CODEWITHSUNDEEP
haskellwebghccabal
nahiluhmot
nahiluhmot

Reputation: 33

Haskell Dependency Conflict

I've been trying to start writing a web app in Scotty, but I'm getting a dependency conflict when I try to run the server. Here's my code:

{-# LANGUAGE OverloadedStrings #-}
module Site where

import Web.Scotty
import Control.Monad.IO.Class
import qualified Data.Text.Lazy.IO as T

-- Controllers
indexController :: ActionM ()
indexController = do
    index <- liftIO $ T.readFile "public/index.html"
    html index

routes :: ScottyM ()
routes = do
    get "/" indexController

main :: IO ()
main = do
    scotty 9901 routes

When I run it using runhaskell Site.hs, I get the following error:

Site.hs:12:10:
    Couldn't match expected type `text-0.11.2.3:Data.Text.Lazy.Internal.Text'
                with actual type `Data.Text.Lazy.Internal.Text'
    In the first argument of `html', namely `index'
    In a stmt of a 'do' block: html index
    In the expression:
      do { index <- liftIO $ T.readFile "public/index.html";
           html index }

Using cabal list text, it tells me that versions 0.11.2.3 and 0.11.3.1 are installed, but 0.11.3.1 is the default. Scotty's scotty.cabal specifies that the text package must be >= 0.11.2.3, which seems to me like the above code should work. Are there any workarounds for this sort of error?

Upvotes: 3

Views: 393

Answers (1)

Daniel Fischer
Daniel Fischer

Reputation: 183978

The error message

Site.hs:12:10:
    Couldn't match expected type `text-0.11.2.3:Data.Text.Lazy.Internal.Text'
                with actual type `Data.Text.Lazy.Internal.Text'

means that your scotty was compiled using version 0.11.2.3 of the text package, but the invocation of runhaskell chose to use version 0.11.3.1 (because that's the newest you have, and you haven't told it to use a different version). The (lazy) Text types of two different package versions are, as far as GHC is concerned, two completely different types, and therefore, you must use the exact version of text used to compile the scotty library to run the code.

runhaskell -package=text-0.11.2.3 Site.hs

ought to work. If you compile the module, you also need to tell GHC to use the right version of text, either directly or via Cabal.

Another option could be to recompile scotty against the newer text version.

Upvotes: 5

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