Data.Variant

data Variant :: Row Type -> Type

Instances

• (RowToList r rl, VariantTags rl, VariantEqs rl) => Eq (Variant r)
• (RowToList r rl, VariantTags rl, VariantEqs rl, VariantOrds rl, VariantBounded rl) => Bounded (Variant r)
• (RowToList r rl, VariantTags rl, VariantEqs rl, VariantOrds rl, VariantBoundedEnums rl) => Enum (Variant r)
• (RowToList r rl, VariantTags rl, VariantEqs rl, VariantOrds rl, VariantBoundedEnums rl) => BoundedEnum (Variant r)
• (RowToList r rl, VariantTags rl, VariantEqs rl, VariantOrds rl) => Ord (Variant r)
• (RowToList r rl, VariantTags rl, VariantShows rl) => Show (Variant r)

inj :: forall proxy sym a r1 r2. Cons sym a r1 r2 => IsSymbol sym => proxy sym -> a -> Variant r2

Inject into the variant at a given label.

intAtFoo :: forall r. Variant (foo :: Int | r)
intAtFoo = inj (Proxy :: Proxy "foo") 42

prj :: forall proxy sym a r1 r2 f. Cons sym a r1 r2 => IsSymbol sym => Alternative f => proxy sym -> Variant r2 -> f a

Attempt to read a variant at a given label.

case prj (Proxy :: Proxy "foo") intAtFoo of
  Just i  -> i + 1
  Nothing -> 0

on :: forall proxy sym a b r1 r2. Cons sym a r1 r2 => IsSymbol sym => proxy sym -> (a -> b) -> (Variant r1 -> b) -> Variant r2 -> 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 rl r r1 r2 r3 b. RowToList r rl => VariantMatchCases rl r1 b => Union r1 r2 r3 => Record r -> (Variant r2 -> b) -> Variant r3 -> b

Match a Variant 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 Variant case.

onMatch
  { foo: \foo -> "Foo: " <> foo
  , bar: \bar -> "Bar: " <> 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 a. Variant () -> a

Combinator for exhaustive pattern matching.

caseFn :: Variant (foo :: Int, bar :: String, baz :: Boolean) -> String
caseFn = case_
 # on (Proxy :: Proxy "foo") (\foo -> "Foo: " <> show foo)
 # on (Proxy :: Proxy "bar") (\bar -> "Bar: " <> bar)
 # on (Proxy :: Proxy "baz") (\baz -> "Baz: " <> show baz)

match :: forall rl r r1 r2 b. RowToList r rl => VariantMatchCases rl r1 b => Union r1 () r2 => Record r -> Variant r2 -> b

Combinator for exhaustive pattern matching using an onMatch case record.

matchFn :: Variant (foo :: Int, bar :: String, baz :: Boolean) -> String
matchFn = match
  { foo: \foo -> "Foo: " <> show foo
  , bar: \bar -> "Bar: " <> bar
  , baz: \baz -> "Baz: " <> show baz
  }

default :: forall a r. a -> Variant r -> a

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

caseFn :: forall r. Variant (foo :: Int, bar :: String | r) -> String
caseFn = default "No match"
 # on (Proxy :: Proxy "foo") (\foo -> "Foo: " <> show foo)
 # on (Proxy :: Proxy "bar") (\bar -> "Bar: " <> bar)

expand :: forall lt a gt. Union lt a gt => Variant lt -> Variant gt

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

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

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

newtype Unvariant r

Constructors

• Unvariant (forall x. Unvariant' r x -> x)

type Unvariant' r x = forall proxy s t o. IsSymbol s => Cons s t o r => proxy s -> t -> x

unvariant :: forall r. Variant r -> Unvariant r

A low-level eliminator which reifies the IsSymbol and Cons constraints required to reconstruct the Variant. This lets you work generically with some Variant at runtime.

revariant :: forall r. Unvariant r -> Variant r

Reconstructs a Variant given an Unvariant eliminator.

class VariantEqs rl  where

Members

• variantEqs :: forall proxy. proxy rl -> List (VariantCase -> VariantCase -> Boolean)

Instances

• VariantEqs Nil
• (VariantEqs rs, Eq a) => VariantEqs (Cons sym a rs)

class VariantOrds rl  where

Members

• variantOrds :: forall proxy. proxy rl -> List (VariantCase -> VariantCase -> Ordering)

Instances

• VariantOrds Nil
• (VariantOrds rs, Ord a) => VariantOrds (Cons sym a rs)

class VariantShows rl  where

Members

• variantShows :: forall proxy. proxy rl -> List (VariantCase -> String)

Instances

• VariantShows Nil
• (VariantShows rs, Show a) => VariantShows (Cons sym a rs)

class VariantBounded rl  where

Members

• variantBounded :: forall proxy. proxy rl -> List (BoundedDict VariantCase)

Instances

• VariantBounded Nil
• (VariantBounded rs, Bounded a) => VariantBounded (Cons sym a rs)

class (VariantBounded rl) <= VariantBoundedEnums rl  where

Members

• variantBoundedEnums :: forall proxy. proxy rl -> List (BoundedEnumDict VariantCase)

Instances

• VariantBoundedEnums Nil
• (VariantBoundedEnums rs, BoundedEnum a) => VariantBoundedEnums (Cons sym a rs)

class Contractable gt lt 

Instances

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

class VariantMatchCases rl vo b | rl -> vo b

Instances

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