Will Yager will.yager at gmail.com
Sat Jul 9 18:25:52 UTC 2016

```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
>>
>>
>> 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> escribiÃ³:
>>>>
>>>>> 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 ()
>>>>
>>>>
>>>> do
>>>>    s <- get
>>>>    put (s + 5)
>>>>
>>>> You say:
>>>>
>>>> modify (+5)
>>>>
>>>>
>>>> Tom
>
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