haskell facebook example - facebook

Question

I am stuck with haskell types.
{-# LANGUAGE OverloadedStrings #-}
module Main (
main
) where
import qualified Facebook as FB
import Network.HTTP.Conduit (withManager)
import Control.Monad.IO.Class (liftIO)
import System.IO
app :: FB.Credentials
app = FB.Credentials "localhost" "249348058430770" "..."
url :: FB.RedirectUrl
url = "http://localhost/fb"
perms :: [FB.Permission]
perms = ["user_about_me", "email"]
main :: IO ()
main = do
fbAuthUrl <- FB.getUserAccessTokenStep1 url perms
liftIO $ print fbAuthUrl
argument <- readLn
token <- FB.getUserAccessTokenStep2 url [argument]
withManager $ \manager -> do
FB.runFacebookT app manager $ do
u <- FB.getUser "me" [] token
liftIO $ print (FB.userEmail u)
error
src/Main.hs:23:18:
Couldn't match expected type `IO t0'
with actual type `FB.FacebookT
FB.Auth m0 text-0.11.2.0:Data.Text.Internal.Text'
In the return type of a call of `FB.getUserAccessTokenStep1'
In a stmt of a 'do' block:
fbAuthUrl <- FB.getUserAccessTokenStep1 url perms
In the expression:
do { fbAuthUrl <- FB.getUserAccessTokenStep1 url perms;
liftIO $ print fbAuthUrl;
argument <- readLn;
token <- FB.getUserAccessTokenStep2 url [argument];
.... }
package http://hackage.haskell.org/package/fb

Answer 1

First, let me preface this answer by the disclaimer that I've never actually used the Facebook API or the Conduits library, so I'm not sure if this code actually does anything sensible, but by going with just the type information, I think this is what you were trying to do
main :: IO ()
main = withManager $ \manager -> FB.runFacebookT app manager $ do
fbAuthUrl <- FB.getUserAccessTokenStep1 url perms
liftIO $ print fbAuthUrl
argument <- liftIO $ readLn
token <- FB.getUserAccessTokenStep2 url [argument]
u <- FB.getUser "me" [] (Just token)
liftIO $ print (FB.userEmail u)
The main pitfall is that main in Haskell must always have the type IO a, but you are trying to use a value of type FacebookT Auth m () as your main. Your implementation is on the right track, but the runFacebookT and withManager need to be the first thing in the function.
Type-wise, the actual do-block has the type FacebookT Auth (ResourceT IO) (). The runFacebookT function is used to unwrap the FacebookT transformer, resulting in a ResourceT IO () value, which is in turn processed by withManager to produce a plain old IO ().
One additional problem was that you originally had a readLn in your do-block without liftIO, which was confusing the type-inference. I also added the missing Just to the FB.getUser call.