Data.Accessor

This module provides a simple abstract data type for a piece of a data stucture that can be read from and written to. It provides an automatic Template Haskell routine to scour data type definitions and generate accessor objects for them automatically.

Documentation

data Accessor s a

An Accessor s a is an object that encodes how to get and put a subject of type a out of/into an object of type s.

In order for an instance of this data structure a to be an Accessor, it must obey the following laws:

 getVal a (setVal a x s) = x
 setVal a (getVal a s) s = s

Constructors

Accessor

Fields

getVal :: s -> a

setVal :: a -> s -> s

nameDeriveAccessors :: Name -> (String -> Maybe String) -> Q [Dec]

nameDeriveAccessors n f where n is the name of a data type declared with data and f is a function from names of fields in that data type to the name of the corresponding accessor. If f returns Nothing, then no accessor is generated for that field.

deriveAccessors :: Name -> Q [Dec]

deriveAccessors n where n is the name of a data type declared with data looks through all the declared fields of the data type, and for each field ending in an underscore generates an accessor of the same name without the underscore.

It is nameDeriveAccessors n f where f satisfies

 f (s ++ "_") = Just s
 f x          = x       -- otherwise

For example, given the data type:

 data Score = Score { p1Score_ :: Int
                    , p2Score_ :: Int
                    , rounds   :: Int
                    }

deriveAccessors will generate the following objects:

 p1Score :: Accessor Score Int
 p1Score = Accessor p1Score_ (\x s -> s { p1Score_ = x })
 p2Score :: Accessor Score Int
 p2Score = Accessor p2Score_ (\x s -> s { p2Score_ = x })

getA :: MonadState s m => Accessor s a -> m a

A structural dereference function for state monads.

putA :: MonadState s m => Accessor s a -> a -> m ()

A structural assignment function for state monads.

modA :: MonadState s m => Accessor s a -> (a -> a) -> m ()

A structural modification function for state monads.

(.>) :: Accessor a b -> Accessor b c -> Accessor a c

Accessor composition.

(<.) :: Accessor b c -> Accessor a b -> Accessor a c

Accessor composition the other direction.

 (<.) = flip (.>)

(=:) :: MonadState s m => Accessor s a -> a -> m ()

An assignment operator for state monads.

 (=:) = putA