Haskell Spock IO within GET Route ActionCtxT Error - haskell

Question

I try to return a uuid within a route definition for a web app (Spock Webserver).
A route is pretty simple to define
get("PATH") $ do
text "Hello World"
Now I try to return a uuid via nextRandom from the Data.UUID.V1 module.
The function returns a IO(Maybe UUID) value.
So I thought, since I am in IO and work with another IO I have to do bind the value simply with <-, like so:
get ("id") $ do
uuid<-nextUUID
json . pack $ show $ uuid
But the compiler says no:
Couldn't match type ‘ActionCtxT ctx0 m0’ with ‘IO’
Expected type: IO b0
Actual type: ActionCtxT ctx0 m0 b0
• In a stmt of a 'do' block: json . pack $ show $ uuid
In the second argument of ‘($)’, namely
‘do { uuid <- nextUUID;
json . pack $ show $ uuid }’
Why is it throwing that error?
I could easily create the uuid with a simple print example, but within Spock I don't understand what the ActionCtxT does and why I can't do the uuid IO in it.

Answer 1

So I thought, since I am in IO and work with another IO
That's the trouble here, when you're routing in Spock, you're not in IO. The error message tells you what context you're really in: ActionCtxT ctx0 m0. According to the docs, that's a monad transformer stack that bundles effects and state.
You can "lift" an IO computation into the right type using liftIO.
get ("id") $ do
uuid <- liftIO nextUUID
json . pack $ show $ uuid

Answer 2

Based on Libbys helpful answer I just added the catch for the Nothing of the Maybe UUID. Here the full programm:
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Web.Spock hiding (head)
import Web.Spock.Config
import Data.UUID.V1
import Data.Pool
import Control.Monad.IO.Class
import Database.PostgreSQL.Simple
import Data.Aeson (Value(Null))
import qualified Network.HTTP.Types.Status as Http
type AppAction a = SpockActionCtx () Connection AppSession AppState a
data AppState = EmptyState
data AppSession = EmptySession
main :: IO ()
main =
do pool<-createPool (connect (ConnectInfo "localhost" 5432 "" "" "envelopes") ) close 1 10 10
spockCfg <- defaultSpockCfg EmptySession (PCPool pool) EmptyState
runSpock 8080 (spock spockCfg app)
app :: SpockM Connection AppSession AppState ()
app = do
get ("json/id") $ do
uuid<-liftIO nextUUID
case uuid of
Nothing -> do
setStatus Http.status500
json Null
Just x -> json $ show x