How to display hashmap in Haskell

Question

my task is to create menu with four choices: -add new students -show all students, -delete students by ID -close program. I used Hashmap for this problem, but the problem is that i have no idea how to display a current list of created students.

My code is presented below:

{module StudentApp where
import Data.HashMap.Strict
import System.Exit (exitSuccess)
data Student = Student {name::String, age::Int, id::String}
 deriving (Show, Eq, Read)
type Students = HashMap String Student
addStudent:: Students -> IO(Students)
addStudent students = do
 putStrLn ("Student Name")
 name<-getLine
 putStrLn("Student Age")
 age<-readLn :: IO Int
 putStrLn("Student ID")
 id<-getLine
 let s = Student name age id
 let updatedStudents = insert id s students
 return updatedStudents
showStudent:: Students -> IO (Students)
showStudent students = do
 print students 
 return students
deleteStudent:: Students -> IO (Students)
deleteStudent students = do
 putStrLn ("Give student ID to remove")
 id<-getLine
 let updatedStudents = delete id students
 return updatedStudents
menu :: Students -> IO ()
menu students = do
 putStrLn ("1. Add new student")
 putStrLn ("2. Show all students")
 putStrLn ("3. Delete Student by ID")
 putStrLn ("4. Exit program")
 choice<-getLine
 updatedStudents <- case choice of
 "1" -> addStudent students
 "2" -> showStudent students
 "3" -> deleteStudent students
 "4" -> exitSuccess --System.Exit.exitSuccess (zwraca typ IO students) 
 menu updatedStudents
main :: IO ()
main = do
 let students = empty :: Students
 menu students

}

Answer 1

Given that the keys k and the values v are instances of Show, then a HashMap is an instance of Show as well. Indeed in the source code, we see:

instance (Show k, Show v) => Show (HashMap k v) where
 showsPrec d m = showParen (d > 10) $
 showString "fromList " . shows (toList m)

So we can use print :: Show a => a -> IO () here to print the content of the HashMap. Like:

Prelude Data.HashMap.Strict> print (fromList [("id001", Student "Foo" 21 "id001"), ("id003", Student "Bar" 19 "id003")])
fromList [("id003",Student {name = "Bar", age = 19, id = "id003"}),("id001",Student {name = "Foo", age = 21, id = "id001"})]

So here print contains as argument a HashMap String Student we constructed, and this will print the content of the dictionary.

You can make use of ToList :: HashMap k v -> [(k, v)] as well to convert your HashMap k v into a list of key-value tuples, and then thus show the values with some extra functions.

For example we can print every value (here Student) on a separate line with:

mapM_ (print . snd) (toList myhashmap)

In your menu you can thus write:

menu :: Students -> IO ()
menu students = do
 putStrLn "1. Add new student"
 putStrLn "2. Show all students"
 putStrLn "3. Delete Student by ID"
 putStrLn "4. Exit program"
 choice <- getLine
 updatedStudents <- case choice of
 "1" -> addStudent students
 --"2" -> showstudent
 "3" -> deleteStudent students
 "4" -> exitSuccess --System.Exit.exitSuccess (zwraca typ IO students)
 print updatedStudents
 menu updatedStudents

Advise: In your current program, you constantly use do blocks. Typically in a Haskell program, you write the logic that does not require IO, a State, etc. separately in functions that do a small number of things.

• Adrian Kozień: Thanks for your's response. I understand everything but i still have got problem with priting function in my menu;/ When I add "print updatedStudents program stops.." Can you write whole statement?
• willeM_ Van Onsem: @AdrianKozień: did you by any chance removed the menu updatedStudents?
• Adrian Kozień: I edited my post, now there is my last version, but still doesn't work properly
• willeM_ Van Onsem: @AdrianKozień: shouldn't the do block of your showStudent be indented?
• Adrian Kozień: Yes, of course. I forgot about it while i was writing post.