slides.md

% Applicative Parsing % Julie Moronuki % January 10, 2017

Hour 1: Applicative

Functor

fmap :: (a -> b) -> f a -> f b

Monad

• ... is a kind of functor.

• but the (a -> b) of fmap has become an (a -> f b)

fmap vs bind

fmap :: Functor f => (a -> b) -> f a -> f b

(>>=) :: Monad m  => m a -> (a -> m b) -> m b

flip bind

fmap :: Functor f => (a ->  b)  -> f a -> f b

(=<<) :: Monad m  => (a -> m b) -> m a -> m b

Join

join :: Monad m => m (m a) -> m a

Applicative

Pray I do not alter it further.

Applicative

(<*>) :: Applicative f => f (a -> b) -> f a -> f b

Applicatives vs Monads

• context sensitivity
• composability (applicatives compose; monads need transformers)

Applicative vs Monad

doSomething = do  
  a <- f  
  b <- g  
  c <- h  
  pure (a, b, c)  

doSomething' n = do  
  a <- f n  
  b <- g a  
  c <- h b  
  pure (a, b, c)  

AccValidation

• like an Either, but accumulates error values
• cannot have a Monad instance

ApplicativeDo

• language extension
• allows use of do syntax with applicatives

Hour 2: Electric Boogaloo

In this hour, we'll be working on a small project with the optparse-applicative library.

Example

-- stack new optex simple

Options.Applicative.Builder

Here are some basic argument types we can use: commands, flags, switches.

command :: String -> ParserInfo a -> Mod CommandFields a

Add a command to a subparser option.

Flag and flag'

flag :: a  -> a -> Mod FlagFields a -> Parser a
--     [1]   [2]         [3]              [4]

1. default value

2. active value

3. option modifier

4. Builder for a flag parser

Switch

switch :: Mod FlagFields Bool -> Parser Bool

-- flagEx.hs

helpful builders

subparser :: Mod CommandFields a -> Parser a  

-- Builder for a command parser.  

strArgument :: Mod ArgumentFields String -> Parser String  

-- Builder for a String argument.  

argument :: ReadM a -> Mod ArgumentFields a -> Parser a  

-- Builder for an argument parser.

information we can provide about arguments

short :: HasName f => Char -> Mod f a  

-- Specify a short name for an option.  

long :: HasName f => String -> Mod f a  

-- Specify a long name for an option.  

metavar :: HasMetavar f => String -> Mod f a  

-- Specify a metavariable for the argument.  

Options.Applicative.Extra

execParser :: ParserInfo a -> IO a

Run a program description.

Parse command line arguments. Display help text and exit if any parse error occurs.