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Make the two fib functions consistent
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@ -148,12 +148,12 @@ add 1 2 -- 3
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-- Guards: an easy way to do branching in functions
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fib x
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| x < 2 = x
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| x < 2 = 1
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| otherwise = fib (x - 1) + fib (x - 2)
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-- Pattern matching is similar. Here we have given three different
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-- definitions for fib. Haskell will automatically call the first
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-- function that matches the pattern of the value.
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-- function that matches the pattern of the value.
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fib 1 = 1
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fib 2 = 2
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fib x = fib (x - 1) + fib (x - 2)
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@ -181,7 +181,7 @@ foldl1 (\acc x -> acc + x) [1..5] -- 15
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----------------------------------------------------
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-- partial application: if you don't pass in all the arguments to a function,
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-- it gets "partially applied". That means it returns a function that takes the
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-- it gets "partially applied". That means it returns a function that takes the
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-- rest of the arguments.
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add a b = a + b
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@ -319,13 +319,13 @@ Nothing -- of type `Maybe a` for any `a`
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-- called. It must return a value of type `IO ()`. For example:
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main :: IO ()
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main = putStrLn $ "Hello, sky! " ++ (say Blue)
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main = putStrLn $ "Hello, sky! " ++ (say Blue)
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-- putStrLn has type String -> IO ()
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-- It is easiest to do IO if you can implement your program as
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-- a function from String to String. The function
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-- It is easiest to do IO if you can implement your program as
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-- a function from String to String. The function
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-- interact :: (String -> String) -> IO ()
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-- inputs some text, runs a function on it, and prints out the
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-- inputs some text, runs a function on it, and prints out the
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-- output.
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countLines :: String -> String
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@ -339,43 +339,43 @@ main' = interact countLines
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-- the `do` notation to chain actions together. For example:
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sayHello :: IO ()
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sayHello = do
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sayHello = do
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putStrLn "What is your name?"
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name <- getLine -- this gets a line and gives it the name "name"
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putStrLn $ "Hello, " ++ name
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-- Exercise: write your own version of `interact` that only reads
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-- one line of input.
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-- The code in `sayHello` will never be executed, however. The only
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-- action that ever gets executed is the value of `main`.
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-- To run `sayHello` comment out the above definition of `main`
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-- action that ever gets executed is the value of `main`.
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-- To run `sayHello` comment out the above definition of `main`
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-- and replace it with:
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-- main = sayHello
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-- Let's understand better how the function `getLine` we just
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-- Let's understand better how the function `getLine` we just
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-- used works. Its type is:
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-- getLine :: IO String
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-- You can think of a value of type `IO a` as representing a
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-- computer program that will generate a value of type `a`
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-- computer program that will generate a value of type `a`
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-- when executed (in addition to anything else it does). We can
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-- store and reuse this value using `<-`. We can also
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-- store and reuse this value using `<-`. We can also
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-- make our own action of type `IO String`:
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action :: IO String
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action = do
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putStrLn "This is a line. Duh"
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input1 <- getLine
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input1 <- getLine
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input2 <- getLine
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-- The type of the `do` statement is that of its last line.
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-- `return` is not a keyword, but merely a function
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-- `return` is not a keyword, but merely a function
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return (input1 ++ "\n" ++ input2) -- return :: String -> IO String
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-- We can use this just like we used `getLine`:
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main'' = do
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putStrLn "I will echo two lines!"
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result <- action
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result <- action
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putStrLn result
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putStrLn "This was all, folks!"
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