# Record access

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## Revision as of 02:40, 15 December 2009

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 ($))) {- | Get the value of a field. -} get :: Accessor r a -> r -> a get f = fst . f undefined infixl 9 ^. {- | 'get' as infix operator. This lets us write @record^.field^.subfield@ -} (^.) :: r -> Accessor r a -> a (^.) = flip get {- | 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 infixr 9 $%, ^: {- | 'modify' as infix operator. This lets us write @record$%field^:subfield^:(1+)@ or @record$%field^:subfield^:(const 1)@. -} (^:) :: Accessor r a -> (a -> a) -> (r -> r) (^:) = modify {- | Flipped version of '($)'. -} ($%) :: a -> (a -> b) -> b ($%) = flip ($) infixr 9 .: {- | Combine an accessor with an accessor to a sub-field. Speak \"stack\". -} (.:) :: Accessor a b -> Accessor b c -> Accessor a c (.:) f g cNew aOld = let (bOld, aNew) = f bNew aOld (cOld, bNew) = g cNew bOld in (cOld, aNew) {- * 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") exampleInfix0 :: Int exampleInfix0 = (('b',7),"hallo")^.first^.second exampleInfix1 :: ((Char, Int), String) exampleInfix1 = (('b',7),"hallo")$%first^:second^:(1+) exampleInfix2 :: Int exampleInfix2 = (('b',7),"hallo")^.(first.:second) exampleInfix3 :: ((Char, Int), String) exampleInfix3 = (('b',7),"hallo")$%(first.:second)^:(1+)

It would also be interesting if we could automatically generate a multi-parameter_type_class for each element name, with a functional dependency from record type to element type which allows us to re-use an element name for multiple record types.

## See also

- There is a readily usable implementation as Darcs repository and as installable package from Hackage.
- 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?"
- Another simple implementation http://code.haskell.org/yi/Yi/Accessor.hs
- Conal Elliott describes a very similar idea and call it Semantic editor combinators