Haskell: Import types, name constructor, value constructors

In this post I am going to explain, how to import types, name constructor, value constructors defined in one module to other modules.

For example, My CustomTypes.hs defined some types like below.

CustomTypes.hs

module CustomTypes () where
    type FirstName = String
    type LastName = String
    type EmpId = Integer
    type NoOfReportees = Integer

    {- Define an employee type -}
    data Employee = Engineer FirstName LastName EmpId
                   | Manager FirstName LastName EmpId NoOfReportees
                   | Director FirstName LastName EmpId NoOfReportees
                   deriving (Show)

Define Test.hs like below.

Test.hs

import CustomTypes

emp1 = Engineer "Hari Krishna" "Gurram" 123

 

Try to load Test.hs file.

Prelude> :load Test.hs
[1 of 2] Compiling CustomTypes      ( CustomTypes.hs, interpreted )
[2 of 2] Compiling Main             ( Test.hs, interpreted )

Test.hs:3:8: Not in scope: data constructor ‘Engineer’
Failed, modules loaded: CustomTypes.

Observe above snippet, Haskell clearly telling that Data constructor Engineer is not available in Test.hs file.

Update the CustomTypes.hs file by using following statement.

module CustomTypes (Employee(Engineer)) where

Above statement clearly tells to Haskell that, Data constructor Employee; value constructor Engineer is available to use in outside the world.

CustomTypes.hs

module CustomTypes (Employee(Engineer)) where
    type FirstName = String
    type LastName = String
    type EmpId = Integer
    type NoOfReportees = Integer

    {- Define an employee type -}
    data Employee = Engineer FirstName LastName EmpId
                   | Manager FirstName LastName EmpId NoOfReportees
                   | Director FirstName LastName EmpId NoOfReportees
                   deriving (Show)

Reload Test.hs.

*Main> :load Test.hs
[1 of 2] Compiling CustomTypes      ( CustomTypes.hs, interpreted )
[2 of 2] Compiling Main             ( Test.hs, interpreted )
Ok, modules loaded: CustomTypes, Main.
*Main> 
*Main> emp1
Engineer "Hari Krishna" "Gurram" 123
*Main> 

Now update Test.hs like below and reload.

Test.hs

import CustomTypes

emp1 = Engineer "Hari Krishna" "Gurram" 123
manager1 = Manager "Anand" "Bandaru" 234 5

*Main> :load Test.hs
[1 of 2] Compiling CustomTypes      ( CustomTypes.hs, interpreted )
[2 of 2] Compiling Main             ( Test.hs, interpreted )

Test.hs:4:12:
    Not in scope: data constructor ‘Manager’
    Perhaps you meant variable ‘manager1’ (line 4)
Failed, modules loaded: CustomTypes.
*CustomTypes> 

Above snippet clearly telling data constructor Manager is not available in Test.hs file, to solve the above error we need to update the module statement in CustomTypes.hs like below.

module CustomTypes (Employee(Engineer, Manager)) where

CustomTypes.hs

module CustomTypes (Employee(Engineer, Manager)) where
    type FirstName = String
    type LastName = String
    type EmpId = Integer
    type NoOfReportees = Integer

    {- Define an employee type -}
    data Employee = Engineer FirstName LastName EmpId
                   | Manager FirstName LastName EmpId NoOfReportees
                   | Director FirstName LastName EmpId NoOfReportees
                   deriving (Show)

*CustomTypes> :load Test.hs
[1 of 2] Compiling CustomTypes      ( CustomTypes.hs, interpreted )
[2 of 2] Compiling Main             ( Test.hs, interpreted )
Ok, modules loaded: CustomTypes, Main.
*Main> 
*Main> emp1
Engineer "Hari Krishna" "Gurram" 123
*Main> 
*Main> manager1
Manager "Anand" "Bandaru" 234 5
*Main> 

How can I expose all the value constructors to outside world?

‘Employee(..)’ expose the type and all the constructors of it.

CustomTypes.hs

module CustomTypes (Employee(..)) where
    type FirstName = String
    type LastName = String
    type EmpId = Integer
    type NoOfReportees = Integer

    {- Define an employee type -}
    data Employee = Engineer FirstName LastName EmpId
                   | Manager FirstName LastName EmpId NoOfReportees
                   | Director FirstName LastName EmpId NoOfReportees
                   deriving (Show)

Test.hs

import CustomTypes

emp1 = Engineer "Hari Krishna" "Gurram" 123
manager1 = Manager "Anand" "Bandaru" 234 5
director1 = Director "Sailaja" "PTR" 345 53

*Main> :load Test.hs
[1 of 2] Compiling CustomTypes      ( CustomTypes.hs, interpreted )
[2 of 2] Compiling Main             ( Test.hs, interpreted )
Ok, modules loaded: CustomTypes, Main.
*Main> 
*Main> emp1
Engineer "Hari Krishna" "Gurram" 123
*Main> 
*Main> manager1
Manager "Anand" "Bandaru" 234 5
*Main> 
*Main> director1
Director "Sailaja" "PTR" 345 53
*Main> 

 

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