[Haskell-cafe] Haskell Data Structure design

[Will Yager]

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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