Haskell Course

Monads Labs

• Lab 1
• Lab 2
• Lab 3
• Lab 4-5
• Lab 6
• Lab 7
• Hopefully all of them

Tests + Solutions:

• Var 1
• Var 2
• Var 3
• Var 4
• Var 5
• Var 6
• Var 7

Haskell Code

Class 1

• Mini-Haskell
• Counter
• lambda
• operator $

Class 2

• Statements
• While
• Operators := and :

Class 3

 >= aplica si pastreaza tipul de apel
 >> transforma 
 <=< compunere inversa
 (Just 3) >>= (\ x -> if (x>0) then Just (x*x) else Nothing) => Just 9
 (Just 3) >> (Just 6) => Just 6
 f <=< g $ 3 => Just 10 (il ia pe 3, ii aplica g, si valorii obtinute ii aplica f)

• Maybe Monad

  • Just, Nothing
  • transformations DO
  • tests with QuickCheck module
  • my map

• Writer String Monad

  • log-increment functions

• Writer List Monad

  • log-increment functions
  • normal map
  • my map

• Reader Monad

  • show a Person (data)
  • show a Person (Reader monad)

Class 4-5

* Identity Monad and data Value

   type M = Identity
   newtype Identity a = Identity { runIdentity :: a }
   term0 = (App (Lam "x" (Var "x" :+: Var "x")) (Con 10 :+: Con 11)) => "42"

• data Value
• interpretor mini Haskell for Identity Monad with data Value
• test program

• showM function for Identity monad

  * Maybe Monad and data Value

  type M = Maybe
  term0 = (App (Lam "x" (Var "x" :+: Var "x")) (Con 10 :+: Con 11)) => Just 42

• Maybe Monad (is system defined)
• data Value (without Wrong)
• interpretor mini Haskell for Maybe Monad with data Value
• test programs

• without showM function (system defined for Maybe)

  * Either String Monad and data Value

  type M = Either String
  term0 = (App (Lam "x" (Var "x" :+: Var "x")) (Con 10 :+: Con 11)) => Right 42
  term1 = (App (Con 7) (Con 2)) [] => Left "should be function: 7"

• Either String Monad (is system defined)
• data Value
• interpretor mini Haskell for Either String Monad with data Value
• test programs
• without showM function (system defined for Either String)

• Left “error text” or Left (show Wrong) for errors

  * List Monad and data Value

  -> intoarce lista raspunsurilor posibile
  type M a = [a]
  interp (App (Lam "x" (Var "x" :+: Var "x")) (Amb (Con 1) (Con 2)))) [] => [2,4]

• List Monad (is system defined)
• data Value
• interpretor mini Haskell for List Monad with data Value
• test program
• show function (system defined for List)

• New: Fail and Amb constructors

  * EnvReader Monad and data Value

  -> mediul de evaluare este vazut o stare care este citita atunci când avem nevoie de valorile variabilelor
  -> dar starea nu va fi modificata(imutabila)
  type M = EnvReader
  newtype EnvReader a = Reader { runEnvReader  Environment -> a }
  term0 = (App (Lam "x" (Var "x" :+: Var "x")) (Con 10 :+: Con 11)) => "42"

• Reader Monad with environment
• interpretor function with one argument (it contains the environment)
• ask, local - 2 auxliary functions for EnvReader
• new function for lookup

• showM function

  * StringWriter Monad and data Value

  -> afisare de rezultate intermediare
  type M = StringWriter
  newtype StringWriter a = StringWriter { runStringWriter  (a, String) }
  interp (Out (Con 41) :+: Out (Con 1)) [] => "Output: 41; 1; Value: 42"

• tell auxiliary function for output a message
• interpretor mini Haskell for Writer Monad with data Value
• test program
• showM function
• New: Out constructor and ShowM function

• instance Show-StrinWriter

  * State Monad and data Value

  -> numarul de pasi necesari pentru calcularea rezultatului (numar doar la adunare si aplicare de functie)
  type M = InState
  newtype IntState a = IntState { runIntState  Integer -> (a, Integer) }
  interp ((Con 1 :+: Con 2) :+: Count) [] => "Value: 4; Count: 2"

• modify, get auxiliary functions
• interpretor mini Haskell for State Monad with data Value
• test program
• showM function
• new: Count constructor and modify(+1) for modifying the counter

  Cand le folosesc?
   modify(+1) >> se foloseste fara DO
   modify(+1) se foloseste in DO

Class 6

• Module SIMPLE
• Simple Type Checker

Class 7

• Module SIMPLE
• Interpretor SIMPLE Module