Merge pull request #3998 from uvtc/patch-1

[janet/en] initial version

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+---
+language: Janet
+filename: learnJanet.janet
+contributors:
+    - ["John Gabriele", "http://www.unexpected-vortices.com/"]
+---
+
+[Janet](https://janet-lang.org/) is a Lisp-like (Clojure-like),
+lexically-scoped, dynamically-typed, garbage-collected, C-based, high-level
+language. The entire language (core library, interpreter, compiler, assembler,
+PEG) is about 300-500 kB and should run on many constrained systems.
+
+I encourage you to try out the code snippets below in the Janet
+repl (either by [installing Janet](https://janet-lang.org/docs/index.html),
+or else by using the repl embedded in the Janet homepage).
+
+As we only have a scant *y* minutes, we'll survey the basics here and
+leave the remaining details for the manual. So please, keep your arms and
+legs inside the vehicle at all times, and on with the scenic tour!
+
+```janet
+# A comment.
+
+# Some literal values.
+true
+false
+nil
+
+# Typical style for symbols (identifiers-for / names-of things).
+do-stuff
+pants-on-fire!
+foo->bar  # Evidently for converting foos to bars.
+fully-charged?
+_  # Usually used as a dummy variable.
+
+# Keywords are like symbols that start with a colon, are treated like
+# constants, and are typically used as map keys or pieces of syntax in
+# macros.
+:a
+:some-val
+
+# Numbers #####################################################################
+5
+1e3    # => 1000
+1_000  # => 1000
+2e-03  # => 0.002
+0xff   # => 255
+
+# You can specify a radix (base) like so:
+16rff   # => 255 (same as 0xff)
+2r1101  # =>  13
+
+# Some numbers in the math library:
+math/pi  # => 3.14159
+math/e   # => 2.71828
+
+# Strings #####################################################################
+"hello"
+"hey\tthere"  # contains a tab
+
+# For multi-line strings, use one or more backticks. No escapes allowed.
+``a long
+multi-line
+string``  # => "a long\nmulti-line\nstring"
+
+# Strings and data structures in Janet come in two varieties: mutable and
+# immutable. The literal for the mutable variety is written with a `@` in
+# front of it.
+
+# A mutable string (aka "buffer").
+@"this"
+@`a multi-line
+one here`
+
+(string "con" "cat" "enate")  # => "concatenate"
+
+# To get a substring:
+(string/slice "abcdefgh" 2 5)  # => "cde"
+# To find a substring:
+(string/find "de" "abcdefgh")  # => 3
+
+# See the string library for more (splitting, replacement, etc.)
+
+# Arrays and Tuples ###########################################################
+# Arrays are mutable, tuples are immutable.
+
+# Arrays (mutable)
+@(4 5 6)
+@[4 5 6]
+
+# Tuples (immutable)
+# Note that an open paren usually indicates a function call, so if you want a
+# literal tuple with parens, you need to "quote" it (with a starting single
+# quote mark).
+'(4 5 6)
+[4 5 6]  # ... or just use square brackets.
+
+# Tables and Structs (AKA: "maps", "hashmaps", "dictionaries")
+@{:a 1 :b 2 :c 3}  # table  (mutable)
+{:a 1 :b 2 :c 3}   # struct (immutable)
+
+# More about how to work with arrays/tuples and tables/structs below.
+
+# Bindings ####################################################################
+# ... or "Name Some Things!" (that is, bind a value to a symbol)
+(def x 4.7)  # Define a constant, `x`.
+x            # => 4.7
+(quote x)    # => x (the symbol x)
+'x           # => x (the symbol x (shorthand))
+(print x)    # prints 4.7
+
+# Since we used `def`, can't change to what `x` refers:
+(set x 5.6)  # Error, `x` is a constant.
+
+(var y 10)
+(set y 12)  # Works, since `y` was made var.
+
+# Note that bindings are local to the scope they're called in. `let`
+# creates a local scope and makes some bindings all in one shot:
+(let [a 2
+      b 3]
+  (print "Hello from inside this local scope.")
+  (* a b))  # => 6
+
+# Destructuring is supported, both for arrays/tuples ...
+(def a ["foos" "bars" "moos"])
+(let [[s1 _ s2] a]
+  (print s1 s2))  # foosmoos
+
+# ... and for tables/structs.
+(def t {:a "ayy" :b "bee" :c "sea"})
+(let [{:a a :b b} t]
+  (print a b))  # ayybee
+
+# You can even destructure right in a `def`:
+(def [aa1 aa2] a)
+aa1  # => foos
+aa2  # => bars
+
+(def {:c body-of-water :b insect-friend} t)
+body-of-water  # => sea
+insect-friend  # => bee
+
+# Note that keywords evaluate to themselves, whereas symbols evaluate
+# to whatever value they're bound to (unless you quote them).
+
+# Operators ###################################################################
+# Janet supports the usual ensemble of operators.
+# +, -, *, /, and so on. Note:
+(/ 5 3)  # =>  1.66667
+(% 5 3)  # =>  2 (remainder)
+(- 5)    # => -5 (or you can just write `-5`)
+
+(++ i)    # increments
+(-- i)    # decrements
+(+= i 3)  # add 3 to `i`
+(*= i 3)  # triple `i`
+# ... and so on for the other operations on numbers.
