Lecture 24-2

COMP105 Lectures 2020-12-03

Useful IO Functions

This lecture covers many useful functions included with Haskell. I won’t be writing out examples but I will be writing down names and descriptions to search through.

For more detailed examples view the slides for this lecture.

print

This prints out any show-able type.

putStr

This prints a string without adding a new line.

With this you have to manually end the line with:

putStr '\n'

readLn

This function gets a line of input and then calls read. This saves you calling read when getting a line with getLine.

To give an explicit type use the syntax:

x <- readLn :: IO Int

This is due to the fact that when readLn is reading the line it is still in the IO ‘box’.

forever

This repeat an IO action forever. It is in the Control.Monad package. This is a higher order IO action as it takes another IO action as an input.

It can be used to make interactive code that runs a function forever.

sequence

This IO action will run a list of IO actions and will return a list containing the return values of the IO actions.

The IO actions must have the same type otherwise they can’t be put in the same list.

sequence works well with map:

sequence (map print [1..10])

This saves you typing out all the IO actions to sequence on.

sequence_

This IO action is the same as sequence but it will return the type IO (). You would want to use this when the IO actions you are running return IO () themselves.

mapM

You can use this map on monads. This is similar to running sequence in conjunction with map. This is the same as the code from before:

mapM print [1..10]

mapM_

This is similar to sequence_.

when

This executes an IO action if a condition is true.

when True (putStrLn "hi")

This is similar to if in pure functional code.

unless

This is the opposite of when. It will execute if the condition is false.

Exercises

1.  timesTwo = do
        n <- readLn :: IO Int
        print (n * 2)
   

2.  import Control.Monad
    ltFiveOnce = do
        x <- getLine
        when (length x < 5) (putStrLn "yes")
        when (length x >= 5) (putStrLn "no")
    ltFive = forever ltFiveOnce
   

3.  addThree = do
    x <- sequence [readLn,readLn,readLn]
    print (sum x)