an effectful function that can throw an error

tooBig = do
  when (i > 100) $ throwError $ show i
  return i

run the tooBig effect based on a provided Int.

runTooBig i = run . runError $ tooBig i

>>> runTooBig 1
Right 1

>>> runTooBig 200
Left "200"

an effectul computation using state. The state is of type [Int]. This function takes the head off the list, if it is there and return it. If state is the empty list, then it stays the same and returns Nothing.

popState = do
 stack <- get
 case stack of
   []       -> return Nothing
   (x : xs) -> do
     put xs
     return $ Just x

run the popState effectful computation based on initial state. The result-type is the result of the computation Maybe Int together with the state at the end of the computation [Int]

runPopState xs = run . runState xs $ popState

>>> runPopState  [1, 2, 3]
(Just 1,[2,3])

>>> runPopState []
(Nothing,[])

an effect that returns a number one more than the given

oneMore = do
  x <- ask -- query the environment
  return $ x + 1 -- add one to the asked value and return it

Run the oneMore effectful function by giving it a value to read.

runOneMore i = run . runReader i $ oneMore

>>> runOneMore 1
2

An effectful computation with multiple effects:

• A value gets read
• an error can be thrown depending on the read value
• state gets read and transformed

All these effects are composed using the Eff monad using the corresponding Effect types.

something = do
  readValue :: Float <- ask -- read a value from the environment
  when (readValue < 0) $ throwError readValue  -- if the value is negative, throw an error
  modify (l -> (round readValue :: Integer) : l) -- add the rounded read element to the list
  currentState :: [Integer] <- get -- get the state after the modification
  return $ sum currentState -- sum the elements in the list and return that

Run the someting effectful computation given in the previous function. The handlers apply from bottom to top - so this is the reading direction.

runSomething1 initialState newValue =
  run . -- run the Eff-monad with no effects left
  runError . -- run the error part of the effect. This introduces the Either in the result.
  runState initialState . -- handle the state-effect providing an initial state giving back a pair.
  runReader newValue $ -- provide the computation with the dynamic value to read/ask for
  something -- the computation - function

>>> runSomething1 [] (-0.5)
Left (-0.5)

>>> runSomething1 [2] 1.3
Right (3,[1,2])

Run the something effectful computation given above. This has an alternative ordering of the effect-handlers.

The used effect-handlers are the same are used in slightly different order: The runState and runError methods are swapped, which results in a different output type and run-semantics.

runSomething1 initialState newValue =
  run .
  runState initialState .
  runError .
  runReader newValue $
  something -- the computation - function

>>> runSomething2 [4] (-2.4)
(Left (-2.4),[4])

>>> runSomething2 [4] 5.9
(Right 10,[6,4])