Data.Accessor.Basic

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

Documentation

data T r a

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 a

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

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

Set the value of a field.

setMany :: [r -> (a, r)] -> r -> r

Set many fields at once.

This function could also be used for initialisation of record, if record value with undefined fields is provided.

Drawback: Since all types in a list must have the same type, you can set only values of the same type.

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

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

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

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

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

Get the value of a field.

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

get as infix operator. This lets us write record^.field^.subfield

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

Transform the value of a field by a function.

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

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

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

Flipped version of '($)'.

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

Accessor composition the other direction.

 (<.) = flip (.>)

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

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