[Haskell-beginners] Monad problem

[Bob Ippolito]

Great question! Many libraries use a monad transformer stack on top of IO
rather than a direct IO interface. This can be convenient if you are also
using such a stack, but it certainly complicates things a bit if you're
just in IO directly.

If you follow the error messages it says that it's expecting your Monad to
have an instance of MonadResource. IO does not (or else it would've just
worked). This means that you'll need to find a monad transformer that
provides MonadResource. By looking at the name "
Control.Monad.Trans.Resource.Internal.MonadResource" I know that I should
probably start looking for something called "Control.Monad.Trans.Resource".
After a bit of searching on Hackage I found
http://hackage.haskell.org/package/resourcet

The relevant function is `runResourceT` (most monad transformers are going
to have a similarly named run function to "unwrap" a transformer):
http://hackage.haskell.org/package/resourcet-1.1.5/docs/Control-Monad-Trans-Resource.html#v:runResourceT

Something like this should compile:

{-# LANGUAGE OverloadedStrings #-}

import Network.HTTP.Client
import Instagram
import Control.Monad.Trans.Resource

code = "xxx_some_code"
redirectUrl = "http://localhost:9988/instagram/oauth2/callback"
credentials = Credentials "xxx_some_api_id" "xxx_some_api_secret"

main :: IO ()
main = do
    manager <- newManager defaultManagerSettings
    token <- runResourceT . runInstagramT credentials manager $
        getUserAccessTokenURL2 redirectUrl code
    print token





On Sat, Jul 11, 2015 at 5:16 AM, René Klačan <rene.klacan at gmail.com> wrote:

> Hi all,
>
> I've been trying to create working example with "ig"
> https://hackage.haskell.org/package/ig-0.2.2 - library over instagram API
> and I am facing little monad problem.
>
> Can someone advise me please how to make this small piece of code work?
>
> {-# LANGUAGE OverloadedStrings #-}
>
> import Network.HTTP.Client
> import Instagram
>
> code = "xxx_some_code"
> redirectUrl = "http://localhost:9988/instagram/oauth2/callback"
> credentials = Credentials "xxx_some_api_id" "xxx_some_api_secret"
>
> main :: IO ()
> main = do
>     manager <- newManager defaultManagerSettings
>     token <- runInstagramT credentials manager $
>         getUserAccessTokenURL2 redirectUrl code
>     print token
>
>
>
> I am getting following error:
>
>
> src/Main.hs:14:9:
>     No instance for (Control.Monad.Trans.Resource.Internal.MonadResource
>                        IO)
>       arising from a use of ‘getUserAccessTokenURL2’
>     In the second argument of ‘($)’, namely
>       ‘getUserAccessTokenURL2 redirectUrl code’
>     In a stmt of a 'do' block:
>       token <- runInstagramT credentials manager
>                $ getUserAccessTokenURL2 redirectUrl code
>     In the expression:
>       do { manager <- newManager defaultManagerSettings;
>            token <- runInstagramT credentials manager
>                     $ getUserAccessTokenURL2 redirectUrl code;
>            print token }
>
>
>
> Thanks
>
> Rene
>
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