diff options
Diffstat (limited to 'haskell.html.markdown')
| -rw-r--r-- | haskell.html.markdown | 100 |
1 files changed, 57 insertions, 43 deletions
diff --git a/haskell.html.markdown b/haskell.html.markdown index e0489710..34eee748 100644 --- a/haskell.html.markdown +++ b/haskell.html.markdown @@ -1,5 +1,5 @@ --- -language: haskell +language: Haskell contributors: - ["Adit Bhargava", "http://adit.io"] --- @@ -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] @@ -110,7 +119,7 @@ last [1..5] -- 5 -- A tuple: ("haskell", 1) --- accessing elements of a tuple +-- accessing elements of a pair (i.e. a tuple of length 2) fst ("haskell", 1) -- "haskell" snd ("haskell", 1) -- 1 @@ -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) @@ -171,8 +180,8 @@ foldl1 (\acc x -> acc + x) [1..5] -- 15 -- 4. More functions ---------------------------------------------------- --- currying: if you don't pass in all the arguments to a function, --- it gets "curried". That means it returns a function that takes the +-- 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 -- rest of the arguments. add a b = a + b @@ -186,23 +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 `$`. This changes the precedence --- so that everything to the left of it gets computed first and then applied --- to everything on the right. You can use `.` and `$` 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 +even (fib 7) -- false + +-- equivalently +even $ fib 7 -- false + +-- composing functions +even . fib $ 7 -- false --- after -even . fib $ 7 -- true ---------------------------------------------------- -- 5. Type signatures @@ -227,24 +241,24 @@ double :: Integer -> Integer double x = x * 2 ---------------------------------------------------- --- 6. Control Flow and If Statements +-- 6. Control Flow and If Expressions ---------------------------------------------------- --- if statements +-- if expressions haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome" --- if statements 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" --- case statements: Here's how you could parse command line arguments +-- case expressions: Here's how you could parse command line arguments case args of "help" -> printHelp "start" -> startProgram _ -> putStrLn "bad args" --- Haskell doesn't have loops because it uses recursion instead. +-- Haskell doesn't have loops; it uses recursion instead. -- map applies a function over every element in an array map (*2) [1..5] -- [2, 4, 6, 8, 10] @@ -269,7 +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 @@ -307,13 +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 @@ -327,43 +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!" @@ -412,7 +426,7 @@ 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 |