open-union adds type-safe extensible unions to Haskell, which can be used a la:
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}
import Data.OpenUnion
type MyUnion = Union (Char :| Int :| [()])
showMyUnion :: MyUnion -> String
showMyUnion
= (\(c :: Char) -> "char: " ++ show c)
@> (\(i :: Int) -> "int: " ++ show i)
@> (\(l :: [()]) -> "list length: " ++ show (length l))
@> (\(s :: String) -> "string: " ++ s)
@> typesExhausted
main :: IO ()
main = do
putStrLn $ showMyUnion $ liftUnion (4 :: Int)
putStrLn $ showMyUnion $ liftUnion 'a'
putStrLn $ showMyUnion $ liftUnion [(), ()]
which prints:
int: 4
char: 'a'
list length: 2
The original use case for this library was code like this (snipped from some record/playback logic):
type TrackStates = '[Stopped, Recording, Playing]
startRecording
:: Union (TrackStates :\ Recording)
-> ([Note], Union '[Recording])
The (:\) type-level operator is for removal from a set, i.e. startRecording can be
applied to a track in any state except the Recording state.
Casting to an unrelated type does not cause errors;
In the above example,showMyUnion contains a String case despite MyUnion not containing
String - superfluous cases are ignored, for the time being.
typesExhausted is NOT a catchall. It is a Void case, and using it as a catchall
(or forgetting to provide a certain case, for instance) will result in an error like:
example.hs:12:8:
Couldn't match type ‘[()] :| (Void :\ [Char])’ with ‘Void’
Expected type: Union (([()] :| Void) :\ String) -> String
Actual type: Union Void -> String
In the second argument of ‘(@>)’, namely ‘typesExhausted’
In the second argument of ‘(@>)’, namely
‘(\ (s :: String) -> "string: " ++ s) @> typesExhausted’
Types to the right of :\ have been handled.
Trying to lift an incorrect type to a Union will cause an error resembling:
example.hs:20:30:
No instance for (Data.OpenUnion.Internal.LiftToUnion Void [Char])
arising from a use of ‘liftUnion’
In the second argument of ‘($)’, namely ‘liftUnion "asdf"’
In the second argument of ‘($)’, namely
‘showMyUnion $ liftUnion "asdf"’
In a stmt of a 'do' block:
putStrLn $ showMyUnion $ liftUnion "asdf"
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