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354 lines
10 KiB
Markdown
354 lines
10 KiB
Markdown
---
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language: LiveScript
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filename: learnLivescript.ls
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contributors:
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- ["Christina Whyte", "http://github.com/kurisuwhyte/"]
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---
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LiveScript is a functional compile-to-JavaScript language which shares
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most of the underlying semantics with its host language. Nice additions
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comes with currying, function composition, pattern matching and lots of
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other goodies heavily borrowed from languages like Haskell, F# and
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Scala.
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LiveScript is a fork of [Coco][], which is itself a fork of
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[CoffeeScript][]. The language is stable, and a new version is in active
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development to bring a plethora of new niceties!
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[Coco]: http://satyr.github.io/coco/
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[CoffeeScript]: http://coffeescript.org/
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Feedback is always welcome, so feel free to reach me over at
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[@kurisuwhyte](https://twitter.com/kurisuwhyte) :)
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```coffeescript
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# Just like its CoffeeScript cousin, LiveScript uses number symbols for
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# single-line comments.
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/*
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Multi-line comments are written C-style. Use them if you want comments
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to be preserved in the JavaScript output.
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*/
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```
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```coffeescript
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# As far as syntax goes, LiveScript uses indentation to delimit blocks,
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# rather than curly braces, and whitespace to apply functions, rather
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# than parenthesis.
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########################################################################
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## 1. Basic values
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########################################################################
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# Lack of value is defined by the keyword `void` instead of `undefined`
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void # same as `undefined` but safer (can't be overridden)
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# No valid value is represented by Null.
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null
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# The most basic actual value is the logical type:
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true
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false
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# And it has a plethora of aliases that mean the same thing:
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on; off
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yes; no
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# Then you get numbers. These are double-precision floats like in JS.
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10
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0.4 # Note that the leading `0` is required
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# For readability, you may use underscores and letter suffixes in a
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# number, and these will be ignored by the compiler.
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12_344km
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# Strings are immutable sequences of characters, like in JS:
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"Christina" # apostrophes are okay too!
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"""Multi-line
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strings
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are
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okay
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too."""
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# Sometimes you want to encode a keyword, the backslash notation makes
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# this easy:
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\keyword # => 'keyword'
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# Arrays are ordered collections of values.
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fruits =
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* \apple
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* \orange
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* \pear
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# They can be expressed more concisely with square brackets:
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fruits = [ \apple, \orange, \pear ]
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# You also get a convenient way to create a list of strings, using
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# white space to delimit the items.
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fruits = <[ apple orange pear ]>
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# You can retrieve an item by their 0-based index:
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fruits[0] # => "apple"
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# Objects are a collection of unordered key/value pairs, and a few other
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# things (more on that later).
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person =
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name: "Christina"
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likes:
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* "kittens"
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* "and other cute stuff"
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# Again, you can express them concisely with curly brackets:
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person = {name: "Christina", likes: ["kittens", "and other cute stuff"]}
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# You can retrieve an item by their key:
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person.name # => "Christina"
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person["name"] # => "Christina"
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# Regular expressions use the same syntax as JavaScript:
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trailing-space = /\s$/ # dashed-words become dashedWords
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# Except you can do multi-line expressions too!
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# (comments and whitespace just gets ignored)
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funRE = //
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function\s+(.+) # name
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\s* \((.*)\) \s* # arguments
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{ (.*) } # body
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//
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########################################################################
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## 2. Basic operations
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########################################################################
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# Arithmetic operators are the same as JavaScript's:
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1 + 2 # => 3
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2 - 1 # => 1
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2 * 3 # => 6
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4 / 2 # => 2
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3 % 2 # => 1
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# Comparisons are mostly the same too, except that `==` is the same as
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# JS's `===`, where JS's `==` in LiveScript is `~=`, and `===` enables
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# object and array comparisons, and also stricter comparisons:
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2 == 2 # => true
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2 == "2" # => false
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2 ~= "2" # => true
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2 === "2" # => false
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[1,2,3] == [1,2,3] # => false
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[1,2,3] === [1,2,3] # => true
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+0 == -0 # => true
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+0 === -0 # => false
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# Other relational operators include <, <=, > and >=
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# Logical values can be combined through the logical operators `or`,
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# `and` and `not`
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true and false # => false
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false or true # => true
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not false # => true
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# Collections also get some nice additional operators
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[1, 2] ++ [3, 4] # => [1, 2, 3, 4]
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'a' in <[ a b c ]> # => true
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'name' of { name: 'Chris' } # => true
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########################################################################
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## 3. Functions
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########################################################################
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# Since LiveScript is functional, you'd expect functions to get a nice
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# treatment. In LiveScript it's even more apparent that functions are
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# first class:
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add = (left, right) -> left + right
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add 1, 2 # => 3
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# Functions which take no arguments are called with a bang!
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two = -> 2
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two!
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# LiveScript uses function scope, just like JavaScript, and has proper
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# closures too. Unlike JavaScript, the `=` works as a declaration
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# operator, and will always declare the variable on the left hand side.
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# The `:=` operator is available to *reuse* a name from the parent
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# scope.
