A walk through a small project (a REST command line client) that aims at explaining monads and at providing a use case for monad transformers. Shown at Functional Programmers Meetup Paris 2015.

1. Toward Monad Transformers
Antoine Leblanc - @nicuveo
Functional Programmers Paris
2015-04-01

2. Outline
▶ Monads!
▶ Our mission
▶ Transformers
▶ Building RestT
▶ Wrapping up
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3. Introduction
What I Wish I Knew When Learning Haskell
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4. Introduction
What I Wish I Knew When Learning Haskell
1. Don’t read the monad tutorials.
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5. Introduction
What I Wish I Knew When Learning Haskell
1. Don’t read the monad tutorials.
2. No really, don’t read the monad tutorials.
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6. Introduction
What I Wish I Knew When Learning Haskell
1. Don’t read the monad tutorials.
2. No really, don’t read the monad tutorials.
…
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7. Introduction
What I Wish I Knew When Learning Haskell
1. Don’t read the monad tutorials.
2. No really, don’t read the monad tutorials.
…
8. Don’t write monad-analogy tutorials.
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8. Typeclass
class Monad m where
return :: a → m a
(>>=) :: m a → (a → m b) → m b
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9. Typeclass
class Monad m where
return :: a → m a
(>>=) :: m a → (a → m b) → m b
adit.io
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10. Typeclass
class Monad m where
return :: a → m a
(>>=) :: m a → (a → m b) → m b
(>>) :: Monad m ⇒ m a → m b → m b
(>=>) :: Monad m ⇒ (a → m b)
→ (b → m c)
→ (a → m c)
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11. Monad laws
Using (>>=)
return a >>= f ≡ f a
m >>= return ≡ m
(m >>= f) >>= g ≡ m >>= (λx → f x >>= g)
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12. Monad laws
Using (>=>)
return >=> f ≡ f
f >=> return ≡ f
(f >=> g) >=> h ≡ f >=> (g >=> h)
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13. Do notation
do { a ← f ; m } ≡ f >>= λa → m
do { f ; m } ≡ f >> m
do { m } ≡ m
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14. Do sugar
f >>= λa →
g >>= λb →
h >>= λc →
return (a, b, c)
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15. Do sugar
f >>= λa →
g >>= λb →
h >>= λc →
return (a, b, c)
do
a ← f
b ← g
c ← h
return (a, b, c)
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16. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
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17. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
Either a parse :: String → Either Error Value
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18. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
Either a parse :: String → Either Error Value
List allNextTurns :: Board → [Board]
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19. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
Either a parse :: String → Either Error Value
List allNextTurns :: Board → [Board]
IO httpGet :: Url → IO String
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20. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
Either a parse :: String → Either Error Value
List allNextTurns :: Board → [Board]
IO httpGet :: Url → IO String
Reader r runReader :: Reader r a → r → a
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21. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
Either a parse :: String → Either Error Value
List allNextTurns :: Board → [Board]
IO httpGet :: Url → IO String
Reader r runReader :: Reader r a → r → a
Writer w runWriter :: Writer w a → (a, w)
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22. ALL TEH MONADS
Maybe getUser :: Id → Maybe User
Either a parse :: String → Either Error Value
List allNextTurns :: Board → [Board]
IO httpGet :: Url → IO String
Reader r runReader :: Reader r a → r → a
Writer w runWriter :: Writer w a → (a, w)
State s runState :: Reader s a → s → (a, s)
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23. Reader Monad
Reader Green :: ■ → ■
Reader Yellow :: ■ → ■
(>>=) :: ■ → ■
→ (■ → ■ → ■ )
→ ■ → ■
runReader :: ■ → ■ → (■ → ■)
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24. Reader example
data Conf = { color :: Color, size :: Int }
formatter :: String → Reader Conf String
formatter p = do
c ← asks color
s ← asks size
return (undefined) -- TODO
format :: Conf → String → String
format c s = runReader (textFormatter s) c
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25. Writer Monad
Writer Green :: ■ ■
Writer Yellow :: ■ ■
(>>=) :: ■ ■
→ (■ → ■ ■ )
→ ■ ■
runWriter :: ■ ■ → (■ ■)
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26. Writer example
type Log = [String]
type Debug = Writer Log
value :: Int → Debug Int
value x = Writer (x, ["Value"])
plus :: Debug Int → Debug Int → Debug Int
plus x y = do
a ← x
b ← y
Writer (a + b, "Plus")
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27. State Monad
State Green :: ■ → ■ ■
State Yellow :: ■ → ■ ■
(>>=) :: ■ → ■ ■
→ (■ → ■ → ■ ■ )
→ ■ → ■ ■
runState :: ■ → ■ ■
→ (■ → ■ ■) 12 / 34

