Haskell - how to handle multiline input which depends on a variable - haskell

Question

I'm trying to solve a competitive programming challenge problem in Haskell.
This is my code:
module Main (main) where
import System.IO
import Text.Printf
getInt :: IO Int
getInt = readLn
getDouble :: IO Double
getDouble = readLn
getCoordinate :: (IO Double, IO Double, IO Double)
getCoordinate = (getDouble, getDouble, getDouble)
readCoordinates :: Int -> [(IO Double, IO Double, IO Double)] -> [(IO Double, IO Double, IO Double)]
readCoordinates 0 list = list
readCoordinates a list = readCoordinates (a - 1) list ++ [getCoordinate]
main :: IO ()
main = do
limit <- getInt
coordinates <- (readCoordinates limit [])
printf "%.2f\n" (run 0.0 (head coordinates) (tail coordinates))
run :: Double -> (Double, Double, Double) -> [(Double, Double, Double)] -> Double
run curr c1 (c2:cs) = run (curr + (d c1 c2)) c2 cs
run curr c1 [] = curr
d :: (Double, Double, Double) -> (Double, Double, Double) -> Double
d (x1, y1, z1) (x2, y2, z2) = sqrt (sas x1 x2) + (sas y1 y2) + (sas z1 z2)
sas :: Double -> Double -> Double
sas a1 a2 = (a1 - a2) ** 2
So as you might guess I'm reading an integer, which denotes how many 3d coordinates I should read in. Then I try to read all of these and calculate the distance.
I get a lot of errors, here are the error log:
Akvariet.hs:22:19:
Couldn't match type `[]' with `IO'
Expected type: IO (IO Double, IO Double, IO Double)
Actual type: [(IO Double, IO Double, IO Double)]
In the return type of a call of `readCoordinates'
In a stmt of a 'do' block:
coordinates <- (readCoordinates limit [])
In the expression:
do { limit <- getInt;
coordinates <- (readCoordinates limit []);
printf "%.2f" (run 0.0 (head coordinates) (tail coordinates)) }
Akvariet.hs:23:34:
Couldn't match expected type `[(Double, Double, Double)]'
with actual type `(IO Double, IO Double, IO Double)'
In the first argument of `head', namely `coordinates'
In the second argument of `run', namely `(head coordinates)'
In the second argument of `printf', namely
`(run 0.0 (head coordinates) (tail coordinates))'
Akvariet.hs:23:53:
Couldn't match expected type `[(Double, Double, Double)]'
with actual type `(IO Double, IO Double, IO Double)'
In the first argument of `tail', namely `coordinates'
In the third argument of `run', namely `(tail coordinates)'
In the second argument of `printf', namely
`(run 0.0 (head coordinates) (tail coordinates))'
I can't really wrap my head around the IO type actually, I get that it is impure and doesn't return the same thing every time but how do I use it in my program?
I don't understand how the readCoordinates method doesn't compile and why it can't cast IO Double to Double when main still is of type IO ().
Cheers!

Answer 1

First, I recommend you don't read in a number and then cumbersomely input each coordinate seperately. It's easier to read all input in one go (resulting in a string), and then parse it to coordinates without bothering about how it came from IO. This looks something like
main = do
allInput <- getContents
let coordinates = parseCoords $ lines allInput
printf ...
with
type Vect = (Double, Double, Double)
parseCoords :: [String] -> [Vect]
parseCoords (x:y:z:cs) = (read x, read y, read z) : parseCoords cs
parseCoords _ = []
If you prefer to manually read everything, to get precise control over the order or whatever, then you need to properly use IO as a monad. It's little use to combine three getDoubles to a tuple of IO actions; what you really want is a single IO action which yields a pure coordinate tuple.
getCoordinate :: IO Vect
getCoordinate = do
x <- getDouble
y <- getDouble
z <- getDouble
return (x,y,z)
Actually this could be written nicer with Applicative, though I suspect you find the above do writing easier to understand:
getCoordinate' = liftA3 (,,) getDouble getDouble getDouble