Thread overview  


October 07, 2010 Tuple assignment  

 
If expr represents a tuple, we (Andrei and I) were thinking about the syntax: auto (a, b, c, d) = expr; being equivalent to: auto t = expr; auto a = t[0]; auto b = t[1]; auto c = t[2 .. $]; You can also do this with arrays, such that: float[3] xyz; auto (x, y, z) = xyz; The Lithpers among you will notice that this essentially provides a handy car,cdr shortcut for tuples and arrays: auto (car, cdr) = expr; 
October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Walter Bright Attachments:
 On Wed, 20101006 at 23:08 0700, Walter Bright wrote: > If expr represents a tuple, we (Andrei and I) were thinking about the syntax: > > auto (a, b, c, d) = expr; > > being equivalent to: > > auto t = expr; auto a = t[0]; auto b = t[1]; auto c = t[2 .. $]; > > You can also do this with arrays, such that: > > float[3] xyz; > auto (x, y, z) = xyz; > > The Lithpers among you will notice that this essentially provides a handy car,cdr shortcut for tuples and arrays: > > auto (car, cdr) = expr; Python may be the best base to compare things to as tuple assignment has been in there for years. Pythons choice is not a car/cdr approach but an exact match approach. so if t represents a tuple datum or a function returning a tuple: x = t then x is a tuple  remembering that variables are all just references to objects implemented via keys in a dictionary, and: a , b , c = t or ( a , b , c ) = t is tuple assignment where now t is required to be a tuple of length 3. cf. > python Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41) [GCC 4.4.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> t = ( 1 , 'fred' , 2.0 ) >>> x = t >>> print x (1, 'fred', 2.0) >>> a , b , c = t >>> print a , b , c 1 fred 2.0 >>> a , b = t Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: too many values to unpack >>> a , b , c , d = t Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: need more than 3 values to unpack >>>  Russel. ============================================================================= Dr Russel Winder t: +44 20 7585 2200 voip: sip:russel.winder@ekiga.net 41 Buckmaster Road m: +44 7770 465 077 xmpp: russel@russel.org.uk London SW11 1EN, UK w: www.russel.org.uk skype: russel_winder 
October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Russel Winder  Russel Winder wrote:
> Python may be the best base to compare things to as tuple assignment has
> been in there for years.
>
> Pythons choice is not a car/cdr approach but an exact match approach.
> so if t represents a tuple datum or a function returning a tuple:
>
> x = t
>
> then x is a tuple  remembering that variables are all just references
> to objects implemented via keys in a dictionary, and:
>
> a , b , c = t
> or
> ( a , b , c ) = t
>
> is tuple assignment where now t is required to be a tuple of length 3.
> cf.
The first thought was to make it an exact match approach. Andrei thought that the car/cdr one was better, though, and I find it intuitively appealing, too. Perhaps Python missed an important use case?
Or perhaps the ambiguity as to whether the last item gets to be a value or another tuple is too much.

October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Russel Winder  Russel Winder wrote: > Python may be the best base to compare things to as tuple assignment has > been in there for years. Too segue this into the previous thread, how does Python treat (1)? Is it a floor wax or a dessert topping? http://www.nbc.com/saturdaynightlive/video/shimmerfloorwax/1056743/ 
October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Russel Winder  On Thu, 07 Oct 2010 10:43:18 +0400, Russel Winder <russel@russel.org.uk> wrote:
> On Wed, 20101006 at 23:08 0700, Walter Bright wrote:
>> If expr represents a tuple, we (Andrei and I) were thinking about the syntax:
>>
>> auto (a, b, c, d) = expr;
>>
>> being equivalent to:
>>
>> auto t = expr; auto a = t[0]; auto b = t[1]; auto c = t[2 .. $];
>>
>> You can also do this with arrays, such that:
>>
>> float[3] xyz;
>> auto (x, y, z) = xyz;
>>
>> The Lithpers among you will notice that this essentially provides a handy
>> car,cdr shortcut for tuples and arrays:
>>
>> auto (car, cdr) = expr;
>
>
> Python may be the best base to compare things to as tuple assignment has
> been in there for years.
>
> Pythons choice is not a car/cdr approach but an exact match approach.
> so if t represents a tuple datum or a function returning a tuple:
>
> x = t
>
> then x is a tuple  remembering that variables are all just references
> to objects implemented via keys in a dictionary, and:
>
> a , b , c = t
> or
> ( a , b , c ) = t
>
> is tuple assignment where now t is required to be a tuple of length 3.
> cf.
>
>
> > python
> Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41)
> [GCC 4.4.3] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> t = ( 1 , 'fred' , 2.0 )
> >>> x = t
> >>> print x
> (1, 'fred', 2.0)
> >>> a , b , c = t
> >>> print a , b , c
> 1 fred 2.0
> >>> a , b = t
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> ValueError: too many values to unpack
> >>> a , b , c , d = t
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> ValueError: need more than 3 values to unpack
> >>>
>
>
That's because Python is not a strictly typed language. With proper type propagation compiler helps you writing code the way in meant to be. E.g. the following:
(a, b, c, d) = ('tuple', 'of', 'three')
could be statically disabled, but there is nothing wrong with allowing it either: d would be just a noop, you will know it for sure the moment you try using it.

