Haskell/Solutions/Simple input and output
Back to the real world
Back in the Type Basics chapter, we mentioned that the type of the
Back in Type Basics, we said:
You need to open a new window which contains all the options that they can change. Let's look at the type signature for this function:openWindow :: WindowTitle -> WindowSize -> Window
Opening a window, however, implies not just creating it but also drawing it on the screen. The result, therefore, has to have an
IO type, just like
putStrLn has. The real signature should be:
openWindow :: WindowTitle -> WindowSize -> IO Window
A real example of an
openWindow function with such a signature (except for slightly different names) can be found in the HGL package.
Sequencing actions with do
Write a program which asks the user for the base and height of a right angled triangle, calculates its area and prints it to the screen. The interaction should look something like:
The base? 3.3 The height? 5.4 The area of that triangle is 8.91Hint: you can use the function
main = do putStrLn "The base?" base <- getLine putStrLn "The height?" height <- getLine putStrLn ("The area of that triangle is " ++ show (0.5 * read base * read height))
Note that the return type for read is inferred from the type of the expression it is in. 0.5*read("10.0") evaluates to 5.0, as expected. read("10.0") results in an error. In such cases a numeric type must be specified: read "10.0" ::Double, for example.
Write a program that asks the user for his or her name. If the name is one of Simon, John or Phil, tell the user that you think Haskell is a great programming language. If the name is Koen, tell them that you think debugging Haskell is fun (Koen Classen is one of the people who works on Haskell debugging); otherwise, tell the user that you don't know who he or she is.(As far as syntax goes there are a few different ways to do it; write at least a version using
main = do putStrLn "Hello, what is your name?" name <- getLine if name == "Simon" || name == "John" || name == "Phil" then putStrLn "I think Haskell is a great programming language." else if name == "Koen" then putStrLn "I think debugging Haskell is fun." else putStrLn "Sorry, I don't know you."
An alternative using a
main = do putStrLn "Hello, what is your name?" name <- getLine putStrLn (message name) where greatlanguage = "I think Haskell is a great programming language." message "Simon" = greatlanguage message "John" = greatlanguage message "Phil" = greatlanguage message "Koen" = "I think debugging Haskell is fun." message _ = "Sorry, I don't know you."
Actions under the microscope
- A let name = value in thing binding has the same type as thing. Because we want the binding to be an IO action, the thing needs to be an IO action, so we need a do keyword.
- No, not always. Just as in "normal" code, you can omit the do keyword if there's only one IO action following it.
- The GHCi interpreter is like one big do-block with some extra magic, so that it converts normal expressions to IO-actions. As in any do-block, you can omit the