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Lecture 23-1

COMP105 Lectures

So far, we have studied pure functional programming. Pure functions:

  • Have no side effects.
  • Always return a value.
  • Are deterministic.

All computation can be done in pure functional programming.

IO

Sometimes programs need to do non-pure things:

  • Print something to the screen.
  • Read or write a file.
  • …

IO v.s. Pure Functional

graph LR
IO --> p[Pure Functional]
p --> IO
  • Impure IO code talks to the outside world.
  • Pure functional code does the interesting computation.
  • IO code can call pure functions; pure functions cannot call IO.

We can call the “functions” that complete IO: IO actions. This is because they are not functions.

getLine

getLine read a line of input from the console.

> getLine
hello there
"hello there"

> :t getLine
getLine :: IO String

The IO Type

This type marks a value as being impure.

If a function returns an IO type then it is impure:

  • It may have side effects.
  • It may return different values for the same inputs.

The IO type should be thought of as a box:

  • The box hold a value from an impure computation.
  • we can use <- to get an impure value from the IO type.
> x <- getLine
Hello
> x
"Hello"

Values must be unboxed before you use them in a pure function.

getChar

This function is similar to getLine but returns a single Char instead of a string.

putStrLn

This IO action prints a string onto the console.

> putStrLn "Hello"
Hello

There are no quotation marks as we are seeing this being written to the StdOut and not via read.

> :t putStrLn
putStrLn :: String -> IO ()

The unit type has the IO type indicating that it has a side effect.

The Unit Type

The unit type is a type represented by (). It only has one value which is itself.

This is used to indicate that nothing interesting is returned. An example of this is with putStrLn where is doesn’t return a value but does have the side effect of printing to the StdOut.

Exercise

  1. It will ask for two lines. It will then print out the two lines with a space between them back to the StdOut.
  2. It will read in a line with the expectation that there will be a number on it. It will then convert the line into an Int and print out the number + 1.
  3. Error due to mismatched types. putStrLn expects a String but an IO type was given.