Haskell Notes

gistfile1.txt

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Haskell
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http://learnyouahaskell.com/

Great resource for learning Haskell

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Language
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'[Haskell is a] general-purpose purely functional programming language, with non-strict semantics and strong static typing.'

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Basic Usage
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'ghci' to launch interactive mode

'ghci :l my_file.hs' to load interactive mode with the functions from a file

Once in ghci, call ':quit' to exit

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Lists
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let farm_animals = ['cat', 'dog', 'sheep', 'cow']

Can't mix data types
e.g. 
['cat', 3, 2, 1, 'baaa']

The keyword let is used to declare a variable


Can concatenate lists by using the ++ operator
[1,2,3,4] ++ [9,10,11,12]
results in [1, 2, 3, 4, 9, 10, 11, 12]

Can pre-pend a value to the list by using the : operator
5:[1, 2, 3, 4]
becomes [5, 1, 2, 3, 4]

Can return the element at index using !!

['1','2','3','4'] !! 2
returns '2'

Lists can also contain lists
[[1,2,3], [1,2,3], [1,2,3]]

Can compare contents in lexicographical order
[1,2,3] > [4,5,6]
false

[1,2,3] > [1,2]
true

Some functions to call on a list
head
  - returns the first element 
tail
  - returns everything but the first
last
  - returns the last element
init
  - returns everything but the last element

  http://s3.amazonaws.com/lyah/listmonster.png

length, null, reverse, take, drop, maximum, minimum, sum, product, elem

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List Ranges
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[1..5] 
  = [1,2,3,4,5]

['a'..'d'] 
  = ['a', 'b','c','d']

[3,6..20] 
  =[3,6,9,12,15,18]   

BUT DONT USE FLOAT!!!!

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List Comprehension
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[x*2 | x <- [1..10]]  
  = [2,4,6,8,10,12,14,16,18,20]  

x comes from [1..10]
applies 'x*2' on every element

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Tuples
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(1, 2, 3)
  - Better way of storing offset data than a list
  - Is still statically typed!
    - [(1,2),(8,11,5),(4,5)] is invalid type (expected tuple(t1, t2, t3))
    - [('a', 1), ('b', 2), ('c', 3 )]

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Function Declarations
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e.g. doubleMe x = x + x

[function_name] [parameters] = [function_body]

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Pattern Matching
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factorial :: Integer -> Integer // Optional
factorial 0 = 1  
factorial n = n * factorial (n - 1)  

fib n = fibs (0,1) !! n
        where fibs (a,b) = a : fibs (b,a+b)

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Syntax Differences
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'!=' doesn't exist, use '/=' instead