp.width * tmp.height; } I also wish "void" were a first-class type with sizeof==0 for maximum efficiency: int[void] intSet = [2:(), 3:(), 4:()] Ditto for size of empty structs. D code should never need abominations like the C++ EBCO.
Jul 08 2012
prev sibling next sibling parent "David Piepgrass" <qwertie256 gmail.com> writes:
     int[void] intSet = [2:(), 3:(), 4:()]

Jul 08 2012
prev sibling parent "bearophile" <bearophileHUGS lycos.com> writes:
David Piepgrass:

 Or how about:

 auto area = {
     auto tmp = foo.bar(baz);
     tmp.width * tmp.height;
 }

Rust is like that, but I don't see it as a significant improvement.
 I also wish "void" were a first-class type with sizeof==0 for 
 maximum efficiency:

     int[void] intSet = [2:(), 3:(), 4:()]

I prefer: auto intSet = hashSet([2, 3, 4]);
 Ditto for size of empty structs.

There are reasons for D empty structs to be of 1 byte.
 D code should never need abominations like the C++ EBCO.

In D classes are never really empty (2 words is their minimum size, not counting GC bookkeeping), and there is no normal struct inheritance. Bye, bearophile
Jul 15 2012
prev sibling next sibling parent reply Don Clugston <dac nospam.com> writes:
On 07/05/12 19:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void. Why not all statement (that are not expression
 already) are expression ?

assert isn't a statement. It's an expression ( same as is() ). What makes you think it's a statement? The main use for a void expression is so that it can be used in a comma expression; this is why assert is an expression. The curious thing, which may be the source of the confusion, is that static assert() is a statement, while assert() is an expression. Maybe static assert should also be an expression rather than a statement?
May 08 2012
next sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
On 05/08/2012 10:46 AM, Don Clugston wrote:
 On 07/05/12 19:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void. Why not all statement (that are not expression
 already) are expression ?

assert isn't a statement. It's an expression ( same as is() ). What makes you think it's a statement? The main use for a void expression is so that it can be used in a comma expression; this is why assert is an expression. The curious thing, which may be the source of the confusion, is that static assert() is a statement, while assert() is an expression. Maybe static assert should also be an expression rather than a statement?

static assert is a declaration.
May 08 2012
prev sibling parent "Jakob Ovrum" <jakobovrum gmail.com> writes:
On Tuesday, 8 May 2012 at 18:30:08 UTC, Timon Gehr wrote:
 On 05/08/2012 10:46 AM, Don Clugston wrote:
 On 07/05/12 19:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, 
 are
 expression of type void. Why not all statement (that are not 
 expression
 already) are expression ?

assert isn't a statement. It's an expression ( same as is() ). What makes you think it's a statement? The main use for a void expression is so that it can be used in a comma expression; this is why assert is an expression. The curious thing, which may be the source of the confusion, is that static assert() is a statement, while assert() is an expression. Maybe static assert should also be an expression rather than a statement?

static assert is a declaration.

It's a statement when used inside functions, and a "declaration" (DeclarationDefinition seems to be the spec's term for top-level constructs) elsewhere. Other syntactic constructs have a similar duality, e.g. import. Everything inside a function is a statement, including variable declarations; they are declaration statements.
May 08 2012
prev sibling next sibling parent deadalnix <deadalnix gmail.com> writes:
Le 07/05/2012 19:06, deadalnix a crit :
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void. Why not all statement (that are not expression
 already) are expression ?

I want to pop this question. This is an important one. And nothing is documented about that.
May 09 2012
prev sibling parent reply Stewart Gordon <smjg_1998 yahoo.com> writes:
On 07/05/2012 18:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are expression of
type void.
 Why not all statement (that are not expression already) are expression ?

Because it wouldn't make sense. The semantic of an ExpressionStatement is to evaluate the expression. Creating a variable in the scope in which an expression appears, and controlling the flow of the function in which an expression appears, are beyond the scope of the expression evaluation mechanism. To take an example, what sense would it make if return 42 were an expression? What would its type be? You would be able to do something like auto qwert() { return return 105; } to instantly return from the function that called qwert. OK, so assert, or exception throwing more generally, feels like that kind of thing. But exceptions aren't part of the normal flow of program logic. Exceptions can be, and indeed usually are, thrown in the process of evaluating an expression. AssertExpression is an example of this. Though this does suggest an answer of "because it can be" for why an assert is an expression. Indeed, you could well ask why an assert is an expression but a throw isn't. Maybe it was thought worth convoluting assert to make for concise code by including it in a CommaExpression - but not worth convoluting throw in the same way, since a throw throws unconditionally. Not that it would be much more complicated to make throw an expression. But you could always define a function of your own like void throwEx(Throwable t) { throw t; } Stewart.
May 09 2012
parent deadalnix <deadalnix gmail.com> writes:
Le 09/05/2012 14:11, Stewart Gordon a crit :
 On 07/05/2012 18:06, deadalnix wrote:
 Hi,

 Working on D I noticed that some statement, notably assert, are
 expression of type void.
 Why not all statement (that are not expression already) are expression ?

Because it wouldn't make sense. The semantic of an ExpressionStatement is to evaluate the expression. Creating a variable in the scope in which an expression appears, and controlling the flow of the function in which an expression appears, are beyond the scope of the expression evaluation mechanism. To take an example, what sense would it make if return 42 were an expression?

The same as currently.
 What would its type be?

void
 You would be able to do
 something like
 auto qwert() {
 return return 105;
 }
 to instantly return from the function that called qwert.

No it is incorrect. but not grammatically, semantically. Just like 3 + foo() is incorrect if foo's return type is void. It is grammatically correct.
 OK, so assert, or exception throwing more generally, feels like that
 kind of thing.

They don't feel like. It is specified (and even implemented, look in dmd source code) that assert is an expression for instance. Expression that is of type void (and so have no value). As you are unable to explain why such a thing exist, I really doubt you are qualified to answer that question at all. I'm asking here about design decisions. If you have no clue what I'm talking about please do not answer, or ask for more details.
May 09 2012