I’m currently learning Haskell, and as one of my first “proper” programs have created a mathematics quiz. It works like this:

• A question is generated, with two numbers from 1 to 10 which need to be either added, multiplied or subtracted, for example `3 * 6`.
• The question is presented to the user, and they can write their answer and press Enter.
• If the user’s answer is the same as the actual answer to the question, they get one point towards their score. If the answer is incorrect their points stay the same.
• This repeats 10 times, and the user’s points are shown to them at the end.

I’ve implemented the operator part of question generation using an `Operator` data type with a `Random` instance. The numbers are generated with a `randomSensibleInts x gen` function, which generates `x` random numbers between 1 and 10.

The `askQuestion` function uses `IO` to get the user’s answer, returning `True` if the answer was correct and `False` otherwise. Finally, in `main`, the `askQuestion` function is called 10 times, returning a `[Bool]`. `fromEnum` is then used to count `True` as one point and `False` as zero, which combined with `sum` creates a point total.

My code is as follows:

``````import System.Random

randomSensibleInts :: Int -> StdGen -> [Int]
randomSensibleInts quantity = take quantity . randomRs (1, 10)

data Operator = Add | Subtract | Multiply deriving (Eq, Enum)

instance Show Operator where
show op = case op of
Subtract -> "-"
Multiply -> "*"

instance Random Operator where
random gen = case randomR (0, 2) gen of
(item, gen') -> (toEnum item, gen')

randomR (lo, hi) gen = case randomR (fromEnum lo, fromEnum hi) gen of
(item, gen') -> (toEnum item, gen')

genQuestion :: StdGen -> (Int, Operator, Int, StdGen)
genQuestion gen = (numbers !! 0, operator, numbers !! 1, gen')
where numbers = randomSensibleInts 2 gen
(operator, gen') = random gen :: (Operator, StdGen)

askQuestion :: (Int, Operator, Int, StdGen) -> IO Bool
askQuestion (i1, o, i2, gen) = do
putStrLn \$ unwords ["Enter the answer to", show i1, show o, show i2] ++ ": "
let trueAnswer = case o of
Subtract -> i1 - i2
Multiply -> i1 * i2
putStrLn "Well done!"
return True
else do
return False

main = do
gens <- mapM (const newStdGen) [1..10]
let questions = map genQuestion gens
``````

Please let me know if my code can be made more “Haskell-like”, for example if there are builtins to replace any of my custom functions. I’m also interested in pointfree style, so please let me know if any of my functions can be improved to use this. I’m also fairly certain this code can be shortened drastically.

## The `Show` typeclass is for debugging

The `Show` typeclass is really meant to be derived, not implemented yourself. The purpose of `Show` is mostly for debugging, and the closest thing to a law for `Show` is that it should be the inverse of `Read`. The result of `show` should usually be a string that represents a Haskell expression that would produce the value itself.

While it is tempting to create a custom instance of `Show` for your own purposes, I would just convert your instance into a separate function, `showOperator` (which can also be implemented more simply with direct pattern matching rather that explicitly using `case`):

``````showOperator :: Operator -> String
showOperator Subtract = "-"
showOperator Multiply = "-"
``````

## `randomSensibleInts` is poorly-named

What makes integers in the range [1, 10] any more “sensible” than any other integers? Given how incredibly simple it is, and given it’s only used in one place, I would honestly just inline it into the place it’s used. I think it’s clearer that way than any name you could give it.

## Avoid `!!`

The `!!` function is unsafe, since it will crash if the provided index is out of bounds. Admittedly, in your case, you know the length of the list will always be exactly 2, so it’s not dangerous, but in that case, you should just use pattern-matching instead:

``````genQuestion :: StdGen -> (Int, Operator, Int, StdGen)
genQuestion gen = (x, operator, y, gen')
where [x, y] = take 2 \$ randomRs (1, 10) gen
(operator, gen') = random gen
``````

(You also don’t need the type annotation on `random gen`, so I’ve removed it.)

