Module imports form a cycle

Question

Hello i was wondering how do you solve circular dependency in Haskell

I have :
Module for a type.

module Company where

import Tclass

data Worker=Worker{
    age::Int,
    name::String,
    title::Title,
    income::Int
}

data Title=Manager | Dev | Tester deriving (Show)


data Company=Company{
    cname::String,
    people::[Worker],
    yf::Int
}

Module for the instance of the typeclass

module Tclass where
import System.Environment
import Company

class Console a where
    wLine::a->IO()
    rLine::IO String->a

instance Show Worker where
    show w="to be defined"
    print =putStr . show

If i just want to load the type in ghci and use it ; how do i import them without getting the error:

Module imports form a cycle:
         module `Tclass' (.\Tclass.hs)
        imports `Company' (Company.hs)
  which imports `Tclass' (.\Tclass.hs)

P.S I have tried creating a auxiliary module that imports the two modules and the two modules would import it too,but no luck with this one either :

module Au where
 import Tclass
 import Company

module Company
 import Au

module Tclass 
 import Au

It looks like the first module should not import the second, can you remove the line import Tclass and retry? — Olivier Sohn
But if i do not import the TClass inside the Company module i can not use instance of Show for Worker type. — Bercovici Adrian
Then can you move the Show instance to the other module ? defining an instance in another module than the data definition is bad practice (you'll have the orphan instance warning) — Olivier Sohn
You can have the types in a specific module, but with their instances. — Olivier Sohn
It's not part of the question but reading your code I would suggest that you read about strict fields in data records : stackoverflow.com/questions/8576795/… it might be helpful here. — Olivier Sohn

Olivier Sohn Olivier Sohn · Accepted Answer · 2018-07-02T07:34:18

Moving the Show instance to the other module, you can break the cycle:

module Tclass where
import System.Environment
import Company

class Console a where
    wLine::a->IO()
    rLine::IO String->a

and

module Company where

data Worker=Worker{
    age::Int,
    name::String,
    title::Title,
    income::Int
}
instance Show Worker where
    show w="to be defined"
    print =putStr . show 

data Title=Manager | Dev | Tester deriving (Show)

data Company=Company{
    cname::String,
    people::[Worker],
    yf::Int
}

Module imports form a cycle

1 Answers