CODEWITHSUNDEEP
haskell
CSnerd
CSnerd

Reputation: 2239

How to append random number to a list

I wrote a function to generate random number:

drawFloat :: Float -> Float -> IO Float
drawFloat x y = getStdRandom (randomR (x,y))

Now I have a list [1,2,3].

How can I append the random number to this list?

I tried [1,2,3] ++ (drawFloat 2 10) It did not work...

ERROR Message:

    Couldn't match expected type ‘[a]’ with actual type ‘IO a0’
    Relevant bindings include it :: [a] (bound at <interactive>:72:1)
    In the second argument of ‘(++)’, namely ‘(drawFloat 2 10)’
    In the expression: [1, 2, 3] ++ (drawFloat 2 10)

I also tried [1,2,3] ++ [(drawFloat 2 10)]. Still not work..

Upvotes: 0

Views: 194

Answers (2)

bereal
bereal

Reputation: 34292

Because drawFloat returns a monadic type, you'll have to dive into the same monad in order to append:

import System.Random

drawFloat :: Float -> Float -> IO Float
drawFloat x y = getStdRandom (randomR (x,y))

appendRandom :: [Float] -> IO [Float]
appendRandom as = do
  val <- drawFloat 2 10
  return $ as ++ [val]

main = appendRandom [1, 2, 3] >>= print

Upvotes: 1

alternative
alternative

Reputation: 13042

So, the type of drawFloat is Float -> Float -> IO Float. We can only append a [Float]. Try

 appendRand :: [Float] -> IO [Float]
 appendRand lis = (lis ++) . return <$> drawFloat 2 10

Here we are elevating the function (list ++) . return :: Float -> [Float] to a IO Float -> IO [Float] which is exactly what we want. (Here, return is operating on the [] monad, not IO)

Upvotes: 3

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