# agbell's solution

## to Grade School in the Haskell Track

Published at Jul 13 2018 · 3 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given students' names along with the grade that they are in, create a roster for the school.

In the end, you should be able to:

• Add a student's name to the roster for a grade
• "OK."
• Get a list of all students enrolled in a grade
• "Which students are in grade 2?"
• "We've only got Jim just now."
• Get a sorted list of all students in all grades. Grades should sort as 1, 2, 3, etc., and students within a grade should be sorted alphabetically by name.
• "Who all is enrolled in school right now?"
• "Grade 1: Anna, Barb, and Charlie. Grade 2: Alex, Peter, and Zoe. Grade 3…"

Note that all our students only have one name. (It's a small town, what do you want?)

## For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

• If you're working in a language with mutable data structures and your implementation allows outside code to mutate the school's internal DB directly, see if you can prevent this. Feel free to introduce additional tests.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

## Hints

To complete this exercise you need to create the data type `School` and implement the following functions:

• `add`
• `empty`
• `grade`
• `sorted`

You will find a dummy data declaration and type signatures already in place, but it is up to you to define the functions and create a meaningful data type, newtype or type synonym.

## Getting Started

For installation and learning resources, refer to the exercism help page.

## Running the tests

To run the test suite, execute the following command:

``````stack test
``````

#### If you get an error message like this...

``````No .cabal file found in directory
``````

You are probably running an old stack version and need to upgrade it.

#### Otherwise, if you get an error message like this...

``````No compiler found, expected minor version match with...
Try running "stack setup" to install the correct GHC...
``````

Just do as it says and it will download and install the correct compiler version:

``````stack setup
``````

## Running GHCi

If you want to play with your solution in GHCi, just run the command:

``````stack ghci
``````

## Feedback, Issues, Pull Requests

The exercism/haskell repository on GitHub is the home for all of the Haskell exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!

## Source

A pairing session with Phil Battos at gSchool http://gschool.it

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### Tests.hs

``````{-# OPTIONS_GHC -fno-warn-type-defaults #-}

import Test.Hspec        (Spec, it, shouldBe)
import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)

main :: IO ()
main = hspecWith defaultConfig {configFastFail = True} specs

specs :: Spec
specs = do

let fromList = foldr (uncurry add) empty
let fromGrade g = fromList . zip (repeat g)

it "add student" \$
sorted (add 2 "Aimee" empty) `shouldBe` [(2, ["Aimee"])]

it "add more students in same class" \$
sorted (fromGrade 2 ["James", "Blair", "Paul"])
`shouldBe` [(2, ["Blair", "James", "Paul"])]

it "add students to different grades" \$
sorted (fromList [(3, "Chelsea"), (7, "Logan")])
`shouldBe` [(3, ["Chelsea"]), (7, ["Logan"])]

it "get students in a grade" \$
grade 5 (fromList [(5, "Franklin"), (5, "Bradley"), (1, "Jeff")])

it "get students in a non-existent grade" \$
grade 1 empty `shouldBe` []

it "sorted school" \$
sorted (fromList [ (4, "Jennifer"   )
, (6, "Kareem"     )
, (4, "Christopher")
, (3, "Kyle"       ) ] )
`shouldBe` [ (3, ["Kyle"                   ] )
, (4, ["Christopher", "Jennifer"] )
, (6, ["Kareem"                 ] ) ]``````
``````module School (School, sorted, empty, add, grade)
where

import Control.Arrow (second)
import Control.Applicative ((<\$>))
import qualified Data.Map.Strict as Map
import Data.Map.Strict(Map)
import Data.List

type School = Map Int [String]

sorted :: School -> [(Int, [String])]
sorted s = second sort <\$> Map.toList s

empty :: School
empty = Map.empty

add :: Int -> String -> School -> School
add gr name = Map.insertWith (++) gr [name]

grade :: Int -> School -> [String]
grade gr school = sort \$ Map.findWithDefault [] gr school``````

Find this solution interesting? Ask the author a question to learn more.

Not really better or worse, but second is equivalent to fmap for 2-tuples, so you could write this without importing Control.Arrow.

Solution Author
commented over 5 years ago

@etrepum Nice, I did not know that. I would have expected fmap to hit the first element.

It's easier to define it for the second element, since it's the last type variable in (,) a b. In the same way that there's a Functor instance for Maybe or [] there's a Functor instance for (,) a.

### What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

• What compromises have been made?
• Are there new concepts here that you could read more about to improve your understanding?