[Haskell-cafe] Haskell Data Structure design

[Will Yager]

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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