diff options
| -rw-r--r-- | haskell.html.markdown | 66 |
1 files changed, 48 insertions, 18 deletions
diff --git a/haskell.html.markdown b/haskell.html.markdown index a696cb5f..7158e2aa 100644 --- a/haskell.html.markdown +++ b/haskell.html.markdown @@ -45,15 +45,21 @@ not False -- True 1 /= 1 -- False 1 < 10 -- True +-- In the above examples, `not` is a function that takes one value. +-- Haskell doesn't need parentheses for function calls...all the arguments +-- are just listed after the function. So the general pattern is: +-- func arg1 arg2 arg3... +-- See the section on functions for information on how to write your own. + -- Strings and characters "This is a string." 'a' -- character 'You cant use single quotes for strings.' -- error! --- Strings can be added too! +-- Strings can be concatenated "Hello " ++ "world!" -- "Hello world!" --- A string can be treated like a list of characters +-- A string is a list of characters "This is a string" !! 0 -- 'T' @@ -69,14 +75,24 @@ not False -- True -- You can also have infinite lists in Haskell! [1..] -- a list of all the natural numbers --- joining two lists +-- Infinite lists work because Haskell has "lazy evaluation". This means +-- that Haskell only evaluates things when it needs to. So you can ask for +-- the 1000th element of your list and Haskell will give it to you: + +[1..] !! 999 -- 1000 + +-- And now Haskell has evaluated elements 1 - 1000 of this list...but the +-- rest of the elements of this "infinite" list don't exist yet! Haskell won't +-- actually evaluate them until it needs to. + + - joining two lists [1..5] ++ [6..10] -- adding to the head of a list 0:[1..5] -- [0, 1, 2, 3, 4, 5] -- indexing into a list -[0..] !! 5 -- 4 +[0..] !! 5 -- 5 -- more list operations head [1..5] -- 1 @@ -136,12 +152,12 @@ foo (x, y) = (x + 1, y + 2) -- Pattern matching on arrays. Here `x` is the first element -- in the array, and `xs` is the rest of the array. We can write -- our own map function: -map func [x] = [func x] -map func (x:xs) = func x:(map func xs) +myMap func [x] = [func x] +myMap func (x:xs) = func x:(myMap func xs) -- Anonymous functions are created with a backslash followed by -- all the arguments. -map (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7] +myMap (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7] -- using fold (called `inject` in some languages) with an anonymous -- function. foldl1 means fold left, and use the first value in the @@ -180,10 +196,10 @@ foo 5 -- 75 -- of parentheses: -- before -(even (double 7)) -- true +(even (fib 7)) -- true -- after -even . double $ 7 -- true +even . fib $ 7 -- true ---------------------------------------------------- -- 5. Type signatures @@ -198,13 +214,17 @@ True :: Bool -- Functions have types too. -- `not` takes a boolean and returns a boolean: -not :: Bool -> Bool +-- not :: Bool -> Bool -- Here's a function that takes two arguments: -add :: Integer -> Integer -> Integer +-- add :: Integer -> Integer -> Integer + +-- When you define a value, it's good practice to write it's type above it: +double :: Integer -> Integer +double x = x * 2 ---------------------------------------------------- --- 6. Control Flow +-- 6. Control Flow and If Statements ---------------------------------------------------- -- if statements @@ -263,25 +283,35 @@ Just 1 -- 8. Haskell IO ---------------------------------------------------- --- While IO can't be explained fully without explaining monads --- it is not hard to explain enough to get going +-- While IO can't be explained fully without explaining monads, +-- it is not hard to explain enough to get going. --- An IO a value is an IO action: you can chain them with do blocks +-- An `IO a` value is an IO action: you can chain them with do blocks +action :: IO String action = do putStrLn "This is a line. Duh" input <- getLine -- this gets a line and gives it the name "input" input2 <- getLine - return (input1++"\n"++input2) -- This is the result of the whole action + return (input1 ++ "\n" ++ input2) -- This is the result of the whole action -- This didn't actually do anything. When a haskell program is executed --- an IO action called "main" is read and interprete +-- an IO action called "main" is read and interpreted. main = do putStrLn "Our first program. How exciting!" result <- action -- our defined action is just like the default ones putStrLn result putStrLn "This was all, folks!" - + +-- Haskell does IO through a monad because this allows it to be a purely +-- functional language. Our `action` function had a type signature of `IO String`. +-- In general any function that interacts with the outside world (i.e. does IO) +-- gets marked as `IO` in it's 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. + +-- This is a powerful feature, because it's easy to run pure functions concurrently +-- so concurrency in Haskell is very easy. ---------------------------------------------------- |