Merge pull request #62 from egonSchiele/master

Fixing gripes about Haskell
This commit is contained in:
Adam Bard 2013-06-30 14:50:09 -07:00
commit dbc1d70a01

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@ -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.
----------------------------------------------------