Data.Functor.Variant

data VariantF (f :: Row Type) a

Instances

• Functor (VariantF r)
• (RowToList r rl, VariantTags rl, VariantFShows rl a, Show a) => Show (VariantF r a)

inj :: forall r2 r1 a f sym. Cons sym (FProxy f) r1 r2 => IsSymbol sym => Functor f => SProxy sym -> f a -> VariantF r2 a

Inject into the variant at a given label.

maybeAtFoo :: forall r. VariantF (foo :: FProxy Maybe | r) Int
maybeAtFoo = inj (SProxy :: SProxy "foo") (Just 42)

prj :: forall g r2 r1 a f sym. Cons sym (FProxy f) r1 r2 => Alternative g => IsSymbol sym => SProxy sym -> VariantF r2 a -> g (f a)

Attempt to read a variant at a given label.

case prj (SProxy :: SProxy "foo") maybeAtFoo of
  Just (Just i) -> i + 1
  _ -> 0

on :: forall r2 r1 b a f sym. Cons sym (FProxy f) r1 r2 => IsSymbol sym => SProxy sym -> (f a -> b) -> (VariantF r1 a -> b) -> VariantF r2 a -> b

Attempt to read a variant at a given label by providing branches. The failure branch receives the provided variant, but with the label removed.

onMatch :: forall b a r3 r2 r1 r rl. RowToList r rl => VariantFMatchCases rl r1 a b => Union r1 r2 r3 => Record r -> (VariantF r2 a -> b) -> VariantF r3 a -> b

Match a VariantF with a Record containing functions for handling cases. This is similar to on, except instead of providing a single label and handler, you can provide a record where each field maps to a particular VariantF case.

onMatch
 { foo: \foo -> "Foo: " <> maybe "nothing" id foo
 , bar: \bar -> "Bar: " <> snd bar
 }

Polymorphic functions in records (such as show or id) can lead to inference issues if not all polymorphic variables are specified in usage. When in doubt, label methods with specific types, such as show :: Int -> String, or give the whole record an appropriate type.

case_ :: forall b a. VariantF () a -> b

Combinator for exhaustive pattern matching.

caseFn :: VariantF (foo :: FProxy Maybe, bar :: FProxy (Tuple String), baz :: FProxy (Either String)) Int -> String
caseFn = case_
 # on (SProxy :: SProxy "foo") (\foo -> "Foo: " <> maybe "nothing" show foo)
 # on (SProxy :: SProxy "bar") (\bar -> "Bar: " <> show (snd bar))
 # on (SProxy :: SProxy "baz") (\baz -> "Baz: " <> either id show baz)

match :: forall b a r2 r1 r rl. RowToList r rl => VariantFMatchCases rl r1 a b => Union r1 () r2 => Record r -> VariantF r2 a -> b

Combinator for exhaustive pattern matching using an onMatch case record.

matchFn :: VariantF (foo :: FProxy Maybe, bar :: FProxy (Tuple String), baz :: FProxy (Either String)) Int -> String
matchFn = match
 { foo: \foo -> "Foo: " <> maybe "nothing" show foo
 , bar: \bar -> "Bar: " <> show (snd bar)
 , baz: \baz -> "Baz: " <> either id show baz
 }

default :: forall r b a. a -> VariantF r b -> a

Combinator for partial matching with a default value in case of failure.

caseFn :: forall r. VariantF (foo :: FProxy Maybe, bar :: FProxy (Tuple String) | r) Int -> String
caseFn = default "No match"
 # on (SProxy :: SProxy "foo") (\foo -> "Foo: " <> maybe "nothing" show foo)
 # on (SProxy :: SProxy "bar") (\bar -> "Bar: " <> show (snd bar))

expand :: forall a gt mix lt. Union lt mix gt => VariantF lt a -> VariantF gt a

Every VariantF lt a can be cast to some VariantF gt a as long as lt is a subset of gt.

contract :: forall a f gt lt. Alternative f => Contractable gt lt => VariantF gt a -> f (VariantF lt a)

A VariantF gt a can be cast to some VariantF lt a, where lt is is a subset of gt, as long as there is proof that the VariantF's runtime tag is within the subset of lt.

class VariantFShows (rl :: RowList) x  where

Members

• variantFShows :: RLProxy rl -> Proxy x -> List (VariantCase -> String)

Instances

• VariantFShows Nil x
• (VariantFShows rs x, TypeEquals a (FProxy f), Show (f x), Show x) => VariantFShows (Cons sym a rs) x

data SProxy (sym :: Symbol)

A value-level proxy for a type-level symbol.

Constructors

• SProxy

data FProxy (a :: Type -> Type)

Proxy for a Functor.

Constructors

• FProxy

class Contractable gt lt 

Instances

• (RowToList lt ltl, Union lt a gt, VariantTags ltl) => Contractable gt lt

class VariantFMatchCases (rl :: RowList) (vo :: Row Type) a b | rl -> vo a b

Instances

• (VariantFMatchCases rl vo' a b, Cons sym (FProxy f) vo' vo, TypeEquals k (f a -> b)) => VariantFMatchCases (Cons sym k rl) vo a b
• VariantFMatchCases Nil () a b