open-union: Extensible, type-safe unions.
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.
Downloads
- open-union-0.4.0.0.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)
Maintainer's Corner
For package maintainers and hackage trustees
Candidates
Versions [RSS] | 0.1.0, 0.1.0.0, 0.1.0.1, 0.2.0.0, 0.3.0.0, 0.4.0.0 (info) |
---|---|
Dependencies | base (>=4 && <5), open-union, type-fun [details] |
License | MIT |
Author | Zeke Foppa |
Maintainer | benjamin.foppa@gmail.com |
Category | Data |
Home page | https://github.com/bfops/open-union |
Source repo | head: git clone https://github.com/bfops/open-union.git |
Uploaded | by BenFoppa at 2018-04-20T01:08:10Z |
Distributions | NixOS:0.4.0.0 |
Reverse Dependencies | 4 direct, 2 indirect [details] |
Executables | example |
Downloads | 4784 total (15 in the last 30 days) |
Rating | (no votes yet) [estimated by Bayesian average] |
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Status | Docs available [build log] Last success reported on 2018-04-20 [all 1 reports] |