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
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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