Safe Haskell	Safe-Inferred

Lessons.Lesson12

Description

Notes taken by Deimantė Davidavičiūtė

Synopsis

type MyDomain a = Free MyDomainAlgebra a
data MyDomainAlgebra next
- = Calculate Expr (Integer -> next)
- | Store Integer (() -> next)
- | Restore (Integer -> next)
calculte :: Expr -> MyDomain Integer
store :: Integer -> MyDomain ()
restore :: MyDomain Integer
myProgram :: MyDomain Integer
runInIO :: MyDomain a -> IO a
runInState :: MyDomain a -> State Integer a
runState :: State s a -> s -> (a, s)

Documentation

The Free monad over MyDomainAlgebra.

The algebra of our domain:

'Calculate Expr (Integer -> next)': evaluate an arithmetic expression, then pass the resulting Integer to the continuation.

'Store Integer (() -> next)': store a number and continue.

'Restore (Integer -> next)': fetch the last stored number and continue.

Constructors

Instances details

Our algebra must be a Functor to build a Free monad. Demonstration of functorial mapping (not tied to our type):

>>> fmap (+5) (Just 5)
Just 10

Instance details

Methods

Lift a Calculate instruction into the Free program.

Lift a Store instruction.

Lift a Restore instruction.

A sample program composed of our domain actions.

Calculates an expression and binds it to r Restores previously stored value v0. Stores r and then stores 42. Restores v1 and returns the sum.

An interpreter that runs the program in IO. Effects are implemented as printing/prompts and pure evaluation.

runInState :: MyDomain a -> State Integer a Source #

Interpreter that runs the program in pure 'State Integer'. The state holds the latest stored number.

>>> runState (runInState myProgram) 0
(27,42)

runState :: State s a -> s -> (a, s) #