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Labelled Tuples/Enums in GHC >9.2 Haskell (Hackage: https://hackage.haskell.org/package/pa-label)
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE LambdaCase #-}
module Label
( -- * Labels
Label,
label,
label',
getLabel,
mapLabel,
traverseLabel,
-- * Named Tuples
T2 (..),
focusOnField,
monoMapT2,
T3 (..),
monoMapT3,
-- * Named Sums/Enums
E2 (..),
mapE2,
monoMapE2,
monoFoldE2,
monoTraverseE2,
partitionE2,
E3 (..),
mapE3,
)
where
import Data.Data (Proxy (..))
import Data.Either (partitionEithers)
import Data.Function ((&))
import Data.Functor ((<&>))
import Data.Typeable (Typeable)
import GHC.Records (HasField (..))
import GHC.TypeLits (KnownSymbol, Symbol, symbolVal)
-- | A labelled value.
--
-- Use 'label'/'label'' to construct,
-- then use dot-syntax to get the inner value.
newtype Label (label :: Symbol) value = Label value
deriving stock (Eq, Ord)
deriving newtype (Typeable)
instance (KnownSymbol label, Show value) => Show (Label label value) where
showsPrec d (Label val) =
showParen (d > 10) $
showString "label @"
. showsPrec 11 (symbolVal (Proxy @label))
. showString " "
. showsPrec 11 val
-- | Attach a label to a value; should be used with a type application to name the label.
--
-- @
-- let f = label @"foo" 'f' :: Label "foo" Char
-- in f.foo :: Char
-- @
--
-- Use dot-syntax to get the labelled value.
label :: forall label value. value -> Label label value
label value = Label value
-- | Attach a label to a value; Pass it a proxy with the label name in the argument type.
-- This is intended for passing through the label value;
-- you can also use 'label'.
--
--
-- @
-- let f = label' (Proxy @"foo") 'f' :: Label "foo" Char
-- in f.foo :: Char
-- @
--
-- Use dot-syntax to get the labelled value.
label' :: forall label value. (Proxy label) -> value -> Label label value
label' Proxy value = Label value
-- | Fetches the labelled value.
instance HasField label (Label label value) value where
getField :: (Label label value) -> value
getField (Label value) = value
-- | Fetch a value from a record, like 'getField', but also keep it wrapped by its label.
getLabel :: forall label record a. HasField label record a => record -> Label label a
getLabel rec = rec & getField @label & label @label
-- | 'fmap' over the contents of the labbelled value. Helper.
mapLabel :: forall label a b. (a -> b) -> Label label a -> Label label b
mapLabel f (Label a) = Label @label $ f a
-- | 'traverse' over the contents of the labbelled value. Helper.
traverseLabel :: forall label f a b. Functor f => (a -> f b) -> Label label a -> f (Label label b)
traverseLabel fab (Label a) = Label @label <$> fab a
-- | A named 2-element tuple. Since the elements are named, you can access them with `.`.
--
-- @
-- let t2 = T2 (label @"myfield" 'c') (label @"otherfield" True) :: T2 "myfield" Char "otherfield" Bool
-- in (
-- t2.myfield :: Char,
-- t2.otherfield :: Bool
-- )
-- @
data T2 (l1 :: Symbol) t1 (l2 :: Symbol) t2 = T2 (Label l1 t1) (Label l2 t2)
deriving stock (Show, Eq)
-- | Access the first field by label
instance HasField l1 (T2 l1 t1 l2 t2) t1 where
getField (T2 t1 _) = getField @l1 t1
-- | Access the second field by label
instance HasField l2 (T2 l1 t1 l2 t2) t2 where
getField (T2 _ t2) = getField @l2 t2
-- | Given a record with some field, “focus” on that field by pulling it into the first part of the T2,
-- and put the original record into the second part of the T2.
--
-- This can be useful when you have a function that requires something with a field,
-- but the field itself is nested somewhere in the record.
--
-- Example:
--
-- @
-- data Foo = Foo
-- { nested :: Label "myId" Text
-- }
--
-- foo = Foo {nested = "hi"}
--
-- fn :: HasField "myId" rec Text => rec -> Text
-- fn rec = rec.myId <> "!"
--
-- x = fn (focusOnField @"myId" (.nested) foo) == "hi!"
-- @
--
-- Note that you will have to give `focusOnField` a type annotation of which label to use,
-- otherwise it cannot infer it.
focusOnField ::
forall field rec subrec t.
HasField field subrec t =>
(rec -> subrec) ->
rec ->
T2 field t "dat" rec
focusOnField zoom rec = T2 (getLabel @field (rec & zoom)) (label @"dat" rec)
-- | Map a function over all fields in the tuple. All fields have to have the same type.
monoMapT2 :: (t -> t') -> T2 l1 t l2 t -> T2 l1 t' l2 t'
monoMapT2 f (T2 a b) = T2 (mapLabel f a) (mapLabel f b)
-- | A named 3-element tuple. Since the elements are named, you can access them with `.`. See 'T2' for an example.
