Question

I’m trying to parse a JSON string using the json library. This code works perfectly in GHCI:

import Text.JSON as JS
JS.decode  "{}"  :: Result JSValue

But when I use the same code in a compiled program:

case JS.decode "{}" of
  JS.Ok value -> putStrLn value
  JS.Error err -> error err

It is unable to parse the String with:

Unable to read String

I suspect it’s just me doing something stupid but I can’t figure out what….

Any ideas very very welcome!

Update:

I wrote this code to see if it was something in the other part of the app causing the problem:

import qualified Text.JSON as JS

main :: IO ()
main = do
   case JS.decode "{}" of
                JS.Ok value -> putStrLn value
                JS.Error err -> error err

However I get the same error with this:

test: Unable to read String

It’s compiled with GHC 7.0.3 and the source was edited with vim. However even string data passed in from outside the app generate the same error. Really out of ideas now…

Answer 1

Use print instead of putStrLn, which only works on Strings:

import qualified Text.JSON as JS

main :: IO ()
main = do
   case JS.decode "{}" of
            JS.Ok value -> print (value :: JSValue)
            JS.Error err -> error err