[hy/en] Introductory tutorial to hy language

* hy.html.markdown: This is an introductory tutorial to hy language,
  http://hylang.org which is a dialect of lisp built on top of python

hy.html.markdown

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+---
+language: hy
+filename: learnhy.hy
+contributors:
+    - ["Abhishek L", "http://twitter.com/abhishekl"]
+---
+
+Hy is a lisp dialect built on top of python. This is achieved by
+converting hy code to python's abstract syntax tree (ast). This allows
+hy to call native python code or python to call native hy code as well
+
+This tutorial works for hy ≥ 0.9.12
+
+```clojure
+;; this gives an gentle introduction to hy for a quick trial head to
+;; http://try-hy.appspot.com
+;;
+; Semicolon comments, like other LISPS
+
+;; s-expression basics
+; lisp programs are made of symbolic expressions or sexps which
+; resemble
+(some-function args)
+; now the quintessential hello world
+(print "hello world")
+
+;; simple data types
+; All simple data types are exactly similar to their python counterparts
+; which
+42 ; => 42
+3.14 ; => 3.14
+True ; => True
+4+10j ; => (4+10j) a complex number
+
+; lets start with some really simple arithmetic
+(+ 4 1) ;=> 5
+; the operator is applied to all arguments, like other lisps
+(+ 4 1 2 3) ;=> 10
+(- 2 1) ;=> 1
+(* 4 2) ;=> 8
+(/ 4 1) ;=> 4
+(% 4 2) ;=> 0 the modulo operator
+; power is represented by ** operator like python
+(** 3 2) ;=> 9
+; nesting forms will do the expected thing
+(+ 2 (* 4 2)) ;=> 10
+; also logical operators and or not and equal to etc. do as expected
+(= 5 4) ;=> False
+(not (= 5 4)) ;=> True
+
+;; variables
+; variables are set using setv, variable names can use utf-8 except
+; for ()[]{}",'`;#|
+(setv a 42)
+(setv π 3.14159)
+(def *foo* 42)
+;; other container data types
+; strings, lists, tuples & dicts
+; these are exactly same as python's container types
+"hello world" ;=> "hello world"
+; string operations work similar to python
+(+ "hello " "world") ;=> "hello world"
+; lists are created using [], indexing starts at 0
+(setv mylist [1 2 3 4])
+; tuples are immutable data structures
+(setv mytuple (, 1 2))
+; dictionaries are key value pairs
+(setv dict1 {"key1" 42 "key2" 21})
+; :name can be used to define keywords in hy which can be used for keys
+(setv dict2 {:key1 41 :key2 20})
+; use `get' to get the element at an index/key
+(get mylist 1) ;=> 2
+(get dict1 "key1") ;=> 42
+; Alternatively if keywords were used they can directly be called
+(:key1 dict2) ;=> 41
+
+;; functions and other program constructs
+; functions are defined using defn, the last sexp is returned by default
+(defn greet [name]
+  "A simple greeting" ; an optional docstring
+  (print "hello " name))
+
+(greet "bilbo") ;=> "hello bilbo"
+
+; functions can take optional arguments as well as keyword arguments
+(defn foolist [arg1 &optional [arg2 2]]
+  [arg1 arg2])
+
+(foolists 3) ;=> [3 2]
+(foolists 10 3) ;=> [10 3]
+
+; anonymous functions are created using `fn' or `lambda' constructs
+; which are similiar to `defn'
+(map (fn [x] (* x x)) [1 2 3 4]) ;=> [1 4 9 16]
+
+;; Sequence operations
+; hy has some builtin utils for sequence operations etc.
+; retrieve the first element using `first' or `car'
+(setv mylist [1 2 3 4])
+(setv mydict {"a" 1 "b" 2})
+(first mylist) ;=> 1
+
+; slice lists using slice
+(slice mylist 1 3) ;=> [2 3]
+
+; get elements from a list or dict using `get'
+(get mylist 1) ;=> 2
+(get mydict "b") ;=> 2
+; list indexing starts from 0 same as python
+; assoc can set elements at keys/indexes
+(assoc mylist 2 10) ; makes mylist [1 2 10 4]
+(assoc mydict "c" 3) ; makes mydict {"a" 1 "b" 2 "c" 3}
+; there are a whole lot of other core functions which makes working with
+; sequences fun
+
+;; Python interop
+;; import works just like in python
+(import datetime)
+(import [functools [partial reduce]]) ; imports fun1 and fun2 from module1
+(import [matplotlib.pyplot :as plt]) ; doing an import foo as bar
+; all builtin python methods etc. are accessible from hy
+; a.foo(arg) is called as (.foo a arg)
+(.split (.strip "hello world  ")) ;=> ["hello" "world"]
+
+;; Conditionals
+; (if condition (body-if-true) (body-if-false)
+(if (= passcode "moria")
+  (print "welcome")
+  (print "Speak friend, and Enter!"))
+
+; nest multiple if else if clauses with cond
+(cond
+ [(= someval 42)
+  (print "Life, universe and everything else!")]
+ [(> someval 42)
+  (print "val too large")]
+ [(< someval 42)
+  (print "val too small")])
+
+; group statements with do, these are executed sequentially
+; forms like defn have an implicit do
+(do
+ (setv someval 10)
+ (print "someval is set to " someval)) ;=> 10
+
+; create lexical bindings with `let', all variables defined thusly
+; have local scope
+(let [[nemesis {"superman" "lex luther"
+                "sherlock" "moriarty"
+                "seinfeld" "newman"}]]
+  (for [(, h v) (.items nemesis)]
+	(print (.format "{0}'s nemesis was {1}" h v))))
+
+;; classes
+; classes are defined in the following way
+(defclass Wizard [object]
+  [[--init-- (fn [self spell]
+             (setv self.spell spell) ; init the spell attr
+             None)]
+   [get-spell (fn [self]
+              self.spell)]])
+
+;; do checkout hylang.org
+```
+
+### Further Reading
+
+This tutorial is just a very basic introduction to hy/lisp/python.
+
+Hy docs are here: [http://hy.readthedocs.org](http://hy.readthedocs.org)
+
+Hy's Github repo: [http://github.com/hylang/hy](http://github.com/hylang/hy)
+
+On freenode irc #hy, twitter hashtag #hylang