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-rw-r--r--haskell.html.markdown83
1 files changed, 47 insertions, 36 deletions
diff --git a/haskell.html.markdown b/haskell.html.markdown
index 748a29da..936744a0 100644
--- a/haskell.html.markdown
+++ b/haskell.html.markdown
@@ -59,6 +59,7 @@ not False -- True
"Hello " ++ "world!" -- "Hello world!"
-- A string is a list of characters
+['H', 'e', 'l', 'l', 'o'] -- "Hello"
"This is a string" !! 0 -- 'T'
@@ -67,10 +68,21 @@ not False -- True
----------------------------------------------------
-- Every element in a list must have the same type.
--- Two lists that are the same
+-- These two lists are the same:
[1, 2, 3, 4, 5]
[1..5]
+-- Ranges are versatile.
+['A'..'F'] -- "ABCDEF"
+
+-- You can create a step in a range.
+[0,2..10] -- [0, 2, 4, 6, 8, 10]
+[5..1] -- This doesn't work because Haskell defaults to incrementing.
+[5,4..1] -- [5, 4, 3, 2, 1]
+
+-- indexing into a list
+[1..10] !! 3 -- 4
+
-- You can also have infinite lists in Haskell!
[1..] -- a list of all the natural numbers
@@ -90,9 +102,6 @@ not False -- True
-- adding to the head of a list
0:[1..5] -- [0, 1, 2, 3, 4, 5]
--- indexing into a list
-[0..] !! 5 -- 5
-
-- more list operations
head [1..5] -- 1
tail [1..5] -- [2, 3, 4, 5]
@@ -139,12 +148,12 @@ add 1 2 -- 3
-- Guards: an easy way to do branching in functions
fib x
- | x < 2 = x
+ | x < 2 = 1
| otherwise = fib (x - 1) + fib (x - 2)
-- Pattern matching is similar. Here we have given three different
-- definitions for fib. Haskell will automatically call the first
--- function that matches the pattern of the value.
+-- function that matches the pattern of the value.
fib 1 = 1
fib 2 = 2
fib x = fib (x - 1) + fib (x - 2)
@@ -172,7 +181,7 @@ foldl1 (\acc x -> acc + x) [1..5] -- 15
----------------------------------------------------
-- partial application: if you don't pass in all the arguments to a function,
--- it gets "partially applied". That means it returns a function that takes the
+-- it gets "partially applied". That means it returns a function that takes the
-- rest of the arguments.
add a b = a + b
@@ -186,26 +195,28 @@ foo 5 -- 15
-- function composition
-- the (.) function chains functions together.
-- For example, here foo is a function that takes a value. It adds 10 to it,
--- multiplies the result of that by 5, and then returns the final value.
-foo = (*5) . (+10)
+-- multiplies the result of that by 4, and then returns the final value.
+foo = (*4) . (+10)
--- (5 + 10) * 5 = 75
-foo 5 -- 75
+-- (5 + 10) * 4 = 60
+foo 5 -- 60
-- fixing precedence
--- Haskell has another function called `$`. This changes the precedence
--- so that everything to the left of it gets computed first and then applied
--- to everything on the right. You can use `$` (often in combination with `.`)
--- to get rid of a lot of parentheses:
+-- Haskell has another operator called `$`. This operator applies a function
+-- to a given parameter. In contrast to standard function application, which
+-- has highest possible priority of 10 and is left-associative, the `$` operator
+-- has priority of 0 and is right-associative. Such a low priority means that
+-- the expression on its right is applied as the parameter to the function on its left.
-- before
-(even (fib 7)) -- true
-
--- after
-even . fib $ 7 -- true
+even (fib 7) -- false
-- equivalently
-even $ fib 7 -- true
+even $ fib 7 -- false
+
+-- composing functions
+even . fib $ 7 -- false
+
----------------------------------------------------
-- 5. Type signatures
@@ -272,7 +283,7 @@ foldl (\x y -> 2*x + y) 4 [1,2,3] -- 43
foldr (\x y -> 2*x + y) 4 [1,2,3] -- 16
-- This is now the same as
-(2 * 3 + (2 * 2 + (2 * 1 + 4)))
+(2 * 1 + (2 * 2 + (2 * 3 + 4)))
----------------------------------------------------
-- 7. Data Types
@@ -310,13 +321,13 @@ Nothing -- of type `Maybe a` for any `a`
-- called. It must return a value of type `IO ()`. For example:
main :: IO ()
-main = putStrLn $ "Hello, sky! " ++ (say Blue)
+main = putStrLn $ "Hello, sky! " ++ (say Blue)
-- putStrLn has type String -> IO ()
--- It is easiest to do IO if you can implement your program as
--- a function from String to String. The function
+-- It is easiest to do IO if you can implement your program as
+-- a function from String to String. The function
-- interact :: (String -> String) -> IO ()
--- inputs some text, runs a function on it, and prints out the
+-- inputs some text, runs a function on it, and prints out the
-- output.
countLines :: String -> String
@@ -330,43 +341,43 @@ main' = interact countLines
-- the `do` notation to chain actions together. For example:
sayHello :: IO ()
-sayHello = do
+sayHello = do
putStrLn "What is your name?"
name <- getLine -- this gets a line and gives it the name "name"
putStrLn $ "Hello, " ++ name
-
+
-- Exercise: write your own version of `interact` that only reads
-- one line of input.
-
+
-- The code in `sayHello` will never be executed, however. The only
--- action that ever gets executed is the value of `main`.
--- To run `sayHello` comment out the above definition of `main`
+-- action that ever gets executed is the value of `main`.
+-- To run `sayHello` comment out the above definition of `main`
-- and replace it with:
-- main = sayHello
--- Let's understand better how the function `getLine` we just
+-- Let's understand better how the function `getLine` we just
-- used works. Its type is:
-- getLine :: IO String
-- You can think of a value of type `IO a` as representing a
--- computer program that will generate a value of type `a`
+-- computer program that will generate a value of type `a`
-- when executed (in addition to anything else it does). We can
--- store and reuse this value using `<-`. We can also
+-- store and reuse this value using `<-`. We can also
-- make our own action of type `IO String`:
action :: IO String
action = do
putStrLn "This is a line. Duh"
- input1 <- getLine
+ input1 <- getLine
input2 <- getLine
-- The type of the `do` statement is that of its last line.
- -- `return` is not a keyword, but merely a function
+ -- `return` is not a keyword, but merely a function
return (input1 ++ "\n" ++ input2) -- return :: String -> IO String
-- We can use this just like we used `getLine`:
main'' = do
putStrLn "I will echo two lines!"
- result <- action
+ result <- action
putStrLn result
putStrLn "This was all, folks!"