Agda-2.2.8: A dependently typed functional programming language and proof assistant




When lexing by hands (for instance string literals) we need to do some looking ahead. The LookAhead monad keeps track of the position we are currently looking at, and provides facilities to synchronise the look-ahead position with the actual position of the Parser monad (see sync and rollback).


The LookAhead monad

data LookAhead a Source

The LookAhead monad is basically a state monad keeping with an extra AlexInput, wrapped around the Parser monad.


runLookAhead :: (forall b. String -> LookAhead b) -> LookAhead a -> Parser aSource

Run a LookAhead computation. The first argument is the error function.


getInput :: LookAhead AlexInputSource

Get the current look-ahead position.

setInput :: AlexInput -> LookAhead ()Source

Set the look-ahead position.

liftP :: Parser a -> LookAhead aSource

Lift a computation in the Parser monad to the LookAhead monad.

nextChar :: LookAhead CharSource

Look at the next character. Fails if there are no more characters.

eatNextChar :: LookAhead CharSource

Consume the next character. Does nextChar followed by sync.

sync :: LookAhead ()Source

Consume all the characters up to the current look-ahead position.

rollback :: LookAhead ()Source

Undo look-ahead. Restores the input from the ParseState.

match :: [(String, LookAhead a)] -> LookAhead a -> LookAhead aSource

Do a case on the current input string. If any of the given strings match we move past it and execute the corresponding action. If no string matches, we execute a default action, advancing the input one character. This function only affects the look-ahead position.

match' :: Char -> [(String, LookAhead a)] -> LookAhead a -> LookAhead aSource

Same as match but takes the initial character from the first argument instead of reading it from the input. Consequently, in the default case the input is not advanced.