Why does decodeFile from Data.Binary throw the error *** Exception: demandInput: not enough bytes, when using decode and readFile from Data.ByteString.Lazy works as expected?
A minimal example
module Testing where
import Data.Binary
import qualified Data.Map.Strict as Map
import qualified Data.ByteString.Lazy as B
x :: Map.Map Char Int
x = Map.fromList $ zip ['a'..'d'] [1,2,3,4]
main :: IO (Map.Map Char Int)
main = do
encodeFile "testing" (encode x)
r <- decodeFile "testing"
print $ r == x
return r
main2 :: IO (Map.Map Char Int)
main2 = do
B.writeFile "testing2" (encode x)
r <- fmap decode $ B.readFile "testing2"
print $ r == x
return r
I found this potentially relevant question: Using Data.Binary.decodeFile, encountered error "demandInput: not enough bytes" but it does not really answer my question, which is why using decodeFile doesn't work but readFile does.
The output I get:
[1 of 1] Compiling Testing ( testing.hs, interpreted )
Ok, modules loaded: Testing.
*Testing> main
*** Exception: demandInput: not enough bytes
*Testing> main2
True
fromList [('a',1),('b',2),('c',3),('d',4)]
You have
encodeFile "testing" (encode x)
which means you're encoding x twice. Changing that to
encodeFile "testing" x
works as expected.