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jon-thompson's solution

to ETL in the Elm Track

Published at Sep 26 2019 · 0 comments
Instructions
Test suite
Solution

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

ETL

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.

Notes

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.

Elm Installation

Refer to the Installing Elm page for information about installing elm.

Writing the Code

The first time you start an exercise, you'll need to ensure you have the appropriate dependencies installed. Thankfully, Elm makes that easy for you and will install dependencies when you try to run tests or build the code.

Execute the tests with:

$ elm-test

Automatically run tests again when you save changes:

$ elm-test --watch

As you work your way through the test suite, be sure to remove the skip <| calls from each test until you get them all passing!

Source

The Jumpstart Lab team http://jumpstartlab.com

Submitting Incomplete Solutions

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

Tests.elm

module Tests exposing (tests)

import Dict
import Etl
import Expect
import Test exposing (..)


tests : Test
tests =
    describe "Etl.transform"
        [ test "a single letter" <|
            \() ->
                Expect.equal
                    (Dict.fromList [ ( "a", 1 ) ])
                    (Etl.transform <|
                        Dict.fromList [ ( 1, [ "A" ] ) ]
                    )
        , skip <|
            test "single score with multiple letters" <|
                \() ->
                    Expect.equal
                        (Dict.fromList
                            [ ( "a", 1 )
                            , ( "e", 1 )
                            , ( "i", 1 )
                            , ( "o", 1 )
                            , ( "u", 1 )
                            ]
                        )
                        (Etl.transform <|
                            Dict.fromList
                                [ ( 1, [ "A", "E", "I", "O", "U" ] )
                                ]
                        )
        , skip <|
            test "multiple scores with multiple letters" <|
                \() ->
                    Expect.equal
                        (Dict.fromList
                            [ ( "a", 1 )
                            , ( "d", 2 )
                            , ( "e", 1 )
                            , ( "g", 2 )
                            ]
                        )
                        (Etl.transform <|
                            Dict.fromList
                                [ ( 1, [ "A", "E" ] )
                                , ( 2, [ "D", "G" ] )
                                ]
                        )
        , skip <|
            test "multiple scores with differing numbers of letters" <|
                \() ->
                    Expect.equal
                        (Dict.fromList
                            [ ( "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 )
                            ]
                        )
                        (Etl.transform <|
                            Dict.fromList
                                [ ( 1, [ "A", "E", "I", "O", "U", "L", "N", "R", "S", "T" ] )
                                , ( 2, [ "D", "G" ] )
                                , ( 3, [ "B", "C", "M", "P" ] )
                                , ( 4, [ "F", "H", "V", "W", "Y" ] )
                                , ( 5, [ "K" ] )
                                , ( 8, [ "J", "X" ] )
                                , ( 10, [ "Q", "Z" ] )
                                ]
                        )
        ]
module Etl exposing (transform)

import Dict exposing (Dict)


addScore : Int -> String -> Dict String Int -> Dict String Int
addScore score letter dict =
    Dict.insert (String.toLower letter) score dict


addScores : Int -> List String -> Dict String Int -> Dict String Int
addScores score letters dict =
    List.foldl (addScore score) dict letters


transform : Dict Int (List String) -> Dict String Int
transform input =
    Dict.foldl addScores
        Dict.empty
        input

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?