How do I add x tuples into a list x number of times?

Question

I have a question about tuples and lists in Haskell. I know how to add input into a tuple a specific number of times. Now I want to add tuples into a list an unknown number of t

Answer 1

There's a lot of things you could possibly mean. For example, if you want a few copies of a single value, you can use replicate, defined in the Prelude:

replicate :: Int -> a -> [a]
replicate 0 x = []
replicate n | n < 0     = undefined
            | otherwise = x : replicate (n-1) x

In ghci:

Prelude> replicate 4 ("Haskell", 2)
[("Haskell",2),("Haskell",2),("Haskell",2),("Haskell",2)]

Alternately, perhaps you actually want to do some IO to determine the list. Then a simple loop will do:

getListFromUser = do
    putStrLn "keep going?"
    s <- getLine
    case s of
        'y':_ -> do
            putStrLn "enter a value"
            v <- readLn
            vs <- getListFromUser
            return (v:vs)
        _ -> return []

In ghci:

*Main> getListFromUser :: IO [(String, Int)]
keep going?
y
enter a value
("Haskell",2)
keep going?
y
enter a value
("Prolog",4)
keep going?
n
[("Haskell",2),("Prolog",4)]

Of course, this is a particularly crappy user interface -- I'm sure you can come up with a dozen ways to improve it! But the pattern, at least, should shine through: you can use values like [] and functions like : to construct lists. There are many, many other higher-level functions for constructing and manipulating lists, as well.

P.S. There's nothing particularly special about lists of tuples (as compared to lists of other things); the above functions display that by never mentioning them. =)