[Haskell-cafe] Haskell Data Structure design

[Will Yager]

I believe you could use 

foldM (\sum student -> (sum +) <$> totalFeesOwed student) 0 students

Will

> On Jul 9, 2016, at 15:09, Guru Devanla <gurudev.devanla at gmail.com> wrote:
> 
> OK. Thank you. That is what I ended up doing after I understood how mapM worked. But, since I was going through 2 steps, I was wondering if using foldM directly was possible in this case.
> 
> 
>> On Sat, Jul 9, 2016 at 11:25 AM, Will Yager <will.yager at gmail.com> wrote:
>> fees <- mapM totalFeesOwed students
>> let total = sum fees
>> 
>> You can use a fold instead of sum if you want. 
>> 
>> Will
>> 
>>> On Jul 9, 2016, at 13:10, Guru Devanla <gurudev.devanla at gmail.com> wrote:
>>> 
>>> Say, in the above example, I want to add up values returned by  `student_totalFeesOwed`  by using foldM operation.  Is it possible?  
>>> 
>>> For example, here is an expression I have
>>> 
>>> L.foldr (\a  b->  (evalState (student_totalFeesOwed a) $ env) + b) 0 [(RowId 1), (RowId 2)] 
>>> 
>>>> On Sat, Jul 9, 2016 at 9:15 AM, Will Yager <will.yager at gmail.com> wrote:
>>>> I did the same thing when I was learning to generalize my understanding of monads! Very common mistake. 
>>>> 
>>>> I'm not sure I understand your question about #3. Can you give an example using evalState? We'll tell you if you can do it without evalState. 
>>>> 
>>>> I suspect you want something like
>>>> 
>>>> "mapM_ addStudentFee students"
>>>> 
>>>> Will
>>>> 
>>>>> On Jul 9, 2016, at 00:56, Guru Devanla <gurudev.devanla at gmail.com> wrote:
>>>>> 
>>>>> William/Tom,
>>>>> 
>>>>> (1)  Yes, looking into lens and re-factoring my current experimental project in lens will be my next iteration. For now, I plan not to spend time on it.
>>>>> 
>>>>> (2)  Agreed.  Not sure how I missed that.
>>>>> 
>>>>> (3) I see how foldM works now.  I missed the point that foldM not only is a `map` but also does a `sequence` after that.  I got stuck earlier, thinking I will end up with a list of state monads. The sequence steps executes this monadic action.
>>>>>  
>>>>> But, how can I do a foldM in a state monad. Say, I need to map over a list of students and add up all their fees, can I get away not `evalState` inside the foldM step function?
>>>>> 
>>>>> Thanks. this is very exciting as I keep simplifying my code!
>>>>> 
>>>>> Guru
>>>>> 
>>>>> 
>>>>>  
>>>>> 
>>>>>> On Fri, Jul 8, 2016 at 7:55 PM, <amindfv at gmail.com> wrote:
>>>>>> 
>>>>>>> On Fri, Jul 8, 2016 at 9:57 PM, Guru Devanla <gurudev.devanla at gmail.com> wrote:
>>>>>> 
>>>>>> 
>>>>>>>> 1.  I see that almost in every function I deal with state, I have e <- get , expression in the begining. I always ending up having to use the state to query for different values. I guess this is OK.
>>>>>> 
>>>>>> 
>>>>>> El 8 jul 2016, a las 22:07, William Yager <will.yager at gmail.com> escribió:
>>>>>> 
>>>>>>> For #1, look into using the Lens library's support for the State monad. You can often avoid doing a get, and instead write things like `fees += 5`, which will add 5 to the field in the state called "fees".
>>>>>> 
>>>>>> Lens is a pretty heavy extra thing for a beginner to have to learn -- you'll do fine with the 'modify' function:
>>>>>> 
>>>>>> modify :: (s -> s) -> State s ()
>>>>>> 
>>>>>> So instead of writing:
>>>>>> 
>>>>>> do
>>>>>>    s <- get
>>>>>>    put (s + 5)
>>>>>> 
>>>>>> You say:
>>>>>> 
>>>>>> modify (+5)
>>>>>> 
>>>>>> 
>>>>>> Tom
> 
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