Re: The state of functional programming

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 29 Jul 2010 19:19:49 +0000 (UTC), Warren wrote:

> Dmitry A. Kazakov expounded in
> news:a3iznu9uq49d$.1m9cupr81yhut$.dlg@40tude.net: 
> 
>> On Thu, 29 Jul 2010 15:20:48 +0000 (UTC), Warren wrote:
> ..
>>> I think you missed my point - perhaps it wasn't expressed
>>> clearly.
>>> 
>>> As I understand it, a FP tries to determine conclusions 
>>> from a universe of facts, given some inputs. For smaller
>>> problems this can be _exhaustively_ analyzed and results 
>>> obtained.
>> 
>> And so does any declarative language. You declare some facts in
>> whatever form (as relations, as connections of blocks etc, for that
>> matter, as types in a strongly typed languages like Ada). The system
>> infers from them some executable code.
> 
> No, there is a big difference here.
> 
> In a non-FP language (Ada), you can solve _any_ problem so long
> as you code it (you are coding the "how"). 

Not quite. "How" need to be translated into the Ada code first. In some
cases it is not simple or even impossible.

> IOW, you have solved the problem and specified it in code.

No difference here. Any code is a language (Ada language, machine language,
the language of differential equations and so on). The FP code is as code
as Ada code is. The actual problem is that Ada code can be in most cases
effectively translated into machine code (and later into electrical
signals), while for FP code it is much more difficult. There are also other
problems related to maintainability. To be readable and maintainable the
code must be easily translated back into "how". I.e. the programmer must
understand what the code does. Arguably, FP code is farther removed from
programmer's "how" than Ada code.

> In FP, you define the "problem" (instead) and require from 
> it a solution.

Rather you declare a solution. This is how declarative paradigm works. 

(Don't forget that Ada has a declarative parts as well. You declare types
for example, and ask the compiler to solve "range 0..100".)

> But FP cannot always solve that "problem".

Same in Ada. Not every legal Ada program is compilable. I doubt that there
is a higher language where any legal program is compilable. The difference
is in relation, not in principle. Non-compilable Ada programs are less
interesting and numerous than their FP counterparts.

But my point was that this is characteristic for *all* declarative
languages. I think that no language can be 100% declarative. You can have
some well defined parts declarative, as in Ada, but making everything
declarative is like flying faster than light, or going beyond Turing
completeness, name it. Even if that were possible, we wound not be able to
understand what these programs do.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de