Haskell: check the length of IO [Double]

Question

Given a list of type IO [Double], I want to be able to check that the list is of the desired length.

I'm guessing I need to make use of functors here, but I don't understand how to go about defining it. Do I write a functor instance of the length function? Or do I write a functor instance of a data type that uses the length function in fmap?

Monads, functors, etc. are all pretty new to me.

Answer 1

A value of type IO [Double] does not have a length, because it is not a list of Doubles, but rather an IO action which, when performed, will produce a list of Doubles. So, it is impossible to write a function of type IO [a] -> Int.

However, you can easily write a function of type IO [a] -> IO Int, which is as simple as fmap length.

Answer 2

Most likely, what you want to do is obtain the list, and then compute it's length. Generally, you will want to do this with do notation (although it isn't strictly necessary).

Haskell encapsulates all IO in the IO monad, so you can only obtain the list from the IO [Double] in another object of type IO a. The code will look something like this-

main::IO ()
main = do
  theList <- getList --or whatever your object of type IO [Double] is
  let theLength = length theList
  ....  -- Do stuff with theLength here

Answer 3

Both of the answers posted so far are correct and good ways of solving this problem, but I'd like to point a third syntax to accomplish the same effect. This is effectively jamshidh's answer minus the syntactic sugar of do-notation:

ioLength :: IO [Double] -> IO Int
ioLength a = a >>= (\x -> return $ length x)

Or, more concisely:

ioLength a = a >>= (return . length)

Which, if condensed even further, yields amalloy's answer:

ioLength = fmap (length)