October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Walter Bright  On 10/6/2010 11:58 PM, Walter Bright wrote:
> Russel Winder wrote:
>> Python may be the best base to compare things to as tuple assignment has been in there for years.
>>
>> Pythons choice is not a car/cdr approach but an exact match approach. so if t represents a tuple datum or a function returning a tuple:
>>
>> x = t
>>
>> then x is a tuple  remembering that variables are all just references to objects implemented via keys in a dictionary, and:
>>
>> a , b , c = t
>> or
>> ( a , b , c ) = t
>>
>> is tuple assignment where now t is required to be a tuple of length 3. cf.
>
> The first thought was to make it an exact match approach. Andrei thought that the car/cdr one was better, though, and I find it intuitively appealing, too. Perhaps Python missed an important use case?
>
> Or perhaps the ambiguity as to whether the last item gets to be a value or another tuple is too much.
I think the ambiguity should be avoided. There was one language I used ages ago that used a token to signal the use of the last arg as a 'rest' usage. If I remember right, it used:
(a, @b) = aggregate; // a = aggregate[0], b = aggregate[1..$]
It also allowed: (a, @aggregate) = aggregate; // essentially a pop operation.
That said, it was a weakly typed language, so it's application to D has to be taken with an appropriate dose of salt. For D, I think using the @ would clash badly with the attribute syntax, so an alternative that's not horrid:
(a, b...) = aggregate;
Later,
Brad

October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Brad Roberts  On Thu, 07 Oct 2010 11:42:06 +0400, Brad Roberts <braddr@puremagic.com> wrote:
> On 10/6/2010 11:58 PM, Walter Bright wrote:
>> Russel Winder wrote:
>>> Python may be the best base to compare things to as tuple assignment has
>>> been in there for years.
>>>
>>> Pythons choice is not a car/cdr approach but an exact match approach.
>>> so if t represents a tuple datum or a function returning a tuple:
>>>
>>> x = t
>>>
>>> then x is a tuple  remembering that variables are all just references
>>> to objects implemented via keys in a dictionary, and:
>>>
>>> a , b , c = t
>>> or
>>> ( a , b , c ) = t
>>>
>>> is tuple assignment where now t is required to be a tuple of length 3.
>>> cf.
>>
>> The first thought was to make it an exact match approach. Andrei thought that
>> the car/cdr one was better, though, and I find it intuitively appealing, too.
>> Perhaps Python missed an important use case?
>>
>> Or perhaps the ambiguity as to whether the last item gets to be a value or
>> another tuple is too much.
>
> I think the ambiguity should be avoided. There was one language I used ages ago
> that used a token to signal the use of the last arg as a 'rest' usage. If I
> remember right, it used:
>
> (a, @b) = aggregate; // a = aggregate[0], b = aggregate[1..$]
>
> It also allowed: (a, @aggregate) = aggregate; // essentially a pop operation.
>
> That said, it was a weakly typed language, so it's application to D has to be
> taken with an appropriate dose of salt. For D, I think using the @ would clash
> badly with the attribute syntax, so an alternative that's not horrid:
>
> (a, b...) = aggregate;
>
> Later,
> Brad
Interesting idea, I like it!

October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Russel Winder  On 10/7/10 1:43 CDT, Russel Winder wrote:
> On Wed, 20101006 at 23:08 0700, Walter Bright wrote:
>> If expr represents a tuple, we (Andrei and I) were thinking about the syntax:
>>
>> auto (a, b, c, d) = expr;
>>
>> being equivalent to:
>>
>> auto t = expr; auto a = t[0]; auto b = t[1]; auto c = t[2 .. $];
>>
>> You can also do this with arrays, such that:
>>
>> float[3] xyz;
>> auto (x, y, z) = xyz;
>>
>> The Lithpers among you will notice that this essentially provides a handy
>> car,cdr shortcut for tuples and arrays:
>>
>> auto (car, cdr) = expr;
>
>
> Python may be the best base to compare things to as tuple assignment has
> been in there for years.
>
> Pythons choice is not a car/cdr approach but an exact match approach.
So then we'd have the proposed notation not work with dynamic arrays  only with static arrays and tuples.
Andrei

October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Walter Bright  On 10/07/2010 09:03 AM, Walter Bright wrote:
> Russel Winder wrote:
>> Python may be the best base to compare things to as tuple assignment has
>> been in there for years.
>
> Too segue this into the previous thread, how does Python treat (1)? Is
> it a floor wax or a dessert topping?
>
> http://www.nbc.com/saturdaynightlive/video/shimmerfloorwax/1056743/
(1) == 1
(1,) == tuple([1])

October 07, 2010 Re: Tuple assignment  

 
Posted in reply to Walter Bright  On 10/07/2010 08:08 AM, Walter Bright wrote:
> If expr represents a tuple, we (Andrei and I) were thinking about the
> syntax:
>
> auto (a, b, c, d) = expr;
>
> being equivalent to:
>
> auto t = expr; auto a = t[0]; auto b = t[1]; auto c = t[2 .. $];
>
> You can also do this with arrays, such that:
>
> float[3] xyz;
> auto (x, y, z) = xyz;
>
> The Lithpers among you will notice that this essentially provides a
> handy car,cdr shortcut for tuples and arrays:
>
> auto (car, cdr) = expr;
Python 3 uses:
car, *cdr = expr
a, *b, c = [1,2,3,4,5] # leaves a=1, b=[2,3,4], c=5
I would like D to have
(car, cdr...) = expr
(a, b..., c) = [1,2,3,4,5]
for the equivalent.
Our varargs syntax is b..., theirs is *b. So it mirrors a bit, there. :)

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