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authorAditya Bhargava <bluemangroupie@gmail.com>2013-06-30 10:03:42 -0700
committerAditya Bhargava <bluemangroupie@gmail.com>2013-06-30 10:03:42 -0700
commit7ee4774f9b857f4d17fdcb086130bca862ae15f0 (patch)
tree9a155d9dd4b9b23fce5ec7a066cfd04d9c979189 /haskell.html.markdown
parent35a27a333712987e1b613cb5bac8c1275876efaf (diff)
fixing issues listed in "gripes about haskell" (fixes bug #45)
Diffstat (limited to 'haskell.html.markdown')
-rw-r--r--haskell.html.markdown66
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.
----------------------------------------------------