🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of hyphenrf

hyphenrf's solution

to ETL in the OCaml Track

Published at Apr 30 2020 · 0 comments
Test suite

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

  1. Install the Exercism CLI.

  2. Install OCaml.

  3. For library documentation, follow Useful OCaml resources.

Running Tests

A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:


Submitting Incomplete Solutions

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

Feedback, Issues, Pull Requests

The exercism/ocaml repository on GitHub is the home for all of the Ocaml exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue or submit a PR. We welcome new contributors!


The Jumpstart Lab team http://jumpstartlab.com


open OUnit2
open Etl

let ae exp got _test_ctxt =
  let printer xs = String.concat ";" (List.map (fun (ch, n) -> Printf.sprintf "(%c,%d)" ch n) xs) in
  assert_equal exp got ~printer

let tests = [
  "single letter" >::
  ae [('a', 1)]
    (transform [(1, ['A'])]);
  "single score with multiple letters" >::
  ae [('a', 1); ('e', 1); ('i', 1); ('o', 1); ('u', 1)]
    (transform [(1, ['A'; 'E'; 'I'; 'O'; 'U'])]);
  "multiple scores with multiple letters" >::
  ae [('a', 1); ('d', 2); ('e', 1); ('g', 2)]
    (transform [(1, ['A'; 'E']); (2, ['D'; 'G'])]);
  "multiple scores with differing numbers of letters" >::
  ae [
    ('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);
    (transform [
        (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']);

let () =
  run_test_tt_main ("etl tests" >::: tests)
let transform l =
  let rec rtrans = function
    | [] -> []
    | (s, cs) :: scss ->
      List.map (fun c -> (Char.lowercase_ascii c), s) cs
      :: rtrans scss
    rtrans l
    |> List.concat
    |> List.sort (fun (a, _) (b, _) -> Char.compare a b)

Community comments

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

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?