Perfect Numbers in Haskell
Determine if a number is perfect, abundant, or deficient based on Nicomachus' (60  120 CE) classification scheme for natural numbers.
1  exercism fetch haskell perfectnumbers

Perfect Numbers
Determine if a number is perfect, abundant, or deficient based on Nicomachus' (60  120 CE) classification scheme for natural numbers.
The Greek mathematician Nicomachus devised a classification scheme for natural numbers, identifying each as belonging uniquely to the categories of perfect, abundant, or deficient based on their aliquot sum. The aliquot sum is defined as the sum of the factors of a number not including the number itself. For example, the aliquot sum of 15 is (1 + 3 + 5) = 9

Perfect: aliquot sum = number
 6 is a perfect number because (1 + 2 + 3) = 6
 28 is a perfect number because (1 + 2 + 4 + 7 + 14) = 28

Abundant: aliquot sum > number
 12 is an abundant number because (1 + 2 + 3 + 4 + 6) = 16
 24 is an abundant number because (1 + 2 + 3 + 4 + 6 + 8 + 12) = 36

Deficient: aliquot sum < number
 8 is a deficient number because (1 + 2 + 4) = 7
 Prime numbers are deficient
Implement a way to determine whether a given number is perfect. Depending on your language track, you may also need to implement a way to determine whether a given number is abundant or deficient.
Getting Started
For installation and learning resources, refer to the exercism help page.
Running the tests
To run the test suite, execute the following command:
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stack test

If you get an error message like this...
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No .cabal file found in directory

You are probably running an old stack version and need to upgrade it.
Otherwise, if you get an error message like this...
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No compiler found, expected minor version match with... Try running "stack setup" to install the correct GHC... 
Just do as it says and it will download and install the correct compiler version:
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stack setup

Running GHCi
If you want to play with your solution in GHCi, just run the command:
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stack ghci

Feedback, Issues, Pull Requests
The exercism/haskell repository on GitHub is the home for all of the Haskell exercises.
If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!
Source
Taken from Chapter 2 of Functional Thinking by Neal Ford. http://shop.oreilly.com/product/0636920029687.do
Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.