Value constructors in haskell

Question

I have a question about Haskell. I'm new so I don't understand too well, but if anyone could help me I'd really appreciate. I have this exercise from a book that I bought.

Create a type Question with value contructors Sim or Nao. Make a function that:

• listQuest: receive from a parameter a list of Questions and return 0 for Nao or 1 for Sim corresponding the constructors in the list.

I tried this code:

module Question where 

data Question = Yes | No deriving Show

questNum :: Question -> Int
questNum No = 0
questNum Yes = 1

listQuest :: [Question] -> [Int]
listQuest listQuestion =  [ questNum quest | quest <- listQuestion ]

I'm lost in the prompt matter. Don't know what to write to use the function.

Thanks for the help

Answer 1

There are several ways to do that, you did it by list comprehension, another way is using map:

listQuest :: [Question] -> [Int]
listQuest xs = map questNum  xs

shorter

listQuest :: [Question] -> [Int]
listQuest  = map questNum  

using foldr (it is also a way to say map, indeed you can write the general map using foldr):

listQuest :: [Question] -> [Int]
listQuest = foldr (\x rs -> questNum x : rs) []

and the old good pattern matching (is map, but with pattern matching):

listQuest :: [Question] -> [Int]
listQuest []  = []
listQuest (x:xs) = questNum x : listQuest xs  

your way, and all of this, are equivalent.

On the prompt:

Then on the prompt you type something like:

$> :l file_name.hs
$> listQuest [Yes, Yes, No]
$> [1,1,0]