Wrapped and done

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "ep0002"]
+	path = ep0002
+	url = https://github.com/MasseR/katas.git

ep0001/Solution.md

@@ -4,7 +4,29 @@
 
 (Inspired from this exercice: http://www.codewars.com/kata/54dc6f5a224c26032800005c/train/haskell)
 
-## Introduction
+## REPL compatible copy/paste (TL;DR)
+
+```
+import Dict
+
+type Stock = Stock { name : String, count : Int }
+library1 =  [ Stock { name = "ABART", count = 20 } , Stock { name = "CDXEF", count = 50 } , Stock { name = "BKWRK", count = 25 } , Stock { name = "BTSQZ", count = 89 } , Stock { name = "DRTYM", count = 60 } ]
+categories1 = [ 'A', 'B', 'C', 'W' ]
+
+toUplet : List Repl.Stock -> List ( String, Int )
+toUplet = List.map (\x -> case x of Stock v -> (v.name,v.count))
+
+onlyInitials : List (String, Int) -> List (Char, Int)
+onlyInitials = List.map (\x -> case x of (s, i) -> ((Maybe.withDefault '\0' << List.head << String.toList) s,i) )
+
+howMany : Char -> List Char Int -> Int
+howMany c = (List.foldl (\t acc -> acc + Tuple.second t) 0) << List.filter (\t -> (Tuple.first t) == c)
+
+stockList : List Stock -> List Char -> List ( Char, Int )
+stockList sl = List.map (\c -> (c, sl |> toUplet >> onlyInitials >> howMany c))
+```
+
+## Prologue
 
 You are stuck in a library with a very talkative and experimental science-minded IA. It keeps repeating the following:
 
@@ -28,7 +50,8 @@ Since you would really like to have peace, you dare ask:
 
 > — What code? What are you talking about?
 
-> — 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…
+> — 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…
 
 > — Wait!
 
@@ -36,7 +59,10 @@ Since you would really like to have peace, you dare ask:
 
 > This makes no sense, can't you write it down?
 
-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:
+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:
 
     INPUT................:
       LIBRARY COUNT......: [("ABOOK", 12), ("BPDEXU", 45), ("BJLDRSA", 15), ("CLDAVB", 98), ("DAJLDI", 32)]
@@ -104,60 +130,63 @@ main =
 
 ## Walkthrough
 
-Ok, so you'll have to fill the blank. After a few minutes, you identify two
-main components: one is about taking the first letter of every `Stock` name. The
-second is finding the amount of each `Char` in a `List (Char, Int)`.
-> 
-Let's start with the second point and assume we have a value of `List (Char,
-Int)` already available. We know at least to functions to recurse on lists:
-maps and folds. Clearly, we'll have to choose a fold, since some aggregating is
-needed when several identical `Char` are in the list. Let's pick foldl, it does
-not really matter right now. So, the type of `List.foldl` is:
+All is needed is to fill the gaps from `library1`, `categories1` to `stockList`.
+So, what is needed for successfully building a stockList? Let's look at the types:
+We have a `List Stock` but we don't have any function to work on it. We could
+make some functions for this, but we'll just be lazy and turn a List Stock into
+a much lower level `List (String, Int)`:
 
-    foldl : (a -> b -> b) -> b -> List a -> b
+    toUplet : List Stock -> List ( String, Int )
+    toUplet = List.map (\x -> case x of Stock v -> (v.name,v.count))
 
-In fact, you notice you'll need two folds! One inner fold for counting of many
-book for initial X, and one outer fold for going through all the initials
-required by the master.
+We'll need to count stock by initials, so we write a function for that:
 
-### Inner foldl: how many books with Initial X
+    onlyInitials : List (String, Int) -> List (Char, Int)
+    onlyInitials = List.map (\x -> case x of (s, i) -> ((Maybe.withDefault '\0' << List.head << String.toList) s,i) )
 
-You think about using type aliases to make your code easier to read, the IA
-won't mind. To specialize your inner foldl, which is morally a foldlForX, you
-identify `a` as an element of a library stock (even though the title has been
-shortened already to its initial), and `b` as the final index count.
+So, for every tuple, we operate of the left part. There is some strange
+machinery for turning a `String -> Char` involving a Maybe value, because
+`List.head` may fail. Of course in our case, no book should ever have an empty
+title, but Elm doesn't know that. We mitigate this by setting a default `null`
+char if the title is empty.
 
-    type alias Initial = Char
-    type alias Count = Int
-    type alias TitleCount = (Initial, Int) 
-    type alias FinalCount = (Initial, Int)
+Good! We can already chain the two preceding functions since the output type
+of `toUplet` matches the input type of `onlyInitials`. Ain't that a coincidence?
 
-You rewrite your specialized foldl type signature to account for the type
-aliases:
+    > library1 |> toUplet >> onlyInitials
+    [('A',20),('C',50),('B',25),('B',89),('D',60)] : List ( Char, Int )
 
-    foldForX : (TitleCount -> FinalCount -> FinalCount)
-          -> FinalCount
-          -> List TitleCount
-          -> FinalCount
+Oh, but oh. There are two Bs in there! Those should go together and their counts
+added together, and for that, nothing is better than a fold. Let's write a function
+that answers the question: “How many X is there?” where X is a Char.
 
-After a call to this, you'll get how many books have a title that starts with
-the initial X.
+    howMany : Char -> List Char Int -> Int
+    howMany c = List.filter (\t -> (Tuple.first t) == c)
+              >> (List.foldl (\t acc -> acc + Tuple.second t) 0)
 
-You are missing the function with type `(TitleCount -> FinalCount ->
-FinalCount)`, and you reflect a little bit on its name and on its usage. Hmmm,
-it takes a TitleCount and a FinalCount and returns a (new) FinalCount? You
-choose to call it:
+`howMany` is made of two functions: the first is one that only keeps tuples
+of interest (they match our Char c) and then we blindly sum the count of the
+resulting list. We always start the count from 0, so that if we don't find
+anything, it's still correct.
 
-    updateFinalCount : (TitleCount -> FinalCount -> FinalCount)
+Let's try to find how may books that have a title starting with B in the library:
 
-Actually it takes another (hidden) parameter, you change its name to reflect
-that:
+    > library1 |> toUplet >> onlyInitials >> howMany 'B'
+    114 : Int
+
+Nice! Well, we're almost done actually. We just need to do that for every letter
+we were requested in `categories1`. Let's use a map for that:
+
+    stockList sl = List.map (\c -> (c, sl |> toUplet >> onlyInitials >> howMany c) )
+
+And voilà! Let's run this:
+
+    > stockList library1 categories1
+    [('A',20),('B',114),('C',50),('W',0)] : List ( Char, Int )
+
+Mission accomplished.
+
+## Epilogue
 
-    updateFinalCountForX : (TitleCount -> FinalCount -> FinalCount)
-    updateFinalCountForX t f =    
 
 
-NB:
-addBio : String -> User -> User
-addBio bio user =
-  { user | bio = bio }

ep0002

@@ -0,0 +1 @@
+Subproject commit 859fdd499ee97d486846d621235035b2c7ea4383