How to test my own data Types using HUnit in Haskell?

Question

Part of Queue.hs:

module Queue ( Queue, emptyQueue, isEmptyQueue, enQueue, deQueue ) where

data Queue a = EmptyQueue | Single a | Q a ( Queue a ) a deriving (Show, Eq)

emptyQueue :: Queue a
emptyQueue = EmptyQueue

enQueue :: a -> Queue a -> Queue a
enQueue x EmptyQueue = Single x
enQueue x (Single q) = Q q (EmptyQueue) x
enQueue x (Q qf q qb) = Q qf (enQueue qb q) x

I was using print, see if them correct one by one. Now I want to test them using HUint.

Main.hs:

module Main where
import Queue 
import Test.HUnit

test1 = TestCase $ assertEqual "" (Single 'a'::Queue Char) $ enQueue 'a' emptyQueue
tests = TestList [TestLabel "test1" test1]

main = do runTestTT tests

Here's what I got:

*Queue> :r
[2 of 2] Compiling Main             ( Main.hs, interpreted )

Main.hs:5:36: Not in scope: data constructor ‘Single’
Failed, modules loaded: Queue.

So how can I fix it?

How are you exporting the Queue type from Queue.hs? If you have module Queue (Queue) where ... it wouldn't be exporting the constructors, you'd need module Queue (Queue(..)) where to export all constructors of the Queue type. — bheklilr
– bheklilr, Commented Feb 13, 2015 at 5:29
The error still isn't fixed. Add (..) to the second Queue in Queue.hs — ThreeFx
– ThreeFx, Commented Feb 13, 2015 at 6:31
To complete the other two comments: module Queue ( Queue(..), …). — Zeta
– Zeta, Commented Feb 13, 2015 at 7:26

Yuriosity · Accepted Answer · 2015-02-13 11:04:30Z

1

Seems the Queue datatype should be abstract and opaque, so exposing all constructors is not a good idea. However, you can split your implementation into two submodules:

module Queue.Internal where --expose all internal stuff

data Queue = <...>


module Queue (Queue, emptyQueue, <...>) where --expose user API only

import Queue.Internal


module Queue.Tests where

import Queue.Internal --to get access to „guts“

test = <...>


module Main where

import Queue --to just use the type

Some library types (like ByteStrings) are made this way.

answered Feb 13, 2015 at 11:04

Yuriosity

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2 Comments

liubiantao Over a year ago

The file Queue.hs is assignment, only can add implementation below it. Seems ByteString is a good idea, I've tried it, but how could I convert the result of (enQueue 'a' emptyQueue) to ByteString?

Yuriosity Over a year ago

@liubiantao I meant not to use ByteString, but to check how it is organized internally. Well, if you have only a single module, you have to export all Queue constructors (as it was already suggested) or write tests in the Queue module and export a test suit.

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