open-union
Version 0.4.0.0 revision 0 uploaded by BenFoppa.
Package meta
- Synopsis
- Extensible, type-safe unions.
- Description
Extensible, type-safe unions. This package is very new and likely to change.
Basic usage example (language tags ommitted due to https://github.com/haskell/cabal/issues/774)
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
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 null 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 ‘Int : ('[] :\ [Char])’ with ‘'[]’ Expected type: Union ('[Int] :\ String) -> String Actual type: Union '[] -> String In the second argument of ‘(@>)’, namely ‘typesExhausted’ In the second argument of ‘(@>)’, namely ‘(\ (s :: String) -> "string: " ++ s) @> typesExhausted’
The left-hand parts of the `: (think type-level (:)) are the cases that still need to be satisfied.
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 '[] [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"
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.
- Author
- Zeke Foppa
- Bug reports
- n/a
- Category
- Data
- Copyright
- n/a
- Homepage
- https://github.com/bfops/open-union
- Maintainer
- benjamin.foppa@gmail.com
- Package URL
- n/a
- Stability
- n/a