data T3 (l1 :: Symbol) t1 (l2 :: Symbol) t2 (l3 :: Symbol) t3 = T3 (Label l1 t1) (Label l2 t2) (Label l3 t3)
deriving stock (Show, Eq)
-- | Access the first field by label
instance HasField l1 (T3 l1 t1 l2 t2 l3 t3) t1 where
getField (T3 t1 _ _) = getField @l1 t1
-- | Access the second field by label
instance HasField l2 (T3 l1 t1 l2 t2 l3 t3) t2 where
getField (T3 _ t2 _) = getField @l2 t2
-- | Access the third field by label
instance HasField l3 (T3 l1 t1 l2 t2 l3 t3) t3 where
getField (T3 _ _ t3) = getField @l3 t3
-- | Map a function over all fields in the tuple. All fields have to have the same type.
monoMapT3 :: (t -> t') -> T3 l1 t l2 t l3 t -> T3 l1 t' l2 t' l3 t'
monoMapT3 f (T3 a b c) = T3 (mapLabel f a) (mapLabel f b) (mapLabel f c)
-- | A named 2-alternative sum (“'Either' with labels”).
data E2 (l1 :: Symbol) t1 (l2 :: Symbol) t2
= E21 (Label l1 t1)
| E22 (Label l2 t2)
deriving (Eq, Show)
instance (Bounded t1, Bounded t2) => Bounded (E2 l1 t1 l2 t2) where
minBound = E21 (label @l1 minBound)
maxBound = E22 (label @l2 maxBound)
-- TODO: instance for arbitrary Enum types?
instance Enum (E2 l1 () l2 ()) where
toEnum 0 = E21 (label @l1 ())
toEnum 1 = E22 (label @l2 ())
toEnum _ = error "E2: toEnum"
fromEnum (E21 _) = 0
fromEnum (E22 _) = 1
-- | Map a separate function over every possibility in this enum. The label names stay the same.
--
-- Each function has access to its label, this is intentional so that you have to mention the label once (e.g. by using dot-notation), to prevent confusing the cases.
mapE2 ::
forall l1 t1 t1' l2 t2 t2'.
(Label l1 t1 -> t1') ->
(Label l2 t2 -> t2') ->
E2 l1 t1 l2 t2 ->
E2 l1 t1' l2 t2'
mapE2 f1 f2 = \case
E21 lbl -> lbl & getLabel @l1 & f1 & label @l1 & E21
E22 lbl -> lbl & getLabel @l2 & f2 & label @l2 & E22
-- | Map a single function over every possiblity in this enum. All fields have to have the same type.
monoMapE2 :: (t -> t') -> E2 l1 t l2 t -> E2 l1 t' l2 t'
monoMapE2 f = \case
E21 lbl -> lbl & mapLabel f & E21
E22 lbl -> lbl & mapLabel f & E22
-- | If ever branch of this enum has the same type, fold the enum into its contents.
-- This loses the distinction between cases.
monoFoldE2 :: E2 l1 t l2 t -> t
monoFoldE2 = \case
E21 (Label t) -> t
E22 (Label t) -> t
-- | Partition a list of E2 into two lists that each keep their respective label.
-- Like 'partitionEithers', but with labels.
partitionE2 :: forall l1 t1 l2 t2. [E2 l1 t1 l2 t2] -> T2 l1 [t1] l2 [t2]
partitionE2 es =
es
<&> ( \case
E21 (Label t1) -> Left t1
E22 (Label t2) -> Right t2
)
& partitionEithers
& (\(t1s, t2s) -> T2 (label @l1 t1s) (label @l2 t2s))
-- | Map a monadic (actually just a functor-ic) function over each possibility in this enum. All fields have to have the same type.
monoTraverseE2 :: Functor f => (t -> f t') -> E2 l1 t l2 t -> f (E2 l1 t' l2 t')
monoTraverseE2 f = \case
E21 lbl -> lbl & traverseLabel f <&> E21
E22 lbl -> lbl & traverseLabel f <&> E22
-- | A named 3-alternative sum (“'Either' with labels”).
data E3 (l1 :: Symbol) t1 (l2 :: Symbol) t2 (l3 :: Symbol) t3
= E31 (Label l1 t1)
| E32 (Label l2 t2)
| E33 (Label l3 t3)
deriving (Eq, Show)
instance (Bounded t1, Bounded t3) => Bounded (E3 l1 t1 l2 t2 l3 t3) where
minBound = E31 (label @l1 minBound)
maxBound = E33 (label @l3 maxBound)
-- TODO: instance for arbitrary Enum types?
instance Enum (E3 l1 () l2 () l3 ()) where
toEnum 0 = E31 (label @l1 ())
toEnum 1 = E32 (label @l2 ())
toEnum 2 = E33 (label @l3 ())
toEnum _ = error "E3: toEnum"
fromEnum (E31 _) = 0
fromEnum (E32 _) = 1
fromEnum (E33 _) = 2
-- | Map a function over every element in this enum. The label names stay the same.
mapE3 ::
forall l1 t1 t1' l2 t2 t2' l3 t3 t3'.
(Label l1 t1 -> t1') ->
(Label l2 t2 -> t2') ->
(Label l3 t3 -> t3') ->
E3 l1 t1 l2 t2 l3 t3 ->
E3 l1 t1' l2 t2' l3 t3'
mapE3 f1 f2 f3 = \case
E31 lbl -> lbl & getLabel @l1 & f1 & label @l1 & E31
E32 lbl -> lbl & getLabel @l2 & f2 & label @l2 & E32
E33 lbl -> lbl & getLabel @l3 & f3 & label @l3 & E33
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