Record access

Here some proposal for desugared fine functional record field access for HaskellTwo and above.

{- |
In Haskell 98 the name of a record field
is automatically also the name of a function which gets the value
of the according field.
E.g. if we have
@
data Pair a b = Pair {first :: a, second :: b}
@
then
@
first  :: Pair a b -> a
second :: Pair a b -> b
@
However for setting or modifying a field value
we need to use some syntactic sugar, which is often clumsy.
@
modifyFirst :: (a -> a) -> (Pair a b -> Pair a b)
modifyFirst f r@(Pair {first=a}) = r{first = f a}
@

We propose to extend the meaning of the record field names
to a function which allows setting, getting and modifying values easily.
-}
module RecordAccess where

import Control.Monad.State (MonadState, StateT)
import qualified Control.Monad.State as State
import Data.Char (ord)

{- |
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.
-}
type Accessor r a  =  a -> r -> (a, r)

{- *
Access helper functions,
these are similar to State methods and should be in Prelude
-}

{- | Set the value of a field. -}
set :: Accessor r a -> a -> r -> r
set f x = snd . f x

{- | 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.
-}
setMany :: [r -> (a, r)] -> r -> r
setMany = flip (foldl (\x f -> snd (f x)))

{- |
This is a general function,
but it is especially useful for setting many values of different type at once.
-}
compose :: [r -> r] -> r -> r
compose = flip (foldl (flip id))

{- | Get the value of a field. -}
get :: Accessor r a -> r -> a
get f = fst . f undefined

{- | Transform the value of a field by a function. -}
modify :: Accessor r a -> (a -> a) -> (r -> r)
modify f g rOld =
   let (a,rNew) = f (g a) rOld
   in  rNew


{- *
Access helper functions in a State monad.
-}

setState :: MonadState r m => Accessor r a -> a -> m ()
setState f x = State.modify (set f x)

getState :: MonadState r m => Accessor r a -> m a
getState f = State.gets (get f)

modifyState :: MonadState r m => Accessor r a -> (a -> a) -> m ()
modifyState f g = State.modify (modify f g)



{- * Reading records from streams -}

class ReadBin a where
   readBin :: String -> Maybe (a, String)

instance ReadBin Char where
   readBin (c:cs) = Just (c,cs)
   readBin _ = Nothing

instance ReadBin Int where
   readBin (c0:c1:c2:c3:cs) =
      Just (foldl1 (\acc d -> acc*256+d) (map ord [c0,c1,c2,c3]), cs)
   readBin _ = Nothing

type Parser r = (r, String) -> Maybe (r, String)

readField :: ReadBin a => Accessor r a -> Parser r
readField f (r,s) =
   do (x,s') <- readBin s
      return (set f x r, s')

readRecord :: [Parser r] -> Parser r
readRecord ps = flip (foldl (>>=)) ps . Just



{- * Example accessors for the pair type -}

{- | Access to the first value of a pair. -}
first :: Accessor (a,b) a
first xNew (xOld,y) = (xOld, (xNew,y))

{- | Access to the second value of a pair. -}
second :: Accessor (a,b) b
second yNew (x,yOld) = (yOld, (x,yNew))



{- * Example accesses -}

{- | Example of using 'set', 'get', 'modify'. -}
example :: Int
example =
   get second $
   modify second succ $
   set first 'a' $
   ('b',7)

exampleState :: State.State (Char,Int) Int
exampleState =
   do setState first 'a'
      modifyState second succ
      getState second

exampleInit :: (Char,Int)
exampleInit =
   compose [set first 'b', modify first succ, set second 7]
           (undefined,undefined)
--   setMany [first 'b', second 7] (undefined,undefined)

exampleRead :: Maybe ((Char,Int), String)
exampleRead =
   readRecord
      [readField first, readField second]
      ((undefined,undefined), "c\059\154\202\000")

See also

• There is a partial implementation of this using Template Haskell for GHC.
• A similar proposal using "lens" by Apfelmus in Haskell-Cafe. The thread started with "Is there a more concise way to generate helper functions for a datatype built on records?"