Documentation

Basic types for the parser.

Parser a is an ExceptT over State.

State Input carries the remaining input string.

ExceptT ErrorMsg allows failure with an error message.

Parse a single alphabetic character.

Consumes one character from the input state or fails with an error message.

>>> parse parseLetter "a1"
(Right 'a',"1")
>>> parse parseLetter "1a"
(Left "A letter is expected, but got 1","1a")
>>> parse parseLetter ""
(Left "A letter is expected but got empty input","")

Parse two letters using 'do' notation.

>>> parse parseTwoLetters "ds"
(Right "ds","")
>>> parse parseTwoLetters "d5"
(Left "A letter is expected, but got 5","5")

Same as parseTwoLetters, but using applicative style.

>>> parse parseTwoLetters' "ab"
(Right "ab","")

Running the parser:

>>> :t runExceptT parseTwoLetters
runExceptT parseTwoLetters :: State Input (Either ErrorMsg String)

>>> :t runState (runExceptT parseTwoLetters)
runState (runExceptT parseTwoLetters) :: Input -> (Either ErrorMsg String, Input)

>>> parse parseTwoLetters ""
(Left "A letter is expected but got empty input","")
>>> parse parseTwoLetters "ds"
(Right "ds","")
>>> parse parseTwoLetters "545435"
(Left "A letter is expected, but got 5","545435")

Helper to run a Parser on an input string.

A more complex monad stack: IO + State + Except.

Weird a fails with an Int, keeps a String state, and can do IO.

Version using nested lift calls. Interacts with the console and stores the last answer in state.

>>> :t runStateT (runExceptT weird) "init"
runStateT (runExceptT weird) "init" :: IO (Either Int Double, String)

Same behavior, but with liftIO for cleaner IO lifting.

A simple sum type used with QuickCheck.