28. State Example
data GameInfo = GameInfo {
turn :: Int,
score :: Int
}
skipTurn :: State GameInfo ()
skipTurn = do
t ← gets turn
s ← gets score
put $ GameInfo (t + 1) (s `div` 2)
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29. Outline
▶ Monads
▶ Our mission!
▶ Transformers
▶ Building RestT
▶ Wrapping up
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30. REST client
▶ Authenticate
▶ Issue simple commands
▶ That’s it!
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31. Monadic syntax?
main = run $ do
setServerUrl "http://server.com/api"
setCredentials "guest" "guest"
authenticate
result ← request "server.listMyStuff"
print $ format result
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32. Problem
import Network.HTTP.Conduit
simpleHttp :: String → IO ByteString
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33. Problem
import Network.HTTP.Conduit
simpleHttp :: String → IO ByteString
IO!
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34. Problem
import Network.HTTP.Conduit
simpleHttp :: String → IO ByteString
wait…
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35. Problem
import Network.HTTP.Conduit
simpleHttp :: MonadIO m ⇒
String → m ByteString
…MonadIO?
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36. State in IO
▶ MVar?
▶ IORef?
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37. State in IO
▶ MVar?
▶ IORef?
...add State and Either monads to IO?
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38. Outline
▶ Monads
▶ Our mission
▶ Transformers!
▶ Building RestT
▶ Wrapping up
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39. All the way down
▶ Monad generalization
▶ Extra monad parameter
▶ Monad stacks
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40. Monad → MonadT
data State s a = { ... }
get :: State s s
put :: s → State s ()
Mister
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41. Monad → MonadT
data StateT s m a = { ... }
get :: MonadState s m ⇒ m s
put :: MonadState s m ⇒ s → m ()
MisterT
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42. Lifting
example :: CustomT IO Int
example = do
customStuff 1
lift $ putStrLn "Turtles!"
customStuff 2
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43. Lifting
example :: CustomT (MaybeT IO) Int
example = do
customStuff 1
lift $ lift $ putStrLn "Turtles!"
customStuff 2
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44. Instances
class Monad m ⇒ MonadState s m where
get :: m s
put :: s → m ()
instance MonadState s m ⇒
MonadState s (ReaderT r m) where
get = lift get
put = lift . put
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45. n 2
IO ↔ MonadIO ↔ liftIO
StateT ↔ MonadState ↔ get
ReaderT ↔ MonadReader ↔ ask
WriterT ↔ MonadWriter ↔ tell
ErrorT ↔ MonadError ↔ throwError
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46. Outline
▶ Monads
▶ Our mission
▶ Transformers
▶ Building RestT!
▶ Wrapping up
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47. Handling errors: ErrorT
type ErrorMsg = String
type Result m a = m (Either ErrorMsg a)
type RestT m = ErrorT ErrorMsg m
..
m
.
ErrorT ErrorMsg a
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48. Handling errors: ErrorT
type ErrorMsg = String
type Result m a = m (Either ErrorMsg a)
type RestT m = ErrorT ErrorMsg m
run :: RestT m a → Result m a
run = runErrorT
..
m
.
ErrorT ErrorMsg a
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49. Handling errors: ErrorT
type ErrorMsg = String
type Result m a = m (Either ErrorMsg a)
type RestT m = ErrorT ErrorMsg m
run :: RestT m a → Result m a
run = runErrorT
liftRest :: m a → RestT m a
liftRest = lift
..
m
.
ErrorT ErrorMsg a
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50. Adding session: StateT
type ErrorMsg = String
type Token = String
type Result m a = m (Either ErrorMsg a)
type RestT m = StateT Token (ErrorT ErrorMsg m)
..
m
.
ErrorT ErrorMsg
.
StateT Token a
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51. Adding session: StateT
type ErrorMsg = String
type Token = String
type Result m a = m (Either ErrorMsg a)
type RestT m = StateT Token (ErrorT ErrorMsg m)
run :: Token → RestT m a → Result m a
run t x = runErrorT $ evalStateT x t
..
m
.
ErrorT ErrorMsg
.
StateT Token a
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52. Adding session: StateT
type ErrorMsg = String
type Token = String
type Result m a = m (Either ErrorMsg a)
type RestT m = StateT Token (ErrorT ErrorMsg m)
run :: Token → RestT m a → Result m a
run t x = runErrorT $ evalStateT x t
liftRest :: m a → RestT m a
liftRest = lift . lift
..
m
.
ErrorT ErrorMsg
.
StateT Token a
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53. Dependency injection: ReaderT
type ErrorMsg = String
type Token = String
type Getter m = String → m String
type Result m a = m (Either ErrorMsg a)
type RestT m = ReaderT (Getter m) (StateT Token (ErrorT ErrorMsg m))
..
m
.
ErrorT ErrorMsg
.
StateT Token
.
ReaderT (Getter m) a
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54. Dependency injection: ReaderT
type ErrorMsg = String
type Token = String
type Getter m = String → m String
type Result m a = m (Either ErrorMsg a)
type RestT m = ReaderT (Getter m) (StateT Token (ErrorT ErrorMsg m))
run :: Getter m → Token → RestT m a → Result m a
run g t x = runErrorT $ evalStateT (runReaderT x g) t
..
m
.
ErrorT ErrorMsg
.
StateT Token
.
ReaderT (Getter m) a
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55. Dependency injection: ReaderT
type ErrorMsg = String
type Token = String
type Getter m = String → m String
type Result m a = m (Either ErrorMsg a)
type RestT m = ReaderT (Getter m) (StateT Token (ErrorT ErrorMsg m))
run :: Getter m → Token → RestT m a → Result m a
run g t x = runErrorT $ evalStateT (runReaderT x g) t
liftRest :: m a → RestT m a
liftRest = lift . lift . lift
..
m
.
ErrorT ErrorMsg
.
StateT Token
.
ReaderT (Getter m) a
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56. YAY!
request :: String → RestT m String
request method = do
getter ← ask
token ← get
let q = query method token
liftRest $ getter q
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57. AT LAST!
authenticate :: RestT m ()
authenticate = do
token ← get
unless (null token)
(throwError "already logged in")
answer ← request "getToken"
token ← parse answer
put token
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58. WOOOO!
main = void $ run httpGet emptyToken $ do
authenticate
answer ← request "showMyStuff"
liftIO $ print answer
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59. Outline
▶ Monads
▶ Our mission
▶ Transformers
▶ Building RestT
▶ Wrapping up!
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60. What have we learned today?
▶ Transformers: stack monads.
▶ Combine their powers!
▶ Hide stack details.
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61. This is the end. You are free.
Thank you.
Any questions?
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