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Hello,
While trying to combine Get
parsers using asum
, I found out that instance Alternative Get
is not lawful: x <|> empty
is not the same as x
with respect to failure. x <|> empty
errors with "Data.Binary.Get(Alternative).empty"
, overriding the error message from x
.
For me, an unfortunate consequence is that the implementation of asum
leaks through: asum [x, y] = x <|> y <|> empty
provides less helpful parse errors than x <|> y
.
I have attached a minimal working example, also hosted on this Gist: https://gist.github.com/sugarbleat/4f30751feedf8d3e06911deae7ef4a5a, which can be run with cabal run Main.hs
. For reference, a similar test with Parsec shows no violation of identity.
Thanks for the great work on the library, by the way!
#!/usr/bin/env cabal
{- cabal:
build-depends: base ^>= 4.15
, bytestring ^>= 0.10.12
, binary == 0.8.9.1
-}
module Main (main) where
import Control.Applicative (Alternative (..))
import Data.Binary (Get, Word8, get)
import Data.Binary.Get (runGetOrFail)
import qualified Data.ByteString.Lazy as B
import Data.Foldable (asum)
testGet :: Show a => Get a -> B.ByteString -> IO ()
testGet p s = do
putStrLn "p"
print $ runGetOrFail p s
putStrLn "p <|> empty"
print $ runGetOrFail (p <|> empty) s
putStrLn "asum [p]"
print $ runGetOrFail (asum [p]) s
main :: IO ()
main = testGet (get :: Get Word8) B.empty
{-
Output:
p
Left ("",0,"not enough bytes")
p <|> empty
Left ("",0,"Data.Binary.Get(Alternative).empty")
asum [p]
Left ("",0,"Data.Binary.Get(Alternative).empty")
-}
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