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to ETL in the Haskell Track

Published at Jul 13 2018 · 0 comments
Test suite


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.

We are going to do the Transform step of an Extract-Transform-Load.


Extract-Transform-Load (ETL) is a fancy way of saying, "We have some crufty, legacy data over in this system, and now we need it in this shiny new system over here, so we're going to migrate this."

(Typically, this is followed by, "We're only going to need to run this once." That's then typically followed by much forehead slapping and moaning about how stupid we could possibly be.)

The goal

We're going to extract some scrabble scores from a legacy system.

The old system stored a list of letters per score:

  • 1 point: "A", "E", "I", "O", "U", "L", "N", "R", "S", "T",
  • 2 points: "D", "G",
  • 3 points: "B", "C", "M", "P",
  • 4 points: "F", "H", "V", "W", "Y",
  • 5 points: "K",
  • 8 points: "J", "X",
  • 10 points: "Q", "Z",

The shiny new scrabble system instead stores the score per letter, which makes it much faster and easier to calculate the score for a word. It also stores the letters in lower-case regardless of the case of the input letters:

  • "a" is worth 1 point.
  • "b" is worth 3 points.
  • "c" is worth 3 points.
  • "d" is worth 2 points.
  • Etc.

Your mission, should you choose to accept it, is to transform the legacy data format to the shiny new format.


A final note about scoring, Scrabble is played around the world in a variety of languages, each with its own unique scoring table. For example, an "E" is scored at 2 in the Māori-language version of the game while being scored at 4 in the Hawaiian-language version.

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

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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!


The Jumpstart Lab team http://jumpstartlab.com

Submitting Incomplete Solutions

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


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

import Data.Map          (fromList)
import Test.Hspec        (Spec, describe, it, shouldBe)
import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)

import ETL (transform)

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

specs :: Spec
specs =

  describe "transform" $ do

    it "a single letter" $
      transform (fromList [(1, "A")])
      `shouldBe` fromList [('a', 1)]

    it "single score with multiple letters" $
      transform (fromList [(1, "AEIOU")])
      `shouldBe` fromList [('a', 1), ('e', 1), ('i', 1), ('o', 1), ('u', 1)]

    it "multiple scores with multiple letters" $
      transform (fromList [(1, "AE"), (2, "DG")])
      `shouldBe` fromList [('a', 1), ('e', 1), ('d', 2), ('g', 2)]

    it "multiple scores with differing numbers of letters" $
      transform (fromList fullInput)
      `shouldBe` fromList fullOutput


    fullInput = [ ( 1, "AEIOULNRST")
                , ( 2, "DG"        )
                , ( 3, "BCMP"      )
                , ( 4, "FHVWY"     )
                , ( 5, "K"         )
                , ( 8, "JX"        )
                , (10, "QZ"        ) ]

    fullOutput = [ ('a',  1) , ('b',  3) , ('c',  3) , ('d',  2)
                 , ('e',  1) , ('f',  4) , ('g',  2) , ('h',  4)
                 , ('i',  1) , ('j',  8) , ('k',  5) , ('l',  1)
                 , ('m',  3) , ('n',  1) , ('o',  1) , ('p',  3)
                 , ('q', 10) , ('r',  1) , ('s',  1) , ('t',  1)
                 , ('u',  1) , ('v',  4) , ('w',  4) , ('x',  8)
                 , ('y',  4) , ('z', 10) ]
module ETL(transform) where

import Data.Map.Strict(Map)
import qualified Data.Map.Strict as Map
import Data.Char

transform :: Map Int [String] -> Map String Int
transform = Map.foldlWithKey' accumulate Map.empty

accumulate :: Map String Int -> Int -> [String] -> Map String Int
accumulate m i ss = Map.union m . Map.fromList $ pair i ss

pair :: Int -> [String] -> [(String,Int)]
pair i ss = zip (lower ss) (repeat i)
  where lower = fmap (fmap toLower )

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