Write You a Haskell Stephen Diehl

-- Start of layout ( Column: 0 )
fib :: Int -> Int
fib x = truncate $ ( 1 / sqrt 5 ) * ( phi ^ x - psi ^ x ) -- (Column: > 0)
-- Start of new layout ( Column: 2 )
where
-- Indented block ( Column: > 2 )
phi = ( 1 + sqrt 5 ) / 2
psi = ( 1 - sqrt 5 ) / 2
e Parsec monad is itself parameterized over a type variable s which stands for the State layer baked
into the monad allowing us to embed custom parser state inside of our rules. To adopt our parser to
handle sensitive whitespace we will
-- Indentation sensitive Parsec monad.
type IParsec a = Parsec Text ParseState a
data ParseState = ParseState
{ indents :: Column
} deriving (Show)
initParseState :: ParseState
initParseState = ParseState 0
Inside of the Parsec the internal position state (SourcePos) is stored during each traversal, and is accessible
inside of rule logic via getPosition function.
data SourcePos = SourcePos SourceName !Line !Column
getPosition :: Monad m => ParsecT s u m SourcePos
In terms of this function we can write down a set of logic that will allow us to query the current column
count and then either succeed or fail to match on a pattern based on the current indentation level. e
laidout combinator will capture the current indentation state and push it into the indents field in the
State monad.
laidout :: Parsec s ParseState a -> Parsec s ParseState a
laidout m = do
cur