How to extract data using Text.JSON? - json

Question

I'm trying to write a wrapper around ffprobe that extracts value in JSON of the format {"format": {"format_name": value}}. The JSON is output by a created process. Here's what I've gotten to.
import System.Process
import System.Environment
import System.IO
import Text.JSON
main = do
args <- getArgs
(_, Just out, _, p) <- createProcess
(proc "ffprobe" [args!!0, "-of", "json", "-show_format"])
{ std_out = CreatePipe }
s <- hGetContents out
--putStrLn $ show (decode s :: Result JSValue)
--waitForProcess p
--putStrLn $ valFromObj "format_name" format
-- where format = valFromObj "format" rootObj
-- (Ok rootObj) = decode s :: Result (JSObject (JSValue))
let (Ok rootObj) = decode s :: Result (JSObject (JSValue))
let (Ok format) = valFromObj "format" rootObj :: Result (JSObject (JSValue))
putStrLn format_name
where (Ok format_name) = valFromObj "format_name" format
It fails to compile with:
[1 of 1] Compiling Main ( ffprobe.hs, ffprobe.o )
ffprobe.hs:20:59: error:
Variable not in scope: format :: JSObject JSValue
I'm confused about several things, including why I can't get the last line to compile:
Why can't I assert for Ok in the Result after the ::. Like :: Result Ok JSObject JSValue?
Why can't I extract the values in a where clause?
Why is it Result (JSObject (JSValue)) and not Result JSObject JSValue?
Why is format out of scope?
I have a feeling I'm mixing the IO and Result monads together in the same do block or something. Is Result even a monad? Can I extract the value I want in a separate do without crapping all over the IO do?

Answer 1

I think your compile error is because of the position of the where. Try
main = do
...
let (Ok format) = valFromObj "format" rootObj :: Result (JSObject (JSValue))
let (Ok format_name) = valFromObj "format_name" format
putStrLn format_name
The scope of the where is outside the do so it isn't aware of format.

Answer 2

You cannot do this:
main = do
let bar = "only visible inside main? "
return baz
where
baz = bar ++ " yes, this will break!"
This gives:
test.hs:7:11:
Not in scope: ‘bar’
Perhaps you meant ‘baz’ (line 7)
Let bindings unlike function arguments are not available in where bindings. Above bar is not in scope for baz to use it. Compare to your code.