Merge pull request #1 from adambard/master

update

1 files changed, 84 insertions, 57 deletions

diff --git a/haskell.html.markdown b/haskell.html.markdown
index 748a29da..266cf11b 100644
--- a/haskell.html.markdown
+++ b/haskell.html.markdown
@@ -1,5 +1,6 @@
 ---
 language: Haskell
+filename: learnhaskell.hs
 contributors:
     - ["Adit Bhargava", "http://adit.io"]
 ---
@@ -59,18 +60,30 @@ 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'
 
 
 ----------------------------------------------------
--- Lists and Tuples
+-- 2. Lists and Tuples
 ----------------------------------------------------
 
 -- Every element in a list must have the same type.
--- Two lists that are the same
+-- These two lists are equal:
 [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] -- [] (Haskell defaults to incrementing)
+[5,4..1] -- [5, 4, 3, 2, 1]
+
+-- indexing into a list
+[1..10] !! 3 -- 4 (zero-based indexing)
+
 -- You can also have infinite lists in Haskell!
 [1..] -- a list of all the natural numbers
 
@@ -90,9 +103,6 @@ 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 +149,12 @@ 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. 
+-- equations that define fib. Haskell will automatically use the first
+-- equation whose left hand side pattern matches the value.
 fib 1 = 1
 fib 2 = 2
 fib x = fib (x - 1) + fib (x - 2)
@@ -172,7 +182,7 @@ 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
@@ -180,38 +190,40 @@ foo = add 10 -- foo is now a function that takes a number and adds 10 to it
 foo 5 -- 15
 
 -- Another way to write the same thing
-foo = (+10)
+foo = (10+)
 foo 5 -- 15
 
 -- function composition
--- the (.) function chains functions together.
+-- the operator `.` 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
+-- 4*(10+5) = 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 an 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
 ----------------------------------------------------
 
--- Haskell has a very strong type system, and everything has a type signature.
+-- Haskell has a very strong type system, and every valid expression has a type. 
 
 -- Some basic types:
 5 :: Integer
@@ -233,10 +245,10 @@ double x = x * 2
 -- 6. Control Flow and If Expressions
 ----------------------------------------------------
 
--- if expressions
+-- if-expressions
 haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome"
 
--- if expressions can be on multiple lines too, indentation is important
+-- if-expressions can be on multiple lines too, indentation is important
 haskell = if 1 == 1
             then "awesome"
             else "awful"
@@ -248,7 +260,7 @@ case args of
   _ -> putStrLn "bad args"
 
 -- Haskell doesn't have loops; it uses recursion instead.
--- map applies a function over every element in an array
+-- map applies a function over every element in a list
 
 map (*2) [1..5] -- [2, 4, 6, 8, 10]
 
@@ -268,11 +280,11 @@ foldl (\x y -> 2*x + y) 4 [1,2,3] -- 43
 -- This is the same as
 (2 * (2 * (2 * 4 + 1) + 2) + 3)
 
--- foldl is left-handed, foldr is right-
+-- foldl is left-handed, foldr is right-handed
 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
@@ -284,11 +296,10 @@ data Color = Red | Blue | Green
 
 -- Now you can use it in a function:
 
-
 say :: Color -> String
-say Red = "You are Red!"
-say Blue = "You are Blue!"
-say Green =  "You are Green!"
+say Red   = "You are Red!"
+say Blue  = "You are Blue!"
+say Green = "You are Green!"
 
 -- Your data types can have parameters too:
 
@@ -307,16 +318,16 @@ Nothing         -- of type `Maybe a` for any `a`
 -- it is not hard to explain enough to get going.
 
 -- When a Haskell program is executed, `main` is
--- called. It must return a value of type `IO ()`. For example:
+-- called. It must return a value of type `IO a` for some type `a`. 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
@@ -330,51 +341,51 @@ 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 
+-- name 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!"
 
 -- The type `IO` is an example of a "monad". The way Haskell uses a monad to
 -- do IO allows it to be a purely functional language. Any function that
 -- interacts with the outside world (i.e. does IO) gets marked as `IO` in its
--- type signature. This lets us reason about what functions are "pure" (don't
--- interact with the outside world or modify state) and what functions aren't.
+-- type signature. This lets us reason about which functions are "pure" (don't
+-- interact with the outside world or modify state) and which functions aren't.
 
 -- This is a powerful feature, because it's easy to run pure functions
 -- concurrently; so, concurrency in Haskell is very easy.
@@ -390,11 +401,26 @@ main'' = do
 
 let foo = 5
 
--- You can see the type of any value with `:t`:
+-- You can see the type of any value or expression with `:t`:
 
->:t foo
+> :t foo
 foo :: Integer
 
+-- Operators, such as `+`, `:` and `$`, are functions.
+-- Their type can be inspected by putting the operator in parentheses:
+
+> :t (:)
+(:) :: a -> [a] -> [a]
+
+-- You can get additional information on any `name` using `:i`:
+
+> :i (+)
+class Num a where
+  (+) :: a -> a -> a
+  ...
+    -- Defined in ‘GHC.Num’
+infixl 6 +
+
 -- You can also run any action of type `IO ()`
 
 > sayHello
@@ -406,7 +432,7 @@ Hello, Friend!
 
 There's a lot more to Haskell, including typeclasses and monads. These are the
 big ideas that make Haskell such fun to code in. I'll leave you with one final
-Haskell example: an implementation of quicksort in Haskell:
+Haskell example: an implementation of a quicksort variant in Haskell:
 
 ```haskell
 qsort [] = []
@@ -415,8 +441,9 @@ qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
           greater = filter (>= p) xs
 ```
 
-Haskell is easy to install. Get it [here](http://www.haskell.org/platform/).
+There are two popular ways to install Haskell: The traditional [Cabal-based installation](http://www.haskell.org/platform/), and the newer [Stack-based process](https://www.stackage.org/install).
 
 You can find a much gentler introduction from the excellent
-[Learn you a Haskell](http://learnyouahaskell.com/) or
+[Learn you a Haskell](http://learnyouahaskell.com/),
+[Happy Learn Haskell Tutorial](http://www.happylearnhaskelltutorial.com/) or
 [Real World Haskell](http://book.realworldhaskell.org/).