Error (or warning?) "Could not deduce" in Main.hs

General
1

Hi I am getting this message in VSC when looking at the Main.hs file and I wonder if somebody could explain to me what that’s about and if I can fix it in any way.

• Could not deduce (FrontController WebApplication)
    arising from a use of ‘mountFrontController’
  from the context: (?applicationContext::IHP.ApplicationContext.ApplicationContext,
                     ?application::RootApplication,
                     ?context::IHP.Controller.RequestContext.RequestContext)
    bound by the type signature for:
               controllers :: (?applicationContext::IHP.ApplicationContext.ApplicationContext,
                               ?application::RootApplication,
                               ?context::IHP.Controller.RequestContext.RequestContext) =>
                              [RouteParser]
    at /Users/gieseke/Code/IHP/blog/Main.hs:13:3-13
• In the expression: mountFrontController WebApplication
  In the expression: [mountFrontController WebApplication]
  In an equation for ‘controllers’:
      controllers = [mountFrontController WebApplication]

I didn’t really touch Main.hs after generating the example IHP blog:

module Main where

import Config
import IHP.FrameworkConfig
import IHP.Job.Types
import IHP.Prelude
import IHP.RouterSupport
import IHP.Server qualified
import Web.FrontController
import Web.Types

instance FrontController RootApplication where
  controllers =
    [ mountFrontController WebApplication
    ]

instance Worker RootApplication where
  workers _ = []

main :: IO ()
main = IHP.Server.run config

Any help is greatly appreciated.

2

Heya, welcome to IHP. Maybe you can share the link to the repo, so I try to run it locally?

3

Here is the link: https://github.com/TheDarkchip/ihp-test

I guess it is a warning because I am able to run it, but I’m still curious as to why I am getting a warning there.

4

I don’t see errors on VScode, or on terminal itself when I run devenv up. So you should be safe to continue your coding :slight_smile:

5

Hi, I’m new to Haskell and IHP and am seeing the same error when I open the Main.hs file in Neovim, Helix or Zed editors.
(VScode isn’t actually showing the error, but it’s not actually giving anything helpful and I think it’s broken, just hanging, with status “Processing 1/3”)