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Martin Potier
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# FPKATA #01: In the library…
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(Soundtrack: Arch Enemy — War Eternal)
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(Inspired from this exercice: http://www.codewars.com/kata/54dc6f5a224c26032800005c/train/haskell)
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## Introduction
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You are stuck in a library with a very talkative and experimental science-minded IA. It keeps repeating the following:
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> — Pleeease, can you help me?
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> — …
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> — Pleaaase, can you help?
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> — …
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> — Hellooooo?
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> — …
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Since you would really like to have peace, you dare ask:
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> — What's the matter?
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> — My code is incomplete, I've lost my code, please help!
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> — What code? What are you talking about?
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> — I really need to sort the library index and count how many books have the same first-letter reference, but only in from a list…
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> — Wait!
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> — … of characters that my master will provide.
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> This makes no sense, can't you write it down?
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A few seconds pass in total silence. Then, there's a sound coming from your right, something like an inkjet printer work at a fast pace. It's actually a printer. It throws the printed sheet of paper at you in a furious “TWEEEEEEEEEEEP”. You pick up the paper and read:
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INPUT................:
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LIBRARY COUNT......: [("ABOOK", 12), ("BPDEXU", 45), ("BJLDRSA", 15), ("CLDAVB", 98), ("DAJLDI", 32)]
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INDICES............: ['A','B','C','W']
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OUTPUT...............:
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INDEX COUNT........: [('A',12),('B',60),('C',98),('W',0)]
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TIME TO GOOD ANSWER..: 203 years
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You frown:
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> — It took you so long to find this?
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> — Can you help me? I've lost my code, my dedicated code for this task.
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> — Well, you managed to get this one right, didn't you?
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> — Nooooo! Master is going to unplug me if I use bruteforce again! Master was
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very unhappy with my performance! I want to live! Please help me!
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> — Ok, all right, calmn down, there, there… What are programmed with?
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> — Elm.
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Of course it's Elm…
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## Outline
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```
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module Main exposing (..)
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import Html exposing (text)
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type Stock
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= Stock String Int
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library1 : List Stock
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library1 =
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[ Stock "ABART" 20
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, Stock "CDXEF" 50
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, Stock "BKWRK" 25
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, Stock "BTSQZ" 89
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, Stock "DRTYM" 60
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]
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categories1 : List Char
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categories1 =
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[ 'A'
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, 'B'
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, 'C'
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, 'W'
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]
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stockList : List Stock -> List Char -> List ( Char, Int )
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stockList st cs =
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<THIS IS BLANK, FILL ME>
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main =
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text (toString (stockList library1 categories1))
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```
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## Walkthrough
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Ok, so you'll have to fill the blank. After a few minutes, you identify two
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main components: one is about taking the first letter of every `Stock` name. The
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second is finding the amount of each `Char` in a `List (Char, Int)`.
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>
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Let's start with the second point and assume we have a value of `List (Char,
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Int)` already available. We know at least to functions to recurse on lists:
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maps and folds. Clearly, we'll have to choose a fold, since some aggregating is
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needed when several identical `Char` are in the list. Let's pick foldl, it does
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not really matter right now. So, the type of `List.foldl` is:
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foldl : (a -> b -> b) -> b -> List a -> b
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In fact, you notice you'll need two folds! One inner fold for counting of many
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book for initial X, and one outer fold for going through all the initials
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required by the master.
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### Inner foldl: how many books with Initial X
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You think about using type aliases to make your code easier to read, the IA
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won't mind. To specialize your inner foldl, which is morally a foldlForX, you
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identify `a` as an element of a library stock (even though the title has been
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shortened already to its initial), and `b` as the final index count.
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type alias Initial = Char
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type alias Count = Int
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type alias TitleCount = (Initial, Int)
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type alias FinalCount = (Initial, Int)
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You rewrite your specialized foldl type signature to account for the type
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aliases:
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foldForX : (TitleCount -> FinalCount -> FinalCount)
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-> FinalCount
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-> List TitleCount
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-> FinalCount
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After a call to this, you'll get how many books have a title that starts with
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the initial X.
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You are missing the function with type `(TitleCount -> FinalCount ->
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FinalCount)`, and you reflect a little bit on its name and on its usage. Hmmm,
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it takes a TitleCount and a FinalCount and returns a (new) FinalCount? You
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choose to call it:
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updateFinalCount : (TitleCount -> FinalCount -> FinalCount)
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Actually it takes another (hidden) parameter, you change its name to reflect
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that:
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updateFinalCountForX : (TitleCount -> FinalCount -> FinalCount)
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updateFinalCountForX t f =
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NB:
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addBio : String -> User -> User
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addBio bio user =
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{ user | bio = bio }
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