data-accessor- Utilities for accessing and manipulating fields of records



This module defines the Accessor type. It should be imported with qualification.



data T r a Source

The access functions we propose, look very similar to those needed for List.mapAccumL (but parameter order is swapped) and State monad. They get the new value of the field and the record and return the old value of the field and the record with the updated field.


fromSetGet :: (a -> r -> r) -> (r -> a) -> T r aSource

fromLens :: (r -> (a, a -> r)) -> T r aSource

fromWrapper :: (b -> a) -> (a -> b) -> T a bSource

If an object is wrapped in a newtype, you can generate an Accessor to the unwrapped data by providing a wrapper and an unwrapper function. The set function is simpler in this case, since no existing data must be kept. Since the information content of the wrapped and unwrapped data is equivalent, you can swap wrapper and unwrapper. This way you can construct an Accessor that treats a record field containing an unwrapped object like a field containing a wrapped object.

 newtype A = A {unA :: Int}

 access :: Accessor.T A Int
 access = fromWrapper A unA

self :: T r rSource

Access the record itself

null :: T r ()Source

Access a (non-existing) element of type ()

result :: Eq a => a -> T (a -> r) rSource

result a accesses the value of a function for argument a.

Also see semantic editor combinators, that allow to modify all function values of a function at once. Cf.

set :: T r a -> a -> r -> rSource

Set the value of a field.

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

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

compose :: [r -> r] -> r -> rSource

This is a general function, but it is especially useful for setting many values of different type at once.

get :: T r a -> r -> aSource

Get the value of a field.

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

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

modify :: T r a -> (a -> a) -> r -> rSource

Transform the value of a field by a function.

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

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

(.>) :: T a b -> T b c -> T a cSource

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

(<.) :: T b c -> T a b -> T a cSource

Accessor composition the other direction.

 (<.) = flip (.>)

($%) :: a -> (a -> b) -> bSource

Flipped version of '($)'.