Grafting Functional Support on Top of an Imperative Language

[Koroskin Denis]

This reminds me 'The Power of None' presentation by Andrei Alexandrescu:

http://www.nwcpp.org/Meetings/2006/05.html
http://www.nwcpp.org/Downloads/2006/The_Power_of_None.ppt

On Sat, 05 Apr 2008 04:54:26 +0400, Simen Kjaeraas  
<simen.kjaras at gmail.com> wrote:

> On Sat, 05 Apr 2008 02:03:29 +0200, Bill Baxter  
> <dnewsgroup at billbaxter.com> wrote:
>
>> Simen Kjaeraas wrote:
>>> On Sat, 05 Apr 2008 01:09:14 +0200, Jason House  
>>> <jason.james.house at gmail.com> wrote:
>>>
>>>> bearophile wrote:
>>>>> P. 31: >writeln(i);<
>>>>>
>>>>> Can you put (temporary) debugging writeln/putr inside a pure  
>>>>> function?
>>>>
>>>> That's always bothered me about this stuff.  I don't want to lose
>>>> debugging/logging ability!
>>>   Just use multiple return types, and have the caller do the printing.
>>>  --Simen
>>
>>
>> Monads!
>>
>> Or at least I think that's what I read somewhere.  I can't understand  
>> the buggers for the life of me.  I think maybe it's just a fancy word  
>> for "loophole".  If someone here has a good explanation for what a  
>> monad is and how it allows mutable state in FP without making thing  
>> non-FP, I'd love to hear it.  Because I just don't get it.
>>
>> --bb
>
> I read a bit on Wikipedia about monads, and they seem to me to be a  
> fancy way to do multiple return values. I'm not sure if they're usable  
> (hackable) in D at the moment, or if we need to wait for AST macros  
> before we can use them as they should. But one example, the 'maybe'  
> monad, seems to me a bit like this:
>
> struct Maybe(T)
> {
>    bool Nothing;
>    T data;
>
>    static Maybe!(U) opCall(U)(lazy U u)
>    {
>      typeof(return) tmp;
>
>      try
>      {
>        data = u();
>        tmp.Nothing = false;
>      }
>      catch (object o)
>      {
>        tmp.Nothing = true; // something went wrong, data is invalid
>      }
>    }
>
>    T opAdd(T rhs)
>    {
>      if (Nothing)
>        return rhs; // act as if data is nonexistant
>      else
>        return data + rhs;
>    }
>
>    // other operators overloaded in similar ways
> }
>
>
> Disclaimer: This might or might not work. It might contain some value of  
> truth, and it might not. It is merely my understanding after a half-hour  
> of reading at 2:30 in the morning, and so should be sanity-checked  
> before use.
>
> -- Simen