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# You can destructure arguments of a function to quickly get to
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# interesting values inside a complex data structure:
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tail = ([head, ...rest]) -> rest
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tail [1, 2, 3] # => [2, 3]
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# You can also transform the arguments using binary or unary
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# operators. Default arguments are also possible.
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foo = (a = 1, b = 2) -> a + b
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foo! # => 3
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# You could use it to clone a particular argument to avoid side-effects,
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# for example:
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copy = (^^target, source) ->
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for k,v of source => target[k] = v
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target
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a = { a: 1 }
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copy a, { b: 2 } # => { a: 1, b: 2 }
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a # => { a: 1 }
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# A function may be curried by using a long arrow rather than a short
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# one:
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add = (left, right) --> left + right
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add1 = add 1
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add1 2 # => 3
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# Functions get an implicit `it` argument, even if you don't declare
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# any.
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identity = -> it
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identity 1 # => 1
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# Operators are not functions in LiveScript, but you can easily turn
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# them into one! Enter the operator sectioning:
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divide-by-two = (/ 2)
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[2, 4, 8, 16].map(divide-by-two) .reduce (+)
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# Not only of function application lives LiveScript, as in any good
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# functional language you get facilities for composing them:
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double-minus-one = (- 1) . (* 2)
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# Other than the usual `f . g` mathematical formulae, you get the `>>`
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# and `<<` operators, that describe how the flow of values through the
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# functions.
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double-minus-one = (* 2) >> (- 1)
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double-minus-one = (- 1) << (* 2)
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# And talking about flow of value, LiveScript gets the `|>` and `<|`
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# operators that apply a value to a function:
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map = (f, xs) --> xs.map f
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[1 2 3] |> map (* 2) # => [2 4 6]
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# You can also choose where you want the value to be placed, just mark
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# the place with an underscore (_):
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reduce = (f, xs, initial) --> xs.reduce f, initial
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[1 2 3] |> reduce (+), _, 0 # => 6
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# The underscore is also used in regular partial application, which you
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# can use for any function:
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div = (left, right) -> left / right
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div-by-two = div _, 2
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div-by-two 4 # => 2
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# Last, but not least, LiveScript has back-calls, which might help
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# with some callback-based code (though you should try more functional
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# approaches, like Promises):
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readFile = (name, f) -> f name
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a <- readFile 'foo'
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b <- readFile 'bar'
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console.log a + b
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# Same as:
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readFile 'foo', (a) -> readFile 'bar', (b) -> console.log a + b
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########################################################################
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## 4. Patterns, guards and control-flow
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########################################################################
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# You can branch computations with the `if...else` expression:
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x = if n > 0 then \positive else \negative
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# Instead of `then`, you can use `=>`
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x = if n > 0 => \positive
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else \negative
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# Complex conditions are better-off expressed with the `switch`
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# expression, though:
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y = {}
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x = switch
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| (typeof y) is \number => \number
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| (typeof y) is \string => \string
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| 'length' of y => \array
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| otherwise => \object # `otherwise` and `_` always matches.
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# Function bodies, declarations and assignments get a free `switch`, so
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# you don't need to type it again:
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take = (n, [x, ...xs]) -->
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| n == 0 => []
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| _ => [x] ++ take (n - 1), xs
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########################################################################
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## 5. Comprehensions
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########################################################################
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# While the functional helpers for dealing with lists and objects are
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# right there in the JavaScript's standard library (and complemented on
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# the prelude-ls, which is a "standard library" for LiveScript),
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# comprehensions will usually allow you to do this stuff faster and with
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# a nice syntax:
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oneToTwenty = [1 to 20]
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evens = [x for x in oneToTwenty when x % 2 == 0]
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# `when` and `unless` can be used as filters in the comprehension.
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# Object comprehension works in the same way, except that it gives you
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# back an object rather than an Array:
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copy = { [k, v] for k, v of source }
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########################################################################
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## 4. OOP
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########################################################################
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# While LiveScript is a functional language in most aspects, it also has
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# some niceties for imperative and object oriented programming. One of
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# them is class syntax and some class sugar inherited from CoffeeScript:
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class Animal
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(@name, kind) ->
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@kind = kind
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action: (what) -> "*#{@name} (a #{@kind}) #{what}*"
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class Cat extends Animal
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(@name) -> super @name, 'cat'
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purr: -> @action 'purrs'
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kitten = new Cat 'Mei'
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kitten.purr! # => "*Mei (a cat) purrs*"
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# Besides the classical single-inheritance pattern, you can also provide
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# as many mixins as you would like for a class. Mixins are just plain
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# objects:
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Huggable =
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hug: -> @action 'is hugged'
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class SnugglyCat extends Cat implements Huggable
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kitten = new SnugglyCat 'Purr'
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kitten.hug! # => "*Mei (a cat) is hugged*"
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```
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## Further reading
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There's just so much more to LiveScript, but this should be enough to
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get you started writing little functional things in it. The
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[official website](http://livescript.net/) has a lot of information on the
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language, and a nice online compiler for you to try stuff out!
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You may also want to grab yourself some
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[prelude.ls](http://gkz.github.io/prelude-ls/), and check out the `#livescript`
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channel on the Freenode network.
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