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. In contrast to Data.Accessor.Basic it is intended for unqualified import.

Documentation

type Accessor r a = T r a

An Accessor r 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 r) = x
 setVal a (getVal a r) r = r

accessor

Arguments

:: (r -> a)

get method

-> (a -> r -> r)

set method

-> Accessor r a

Construct an Accessor from a get and a set method.

setVal

Arguments

:: Accessor r a

record field f

-> a

value x to be set

-> r

original record

-> r

new record with field f changed to x

Set a value of a record field that is specified by an Accessor

(^=) :: T r a -> a -> r -> r

set as infix operator. This lets us write first ^= 2+3 $ second ^= 5+7 $ record.

getVal

Arguments

:: Accessor r a

record field

-> r

record

-> a

value of the field in the record

Get a value from a record field that is specified by an Accessor

(^.) :: r -> T r a -> a

get as infix operator. This lets us write record^.field^.subfield. This imitates Modula II syntax.

(^:) :: T r a -> (a -> a) -> r -> r

modify as infix operator. This lets us write field^:subfield^:(2*) $ record, record$%field^:subfield^:(2*) or record$%field^:subfield^:(const 1).

getA :: Monad m => Accessor r a -> StateT r m a

Deprecated: Data.Accessor.Monad.Trans.State.get from data-accessor-transformers package

A structural dereference function for state monads.

putA :: Monad m => Accessor r a -> a -> StateT r m ()

Deprecated: Data.Accessor.Monad.Trans.State.set from data-accessor-transformers package

A structural assignment function for state monads.

(=:) :: Monad m => Accessor r a -> a -> StateT r m ()

Deprecated: use (Data.Accessor.Monad.Trans.State.%=) from data-accessor-transformers package

An "assignment operator" for state monads.

 (=:) = putA

(%=) :: Monad m => T r a -> a -> StateT r m ()

Infix variant of set.

modA :: Monad m => Accessor r a -> (a -> a) -> StateT r m ()

Deprecated: Data.Accessor.Monad.Trans.State.modify from data-accessor-transformers package

A structural modification function for state monads.

(%:) :: Monad m => T r a -> (a -> a) -> StateT r m ()

Infix variant of modify.

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

Accessor composition: Combine an accessor with an accessor to a sub-field. Speak "stack".

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

Accessor composition the other direction.

 (<.) = flip (.>)

You may also use the (.) operator from Category class.