Re: [Haskell-cafe] Haskell Data Structure design

The State monad makes a lot of sense for this. I was initially hesitant to go down this path *fearing* monads. But, today I was able to change most of my code to work with the same pattern you provided. Also, my initial impression on State monads was that, it was not a good idea to carry a *big blob* of State around. That impression comes from the thought process influenced by imperative programming. After coding up this, it is a lot clear that State monad declares operations and it is not the `state` itself that is carried around. I am elated! Thank you for the help. I may have more questions as I progress down this path. Thanks Guru On Tue, Jul 5, 2016 at 10:30 PM, Michael Burge wrote:

The implicit parameter approach is best if the environment never changes, or at least doesn't change during the computation You can rebind the variable in the middle of a computation, but it's not a good road to go down.

The easiest way to simulate a changing environment is to use the State monad. There are other techniques: lenses, nested patterns, rebinding an implicit parameter, ST monad, generating a list of changes and applying the changes to the original state, etc. But - despite having to change your syntax somewhat - I think you'll find it easiest to use a state monad to manage this.

Here's a somewhat verbose example of using State to track updates. You can make it less verbose, but I chose to keep it simple. In this example, it updates student_feesOwed as part of registering for a class. So we no longer need to calculate anything: It just grabs the value off of the Student.

import Control.Applicative import Control.Monad.Trans.State.Strict import Data.Monoid

import qualified Data.IntMap as M

newtype RowId a = RowId Int deriving (Eq)

data Classroom = Classroom { classroom_id :: RowId Classroom, classroom_extraFees :: Float, classroom_students :: [ RowId Student ] } data Student = Student { student_id :: RowId Student, student_name::String, student_feesOwed::Float}

data Environment = Environment { environment_classroom :: Maybe Classroom, environment_students :: M.IntMap Student }

student_totalFeesOwed :: RowId Student -> State Environment Float student_totalFeesOwed (RowId studentId) = do (Environment mClassroom students) <- get case mClassroom of Nothing -> return 0.0 Just classroom -> do let fees = student_feesOwed $ students M.! studentId return fees

student_addFee :: RowId Student -> Float -> State Environment () student_addFee studentId fee = do modify $ \e -> e { environment_students = M.map (addFee studentId fee) $ environment_students e } where addFee studentId fee student = if studentId == student_id student then student { student_feesOwed = student_feesOwed student + fee } else student

environment_addStudent :: Student -> State Environment () environment_addStudent student = do let (RowId key) = student_id student value = student modify $ \e -> e { environment_students = M.insert key value (environment_students e) }

classroom_addStudent :: Classroom -> RowId Student -> State Environment () classroom_addStudent classroom studentId = do modify $ \e -> e { environment_classroom = addStudent studentId <$> environment_classroom e } where addStudent :: RowId Student -> Classroom -> Classroom addStudent studentId classroom = classroom { classroom_students = studentId : (classroom_students classroom) }

student_registerClass :: RowId Student -> Classroom -> State Environment () student_registerClass studentId classroom = do student_addFee studentId (classroom_extraFees classroom) modify $ \e -> e { environment_classroom = Just classroom } classroom_addStudent classroom studentId

main = do let studentId = RowId 1 student = Student studentId "Bob" 250.00 classroom = Classroom (RowId 1) 500.00 [] initialEnvironment = Environment Nothing mempty let totalFeesOwed = flip evalState initialEnvironment $ do environment_addStudent student student_registerClass studentId classroom totalFeesOwed <- student_totalFeesOwed studentId return totalFeesOwed putStrLn $ show totalFeesOwed

On Tue, Jul 5, 2016 at 9:44 PM, Guru Devanla wrote:

...

Hi Michael,

That is excellent. I read about Implicit parameters after reading your post. I like this approach better than Reader monad for my current use case. I wanted to stay away from Reader Monad given that this is my first experimental project and dealing with Reader Monads into levels of nested function calls involved lot more head-ache for me.

That said, I plan to try this approach and also see how I can enable this set up in my HUnit tests as well.

One other question, I have regarding this design is as follows: Say, during the progress of the computation, the `student_feesOwed` changes, and therefore we have a new instance of classroom with new instance of student in it (with the updated feesOwed). I am guessing, this would mean, wrapping up this new instance into the environment from there on and calling the subsequent functions. Is that assumption, right. Nevertheless, I will play with approach tomorrow and report back!

Thanks Guru

On Tue, Jul 5, 2016 at 7:18 PM, Michael Burge wrote:

...

When I have functions that are pure but depend on some common state(say in a config file, or retrieved from a database at startup), I like to use implicit parameters to hide it. You can use a type alias to avoid it cluttering up most signatures. Below, a value of type 'Environmental Float' means 'A float value, dependent on some fixed environment containing all students and the single unique classroom'. If you have a deep chain of 'Environmental a' values, the implicit parameter will be automatically propagated to the deepest parts of the expression.

You could also use a Reader monad, but they seem to require more invasive syntactic changes: They are better if you later expect to need other monads like IO, but if you're just doing calculations they're overkill. You could also define a type alias 'Environmental a = Environment -> a', but then if you have multiple such states they don't compose well(they require you to apply the implicit state in the correct order, and it can be a little awkward to propagate the parameter).

Here's how I would start to structure your example in a larger project:

{-# LANGUAGE ImplicitParams,RankNTypes #-}

import qualified Data.IntMap as M

newtype RowId a = RowId Int

data Classroom = Classroom { classroom_id :: RowId Classroom, classroom_extraFees :: Float, classroom_students :: [ RowId Student ] } data Student = Student { student_id :: RowId Student, student_name::String, student_feesOwed::Float}

data Environment = Environment { environment_classroom :: Classroom, environment_students :: M.IntMap Student }

type Environmental a = (?e :: Environment) => a

classroom :: (?e :: Environment) => Classroom classroom = environment_classroom ?e

students :: (?e :: Environment) => M.IntMap Student students = environment_students ?e

student_totalFeesOwed :: RowId Student -> Environmental Float student_totalFeesOwed (RowId studentId) = classroom_extraFees classroom + (student_feesOwed $ students M.! studentId)

main = do let student = Student (RowId 1) "Bob" 250.00 let ?e = Environment { environment_classroom = Classroom (RowId 1) 500.00 [ RowId 1 ], environment_students = M.fromList [ (1, student) ] } putStrLn $ show $ student_totalFeesOwed $ RowId 1

On Tue, Jul 5, 2016 at 6:26 PM, Guru Devanla wrote:

...

Hello All,

I am just getting myself to code in Haskell and would like to design advice. Below, I have a made up example:

data ClassRoom = ClassRoom { classRoomNo:: Integer, extra_fees::Float, students: Map StudentId Student} data Student = Student {name::String, feesOwed::Float} data StudentId = Integer

get_fees_owed classroom student_id = extra_fees + feesOwed $ (students classroom) M.! studentid

Here the `get_fees_owed` needs information from the container 'classroom'.

Here is my question/problem:

I believe I should model most of my code as expressions, rather than storing pre-computed values such as `fees_owed`. But, defining expressions involve passing the container objects all over. For example, deep down in a function that deals with just one `student`, I might need the fees owed information. Without, having a reference to the container, I cannot call get_fees_owed.

Also, I think it hinders composing of functions that just deal with one student at a time, but end up with some dependency on the container.

I have several questions related to this design hurdle, but I will start with the one above.

Thanks! Guru

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