Stream programming

Kiuhnm kiuhnm03.4t.yahoo.it
Sat Mar 24 01:41:38 CET 2012


On 3/24/2012 0:32, Ray Song wrote:
> On Fri, Mar 23, 2012 at 05:00:23PM +0100, Kiuhnm wrote:
>> I've been writing a little library for handling streams as an excuse for
>> doing a little OOP with Python.
>>
>> I don't share some of the views on readability expressed on this ng.
>> Indeed, I believe that a piece of code may very well start as complete
>> gibberish and become a pleasure to read after some additional
>> information is provided.
>>
>> I must say that imposed indentation is a pain when one is trying to
>> define some sort of DSL (domain specific language). Also, Python's
>> operator overloading is a bit limited, but that makes for a more
>> rewarding experience in my case.
>>
>> Here's an example of what you can write:
>>
>> numbers - push - avrg - 'med' - pop - filter(lt('med'), ge('med'))\
>>       - ['same', 'same'] - streams(cat) - 'same'
>>
>> Ok, we're at the "complete gibberish" phase.
>>
>> Time to give you the "additional information".
>>
>> I will use "<=>" to mean "is equivalent to". That's not part of the DSL.
>> A flow has one or more streams:
>>     1 stream:
>>       [1,2,3]
>>     2 streams:
>>       [1,3,5] | [2,4,6]
>> Two flows can be concatenated:
>>     [1,2,3] + [4,5,6]<=>  [1,2,3,4,5,6]
>>     [0] + ([1,2] | [3,4]) + [10]<=>  [0,1,2,10] | [0,3,4,10]
>>     ([1,2] | [10,20]) + ([3,4] | [30,40])<=>  [1,2,3,4] | [10,20,30,40]
>> A flow can be transformed:
>>     [1,2] - f<=>  [f(1),f(2)]
>>     ([1,2] | [3,4]) - f<=>  [f(1,3),f(2,4)]
>>     ([1,2] | [3,4]) - [f]<=>  [f(1),f(2)] | [f(3),f(4)]
>>     ([1,2] | [3,4]) - [f,g]<=>  [f(1),f(2)] | [g(3),g(4)]
>>     [1,2] - [f,g]<=>  [f(1),f(2)] | [g(1),g(2)]
>> Some functions are special and almost any function can be made special:
>>     [1,2,3,4,5] - filter(isprime)<=>  [2,3,5]
>>     [[],(1,2),[3,4,5]] - flatten<=>  [1,2,3,4,5]
>> Note that 'filter' is not really necessary, thanks to 'flatten'.
>> Flows can be named, remembered and used
>>     as a value:
>>       [1,2,3,4,5] - 'flow' + val('flow')<=>  [1,2,3,4,5]*2
>>     as a transformation chain:
>>       [1,2,3] - skipfirst - 'again' | [4,5,6] - func('again')
>>         <=>  [2,3] | [5,6]
>>     Recursion is also possible and stops when a function is applied to an
>> empty sequence.
>> Flows can be saved (push) and restored (pop) :
>>     [1,2,3,4] - push - by(2) - 'double' - pop | val('double')
>>         <=>  [1,2,3,4] | [2,4,6,8]
>> There are easier ways to achieve the same result, of course:
>>     [1,2,3,4] - [id, by(2)]
>>
>> Let's go back to our example. I didn't tell you anything but you should
>> be able to understand it anyway.
>>
>> numbers - push - avrg - 'med' - pop - filter(lt('med'), ge('med'))\
>>       - ['same', 'same'] - streams(cat) - 'same'
>>
>> It reads as
>>
>> "take a list of numbers - save it - compute the average and named it
>> 'med' - restore the flow - create two streams which have, respect., the
>> numbers less than 'med' and those greater or equal to 'med' - do the
>> /entire/ 'same' process on each one of the two streams - concat the
>> resulting streams - name all this /entire/ process 'same'.
>> Not readable enough? Replace 'same' with 'qsort'.
>>
>> Is that readable or am I going crazy? [note: that's a rhetorical
>> question whose answer is "That's very readable!"]
>>
>
> This sounds like a concatenative programming and i've found many cool
> examples from Haskell and Ruby. I was pleased with Python's rigid
> off-side rule at first but frustrated when I realized that this is a
> two-edged blade as the rule makes a DSL difficult.

Yes, there is that. I also noticed that I couldn't use the comparison 
operators the way I liked.
Precedence is also an issue. I had to use '+' and '-' instead of '+' and 
'>>' or '>' because of that. I didn't want to introduce extra parentheses.

> Sorry for former mail to you (my damn Android gmail client doesn't
> understand mailing lists well).

Don't worry, my email address is invalid!

Kiuhnm



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