Haskell wikibook: Type basics II
Here are my answers for the chapter called Next Steps:
We cheated a little when moving from the second version of pts to the third one: they do not do exactly the same thing. Can you spot what the difference is?
pts :: Int -> Int
pts 1 = 10
pts 2 = 6
pts 3 = 4
pts 4 = 3
pts 5 = 2
pts 6 = 1
pts _ = 0
> pts :: Int -> Int
> pts 1 = 10
> pts 2 = 6
> pts x
> | x <= 6 = 7 - x
> | otherwise = 0
Gives different answers if the parameter to the function is negative.
And the chapter Simple Input and Output:
> 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 8.91
>
> You will need to use the function `read` to convert user strings like "3.3" into numbers like 3.3 and the function `show` to convert a number into string.
main = do
putStrLn "The base?"
b <- getLine
putStrLn "The height?"
h <- getLine
let m = "The area of that triangle is "
putStrLn (m ++ (show (area (read b) (read h))))
area b h = b \* h / 2
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 if / then / else.)
main = do
putStrLn "What's your name?"
n <- getLine
putStrLn (resp n)
{-
resp :: String -> String
resp n
| elem n ["Simon", "John", "Phil"] = "Haskell is marvelous!"
| n == "Koen" = "Debugging Haskell is fun."
| otherwise = "Name not recognized."
-}
resp n = if elem n ["Simon", "John", "Phil"]
then "Haskell is marvelous!"
else if n == "Koen"
then
"Debugging Haskell is fun."
else
"Name not recognized."
+-----------------------------------+-----------------------------------+
| sweet | unsweet |
+===================================+===================================+
|
| dir="ltr"> |
| | |
| do name <- getLine | do name <- getLine |
| let loudName = makeLoud n | let loudName = makeLoud n |
| ame | ame |
| putStrLn ("Hello " ++ lou | in do putStrLn ("Hello " |
| dName ++ "!") | ++ loudName ++ "!") |
| putStrLn ( | putStrLn ( |
| "Oh boy! Am I excited | "Oh boy! Am I |
| to meet you, " | excited to meet you, " |
| ++ loudName) | ++ loudNam |
| | e) |
|
| |
| |
|
+-----------------------------------+-----------------------------------+
+-----------------------------------------------------------------------+
| Exercises |
+=======================================================================+
| 1. Why does the unsweet version of the let binding require an extra |
| do keyword?\ |
| Because an 'in' clause can contain any Haskell, so we have to use |
| the 'do' keyword to say that we're going to execute actions. |
| 2. Do you always need the extra do?\ |
| Only if you're doing actions. |
| 3. (extra credit) Curiously, let without in is exactly how we |
| wrote things when we were playing with the interpreter in the |
| beginning of this book. Why is it ok to omit the in keyword in |
| the interpreter but needed (outside of do blocks) in a source |
| file?\ |
| Perhaps because lines entered in the interpreter are implicitly |
| within a 'do' block. |
+-----------------------------------------------------------------------+