Haskell: Hiding definitions from a module

In previous post, I explained the use of qualified imports. IN this post, I am going to explain about the usage of hiding definitions.

For example, you had 2 modules, Module1, Module2. Each module has a function 'processData x'.

Module1.hs

module Module1 where
    processData :: Integer -> Integer
    processData x = 10 * x

  Module2.hs

module Module2 where
    processData :: Integer -> Integer
    processData x = 100 * x

test.hs

import Module1
import Module2

processInfo :: Integer -> Integer
processInfo x = processData x

test.hs imports both the modules Module1, Module2. processInfo method wants to use processData method of Module1, but processData method exist in both the modules Module1, Module2. There is an ambiguity, Haskell don’t have any clue to call processData method. When you try to load test.hs, it will fail.

*Module1> :load test.hs
[1 of 3] Compiling Module2          ( Module2.hs, interpreted )
[2 of 3] Compiling Module1          ( Module1.hs, interpreted )
[3 of 3] Compiling Main             ( test.hs, interpreted )

test.hs:5:17:
    Ambiguous occurrence ‘processData’
    It could refer to either ‘Module1.processData’,
                             imported from ‘Module1’ at test.hs:1:1-14
                             (and originally defined at Module1.hs:3:5-15)
                          or ‘Module2.processData’,
                             imported from ‘Module2’ at test.hs:2:1-14
                             (and originally defined at Module2.hs:3:5-15)
Failed, modules loaded: Module1, Module2.
*Module1>

How to solve above problem?

One way to solve this problem is using qualified imports, I already explained in previous post. Other way to solve the problem is by hiding the function definition of Module2.

Syntax

Import ModuleName hiding (function1, … functionN)

test.hs

import Module1
import Module2 hiding(processData)

processInfo :: Integer -> Integer
processInfo x = processData x

*Main> :load test.hs
[1 of 3] Compiling Module2          ( Module2.hs, interpreted )
[2 of 3] Compiling Module1          ( Module1.hs, interpreted )
[3 of 3] Compiling Main             ( test.hs, interpreted )
Ok, modules loaded: Main, Module1, Module2.
*Main> 
*Main> processInfo 10
100
*Main>

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