From e56c43d73ddc2e6764f486f70b522a079cc38bcd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20Hurlin?= <clement.hurlin@moduscreate.com>
Date: Fri, 11 Oct 2024 12:03:29 +0200
Subject: [PATCH] Finish course 5. Introduce GADTs before type families.

---
 slides/Course05.hs  | 30 ++++++++++++--
 slides/course-05.md | 98 ++++++++++++++++++++++++++++++++++++++++-----
 tn-fp-course.cabal  |  2 +
 tps/Scratch.hs      | 43 +++++++++++++++++++-
 4 files changed, 157 insertions(+), 16 deletions(-)

diff --git a/slides/Course05.hs b/slides/Course05.hs
index 030ac1a..5f04253 100644
--- a/slides/Course05.hs
+++ b/slides/Course05.hs
@@ -56,11 +56,33 @@ class (Monoid w, Monad m) => MonadWriter w m | m -> w where
 
 writerExample :: MonadWriter String m => m (Maybe Int)
 writerExample = do
-  a <- pure $ Just 1 -- Could be an IO action
-  tell ("Producing a" :: String)
-  b <- pure $ Just 2
+  a <- pure (Just 1) -- Could be an IO action
+  tell "Producing a"
+  b <- pure Nothing
   tell "Producing b"
-  return $ (+) <$> a <*> b
+  let r = (+) <$> a <*> b
+  return r
+
+data Mix = MkMix { file :: FilePath, hash :: Int }
+
+-- Phantom types
+data File
+data Hash
+
+data Role a where
+  FileRole :: Role File
+  HashRole :: Role Hash
+
+type family DataKind a where
+  DataKind File = FilePath
+  DataKind Hash = Int
+
+-- | @getData m r@ can return either a value of type @FilePath@
+--   or a value of type @Int@!
+getData :: Mix -> Role a -> DataKind a
+getData mix = \case
+  FileRole -> mix.file
+  HashRole -> mix.hash
 
 class Monad m => MonadState s m | m -> s where
   -- | Return the state
diff --git a/slides/course-05.md b/slides/course-05.md
index 4038b4c..5adf500 100644
--- a/slides/course-05.md
+++ b/slides/course-05.md
@@ -156,14 +156,8 @@ class Applicative m => Monad m where
 --
 
 <br/>
-
 <center>
-<b>Generic</b> model of computation
-
-<br/>
-<br/>
-
-Plug monadic code with <code>pure</code> code
+  <b>Generic</b> model of interruptible computation
 </center>
 