## Package tuples into named datatypes

The long tuple return type for `genQuestion` is a little bit suspicious. The name even provides a good suggestion for naming the datatype: `Question`. Create a datatype for that instead of having such a large tuple:

``````data Question = Question Int Operator Int

genQuestion :: StdGen -> (Question, StdGen)
genQuestion gen = (Question x operator y, gen')
where ...

askQuestion :: (Question, StdGen) -> IO Bool
askQuestion (Question i1 o i2, gen) = ...
``````

## Avoid using booleans directly

Using `Bool` as a return type of a non-predicate function can be confusing (this is referred to as “boolean blindness”). Using a separate, named type can be both more readable and more scalable if the number of cases ever changes:

``````data AnswerResult = Correct | Incorrect
deriving (Eq)

``````

This also allows adjusting the score summing logic to be a bit more reliable. Using `toEnum` and assigning significance to the indices is both unnecessary and mixing concerns.

``````putStrLn \$ "Your score was " ++ show (length \$ filter (== Correct) answers)
``````

## Avoid mixing pure and impure logic

The `askQuestion` function has too many responsibilities: formats a prompt for input, displays it to the user, calculates the answer to the question, validates the answer to the question, and prints out a response based on the answer. Three of those responsibilities are totally pure, and they should ideally be pulled into their own functions:

``````questionPrompt :: Question -> String
questionPrompt (Question x op y) = "Enter the answer to " ++ show x ++ showOperator op ++ show y ++ ": "

questionAnswer (Question x op y) = operatorFunction op x y
operatorFunction Subtract = (-)
operatorFunction Multiply = (*)

then Correct
else Incorrect

putStrLn \$ questionPrompt question
Correct -> putStrLn "Well done!"
Incorrect -> putStrLn \$ "Wrong - the answer was " ++ show (questionAnswer question)
``````

## Eliminate unused variables

If you turn on `-Wall`, GHC will tell you that `gen` is never used in `askQuestion` at all, so it can be removed:

``````genQuestion :: StdGen -> Question
genQuestion gen = Question x operator y
where [x, y] = take 2 \$ randomRs (1, 10) gen
(operator, _) = random gen

putStrLn \$ questionPrompt question
Correct -> putStrLn "Well done!"
Incorrect -> putStrLn \$ "Wrong - the answer was " ++ show (questionAnswer question)

main :: IO ()
main = do
gens <- mapM (const newStdGen) [1..10]
let questions = map genQuestion gens
putStrLn \$ "Your score was " ++ show (length \$ filter (== Correct) answers)
``````

## Use `where` or `let` instead of `case` when there is only one pattern

In the `Random` instance for `Operator`, it’s possible to avoid `case` if you’re simply destructuring rather than matching against multiple patterns:

``````instance Random Operator where
random gen = (toEnum item, gen')
where (item, gen') = randomR (0, 2) gen

randomR (lo, hi) gen = (toEnum item, gen')
``````

# Final result

``````import System.Random

data Operator = Add | Subtract | Multiply
deriving (Eq, Enum)

data Question = Question Int Operator Int

data AnswerResult = Correct | Incorrect
deriving (Eq)

instance Random Operator where
random gen = (toEnum item, gen')
where (item, gen') = randomR (0, 2) gen

randomR (lo, hi) gen = (toEnum item, gen')

showOperator :: Operator -> String
showOperator Subtract = "-"
showOperator Multiply = "-"

questionPrompt :: Question -> String
questionPrompt (Question x op y) = "Enter the answer to " ++ show x ++ showOperator op ++ show y ++ ": "

questionAnswer (Question x op y) = operatorFunction op x y
operatorFunction Subtract = (-)
operatorFunction Multiply = (*)

then Correct
else Incorrect

genQuestion :: StdGen -> Question
genQuestion gen = Question x operator y
where [x, y] = take 2 \$ randomRs (1, 10) gen
(operator, _) = random gen

putStrLn \$ questionPrompt question
Correct -> putStrLn "Well done!"
Incorrect -> putStrLn \$ "Wrong - the answer was " ++ show (questionAnswer question)