[Haskell-beginners] Doubts about functional programming paradigm

Tue Dec 15 17:00:06 UTC 2015

Hey guys,
This conversation is really interesting. I am not a haskell expert (I 
haven't progressed into monads in haskell yet, and have only briefly 
studied the state monad in scala. Studying fp concepts has changed the 
way I think about problems that are complicated. I am not quite there 
yet, (I still catch myself updating states when I don't need to) and 
wondering how the hell to break a problem into recursive little bits. I 
now notice that I am less tempted to do things that would have bugs in 
complicated data structures , prefer map to do something to a list, and 
folds to for loops that update a ``state'' variable, and finally, I can 
reason about where the weak points in my code may be. The learning I 
have achieved do to fp, and my principals of programming languages class 
that relied on fp, have made doing things with code easier because I 
don't try to do stupid things like overrun the end of an array with a 
misplaced loop counter variable. It is way too easy to learn fold, 
filter, map, and list comprehensions, you then have a powerful weapon to 
use against those ugly off by one errors.
Also, I learned how people who learned programming in non-traditional 
languages might think to approach problems earlier this year. My stats 
professor was showing us things in r. She showed us a complicated set of 
formula that we needed to do, and then explained we were calculating 
something to each element of a list. She showed a function 
sapply(vector, function(elem)) that returns a vector. She said to think 
about how sapply applies that function to each vector element, and 
returns the transformed list. She didn't approach it as if it were this 
big bad function that takes a function, mainly i think because she 
hadn't learned programming from people who insist on the idea of c. It 
also really made sense to the class who mainly had little to no 
programming experience, where explaining a for loop in all it's glory 
would normally take a couple of lectures. She is a really solid 
programmer and really understands how to use programming to solve real 
world problems, so I am not saying that she didn't know enough to have 
not learned for loops, just that she immediately realized that the 
sapply function really was better for the situation. If we teach people 
these patterns from the get go, I think some of the horror of learning 
functional programming would be solved because the number of 
applications that a generic function can be applied in far outnumbers 
the number of cases a for loop or state momad is needed. I would also 
now argue that all data structures classes should be taught in 
functional programming languages. I never solidly understood trees and 
could confidently traverse and change them until I actually got 
introduced to pattern matching and folds. I was taught binary search 
trees, and red-black trees, and worked with tree like structures, but it 
was always hard for me to comprehend how to do things with them. I 
learned linked lists in c++ but hated them with a passion because I had 
to write forloops that updated a temporary variable and write while 
loops that did the same, but often jumped off the end of the list and 
then I couldn't go back. The beauty of recursion and lists is that 
recursion allows you to backtrack when things go wrong (as they always 
will for any real input in a program). The second I learned about 
Haskell for the first time, linked list traversing became second nature 
to me, even in non-functional (inferier) c++. The argument that 
"recursion results in overriding the stack" is kind of a flawed one 
since the compiler wizards have figured out ways to optimize tail 
recursive functions to do exactly what we humans are bad at (running 
recursive functions as if they were unrolled, with large inputs, and fast).
Thanks,
Derek

On 12/11/2015 11:32 AM, Thomas Jakway wrote:
> Building on that, I think coming to Haskell with a very specific goal in mind (like swap Haskell for Java in my map reduce problem) kind of misses the point.  Haskell may or may not be faster/better suited to map reduce vs Java, but the real reason to use/learn Haskell is elegance and correctness.  The lack of side effects and referential transparency means you're far more likely to prevent bugs.  And there's a pretty substantial learning curve coming from imperative languages so if you need to speed up map reduce on a deadline you will be more productive in the imperative language of your choice (for now).
>
> Dont take this as discouragement, I think Haskell (and FP in general) is very well suited to that kind of problem.  I'm a beginner in Haskell and it's already had a huge impact on how I think about all the code I write, not just the occasional toy Haskell project.
>
> On Dec 11, 2015 1:08 PM, MJ Williams <matthewjwilliams101 at gmail.com> wrote:
>> A pure functional language enables you to reason about your code,
>> something you can't easily achieve with your average C or Java. And by
>> `reason' I am referring to mathematical proof. Haskell makes it very
>> simple, actually.  Why should you want to reason about your code?
>> Think the hassle you could avoid if you knew what your code really
>> meant and did when executed.
>>
>> The absence of side effects is part of another concept in FP, namely,
>> `referential transparency'.  If your function `f' maps a value `x' to
>> a value `y' then `f x' will always equal `y' and no more. In other
>> words, your function `f' won't change anything e.g. assign to
>> variables, or other state changes as well as mapping `x' to `y', and
>> that's an absolute certainty, in theory, at any rate.
>>
>> That's a very crude overview of at least part of what functional
>> programming is about.  I'm hoping it'll encourage others on this list
>> with far more in-depth knowledge of the subject matter to come in and
>> fill in the gaps and iron out the ambiguities.
>>
>> Matthew
>>
>>
>> On 11/12/2015, Daniel Bergey <bergey at alum.mit.edu> wrote:
>>> On 2015-12-11 at 10:07, Abhishek Kumar <abhishekkmr18 at gmail.com> wrote:
>>>> I am a beginner in haskell.I have heard a lot about haskell being great
>>>> for
>>>> parallel programming and concurrency but couldn't understand why?Aren't
>>>> iterative algorithms like MapReduce more suitable to run parallely?Also
>>>> how
>>>> immutable data structures add to speed?I'm having trouble understanding
>>>> very philosophy of functional programming, how do we gain by writing
>>>> everything as functions and pure code(without side effects)?
>>>> Any links or references will be a great help.
>>> Functional languages make it easy to decompose problems in the way that
>>> MapReduce frameworks require.  A few examples (fold is another name for
>>> reduce):
>>>
>>> sum :: [Double] -> Double
>>> sum xs = foldr (+) 0 xs
>>>
>>> sumSquares :: [Double] -> Double
>>> sumSquares xs = foldr (+) 0 (map (**2) xs)
>>>
>>> -- foldMap combines the map & fold steps
>>> -- The Monoid instance for String specifies how to combine 2 Strings
>>> -- Unlike numbers, there's only one consistent option
>>> unlines :: [Text] -> Text
>>> unlines xs = foldMap (`snoc` '\n') xs
>>>
>>> We'd need a few changes[1] to make this parallel and distribute across many
>>> computers, but expressing the part that changes for each new MapReduce
>>> task should stay easy.
>>>
>>> Immutable data by default helps with concurrency.  Speed may or may not be
>>> the goal.  We want to be able to distribute tasks (eg, function calls)
>>> across processor cores, and run them in different order, without
>>> introducing race conditions.
>>>
>>> Simon Marlow's book is great at explaining parallel & concurrent
>>> concepts, and the particular tools for applying them in Haskell:
>>> http://chimera.labs.oreilly.com/books/1230000000929
>>>
>>> bergey
>>>
>>> Footnotes:
>>> [1]  OK, many changes.
>>>
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    Derek Riemer

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