 ---
@@ -233,25 +227,107 @@ writerExample = do
   tell "Producing a"
   b <- pure Nothing
   tell "Producing b"
-  return $ (+) <$> a <*> b
+  let r = (+) <$> a <*> b
+  return r
+```
+
+---
+
+# Generalized Algebraic Datatypes (GADTs)
+
+The problem:
+
+<!-- exdown-skip -->
+```hs
+data Expr a b =
+    IntExpr Int
+  | BoolExpr Bool
+  | PairExpr a b
+  | IfThenElseExpr Bool (Expr a b) (Expr a b)
+
+eval :: Expr a b -> ?
+```
+
+<center>
+🤮
+</center>
+
+--
+
+GADTs to the rescue!
+
+<!-- exdown-skip: 7 -->
+```hs
+data Expr t where
+  IntExpr :: Int -> Expr Int
+  BoolExpr :: Bool -> Expr Bool
+  Pair :: Expr a -> Expr b -> Expr (a, b)
+  IfThenElse :: Expr Bool -> Expr a -> Expr a -> Expr a
+
+eval :: Expr a -> a
 ```
 
+<center>
+😎
+</center>
+
 ---
 
 # Type Families
 
+* Type families are type-level functions
+* Functions that takes types as inputs and produce types
+
+<center>
+Type families' main usage is to work with GADTs
+</center>
+
+--
+
+```hs
+data Mix = MkMix { file :: FilePath, hash :: Int }
+
+-- Phantom types
+data File
+data Hash
+
+data Role a where
+  FileRole :: Role File
+  HashRole :: Role Hash
+
+type family DataKind a where
+  DataKind File = FilePath
+  DataKind Hash = Int
+
+-- | @getData m r@ can return either a value of type @FilePath@
+--   or a value of type @Int@!
+getData :: Mix -> Role a -> DataKind a
+getData mix = \case
+  FileRole -> mix.file
+  HashRole -> mix.hash
+```
+
 ---
 
 # Recap
 
-TBD
+* Monads
+  * Generic sequencing operation, supporting interruption
+  * Safely bring back sequencing programming to functional programming 👑
+* GADTs
+  * Existential types within data constructors
+  * Fine-grained typing of different constructors
+* Type families
+  * Uniform treatment of non-uniform types
 
 ---
 
 # Recommended reading
 
-* [MonadWriter](https://hackage.haskell.org/package/mtl-2.3.1/docs/Control-Monad-Writer-CPS.html#t:MonadWriter)
-* [MonadState](https://hackage.haskell.org/package/mtl-2.3.1/docs/Control-Monad-State-Class.html#t:MonadState)
+* On Hackage:
+  * [MonadWriter](https://hackage.haskell.org/package/mtl-2.3.1/docs/Control-Monad-Writer-CPS.html#t:MonadWriter)
+  * [MonadState](https://hackage.haskell.org/package/mtl-2.3.1/docs/Control-Monad-State-Class.html#t:MonadState)
+* Type families: https://serokell.io/blog/type-families-haskell
 
 ---
 
diff --git a/tn-fp-course.cabal b/tn-fp-course.cabal
index d2acd2f..8b5bd77 100644
--- a/tn-fp-course.cabal
+++ b/tn-fp-course.cabal
@@ -44,6 +44,7 @@ common common-all
                          FlexibleContexts
                          FlexibleInstances
                          FunctionalDependencies
+                         GADTs
                          MultiParamTypeClasses
                          NamedFieldPuns
                          LambdaCase
@@ -52,6 +53,7 @@ common common-all
                          RecordWildCards
                          ScopedTypeVariables
                          TypeApplications
+                         TypeFamilies
 
 library slides
     import:           common-all
diff --git a/tps/Scratch.hs b/tps/Scratch.hs
index ee00421..92a365b 100644
--- a/tps/Scratch.hs
+++ b/tps/Scratch.hs
@@ -2,6 +2,8 @@
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# OPTIONS_GHC -Wno-noncanonical-monad-instances #-}
 {-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeFamilies #-}
 -- | Module for live trial and error
 module Scratch where
 
@@ -153,4 +155,43 @@ monadWriterExample = do
   tell "Producing a"
   b <- pure $ Nothing
   tell "Producing b"
-  return $ (+) <$> a <*> b
\ No newline at end of file
+  return $ (+) <$> a <*> b
+
+data Expr t where
+  IntExpr :: Int -> Expr Int
+  BoolExpr :: Bool -> Expr Bool
+  Pair :: Expr a -> Expr b -> Expr (a, b)
+  IfThenElse :: Expr Bool -> Expr a -> Expr a -> Expr a
+
+
+eval :: Expr a -> a
+eval = \case
+  IntExpr x -> x
+  BoolExpr x -> x
+  Pair a b -> (eval a, eval b)
+  IfThenElse cond a b -> if eval cond then eval a else eval b
+
+-- data Expr a b =
+--     IntExpr Int
+--   | BoolExpr Bool
+--   | PairExpr a b
+--   | IfThenElseExpr Bool (Expr a b) (Expr a b)
+
+data Mix = MkMix { file :: FilePath, hash :: Int }
+
+-- Phantom types
+data File
+data Hash
+
+data Role a where
+  FileRole :: Role File
+  HashRole :: Role Hash
+
+type family DataKind a where
+  DataKind File = FilePath
+  DataKind Hash = Int
+
+getData :: Mix -> Role a -> DataKind a
+getData mix = \case
+  FileRole -> mix.file
+  HashRole -> mix.hash
\ No newline at end of file