CODEWITHSUNDEEP
jsonhaskellfunctional-programming
qleguennec
qleguennec

Reputation: 544

Can't understand this module/type error

I'm trying to use the Aeson JSON library in haskell. Right now, i just need to use "decode" to read a JSON dump.

import Data.Aeson
import Data.ByteString as BS
import Control.Applicative

main :: IO ()
main = print $ decode <$> BS.readFile "json"

I got the following error when trying to compile/run it:

Couldn't match type 'ByteString'
with 'Data.ByteString.Lazy.Internal.ByteString'
NB: 'ByteString is defined in 'Data.ByteString.Internal'
    'Data.ByteString.Lazy.Internal.ByteString'
     is defined in 'Data.ByteString.Lazy.Internal.ByteString

This error doesn't make sense to me. I tried importing the files described by ghc, but the import either fails or doesn't solve the problem. Thanks

Upvotes: 1

Views: 68

Answers (1)

Joachim Breitner
Joachim Breitner

Reputation: 25782

There are two variants of ByteString: A strict (the default one), exported by Data.ByteString, and a lazy one, exported by Data.ByteString.Lazy.

Aeson works on top of lazy byte string, so you should change your second line to

import Data.ByteString.Lazy as BS

Upvotes: 2

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