+
+# Comparison
+# =  <  >  not=  <=  >=
+(< 2 7 12)  # => true
+
+# Functions ###################################################################
+# Call them:
+(- 5 3)  # => 2 (Yes, operators and functions work the same.)
+(math/sin (/ math/pi 2))  # => 1
+(range 5)  # => @[0 1 2 3 4]
+
+# Create them:
+(defn mult-by-2
+  ``First line of docstring.
+
+  Some more of the docstring.
+
+  Possibly more!``
+  [x]
+  (print "Hi.")
+  (print "Will compute using: " x)
+  (* 2 x))
+
+(print (mult-by-2 6))  # => 12 (after printing "Hi" and so forth)
+
+# If you have a function named "main" in your file, `janet` will automatically
+# call it for you when you run the file.
+
+# Interactively read a function's docs from within the repl:
+(doc mult-by-2)
+
+# Note, functions have to be defined before they can be used in a function,
+# so if you design top-down, you'll need to write your functions from the
+# bottom of the file up.
+
+# You can make anonymous functions as well:
+(fn [x] (+ x x))
+(fn my-func [x] (+ x x))  # This one's less anonymous.
+
+# Use `do` to make some side-effecting calls and then evaluate to
+# the last form in the `do`:
+(def n (do
+         (print "hi")
+         (do-some-side-effecting 42)
+         3))
+n  # => 3
+
+# You might say that function bodies provide an "implicit do".
+
+# Operations on data structures ###############################################
+# (Making all these mutable so we can ... mutate them.)
+(def s @"Hello, World!")
+(def a @[:a :b :c :d :e])
+(def t @{:a 1 :b 2})
+
+(length s)  # => 13
+(length a)  # =>  5
+(length t)  # =>  2
+
+# Getting values:
+(s 7)   # => 87 (which is the code point for "W")
+(a 1)   # => :b
+(t :a)  # => 1
+(keys t)    # => @[:a :b]
+(values t)  # => @[1 2]
+
+# Changing values (for mutable data structures):
+(put s 2 87)   # @"HeWlo, World!"
+(put a 2 :x)   # @[:a :b :x :d :e]
+(put t :b 42)  # @{:a 1 :b 42}
+
+# Adding & removing values (again, for mutable data structures):
+(buffer/push-string s "??")  # @"HeWlo, World!??"
+(array/push a :f)  # @[:a :b :x :d :e :f]
+(array/pop a)      # => :f, and it's also removed from `a`.
+(put t :x 88)      # @{:a 1 :b 42 :x 88}
+
+# See the manual for a wide variety of functions for working with
+# buffers/strings, arrays/tuples, and tables/struct.
+
+# Flow control ################################################################
+(if some-condition
+  42
+  38)
+
+# Only `nil` and `false` are falsey. Everything else is truthy.
+
+(if got-it?
+  71)  # No false-branch value. Returns `nil` if `got-it?` is falsey.
+
+(var i 10)
+(while (pos? i)
+  (print "... " i)
+  (-- i))
+# Now `i` is 0.
+
+# `case` compares the dispatch value to each of the options.
+(var x 2)
+(case x
+  1 "won"
+  2 "too"
+  3 "tree"
+  "unknown")  # => "too"
+
+# `cond` evaluates conditions until it gets a `true`.
+(set x 8)
+(cond
+  (= x 1) "won"
+  (= x 2) "too"
+  (< x 10) "tree"
+  "oof!")  # => "tree"
+
+(when (avoided-wipeout?)
+  (do-side-effecty-thing 88)
+  (smell-the-roses)
+  (paint-fencepost-error))
+
+# Pattern matching.
+# `match` is like a high-powered switch expression. If you switch on a data
+# structure, it can look inside to try and match on its contents. For example,
+# matching on a table or struct:
+(def t {:a 1 :b 2 :c 3})
+(match t
+  {:yar v} (print "matches key :yar! " v)
+  {:moo v} (print "matches key :moo! " v)
+  {:c   v} (print "matches key :c! "   v)
+  _ (print "no match"))  # => prints "matches key :c! 3"
+
+# Iterating ###################################################################
+# Iterate over an integer range:
+(for i 0 5
+  (print i))  # prints 0, 1, 2, 3, 4
+
+# There's also the more general `loop`:
+(loop [i :range [0 10] :when (even? i)]
+  (print i))
+
+# Loop over an array/tuple:
+(def words ["foo" "bar" "baz"])
+(each word words
+  (print word))
+
+# Loop over a table/struct:
+(def t {:a 1 :b 2})
+(eachp [k v] t  # Loop over each pair in `t`.
+  (print k " --> " v))
+
+# Can also use `eachk` to loop over keys in a table or struct.
+
+# Functional programming ######################################################
+# You'll find many familiar old friends here.
+(filter even?
+        (map (fn [x]
+               (* x x))
+             (range 10)))  # => @[0 4 16 36 64]
+
+(reduce + 0 (range 5))  # => 10
+
+# ...and lots more (see the API docs).
+
+# Errata ######################################################################
+(type a)                # => the type of `a` (as a keyword)
+(describe a)            # => a human-readable description of `a`
+(string/format "%j" a)  # => Janet values, nicely-formatted
+```
+
+This tour didn't cover a number of other features such as modules, fibers,
+PEGs, macros, etc., but should give you a taste of what Janet is like. See
+the [Janet manual](https://janet-lang.org/docs/index.html) and the [Janet API
+docs](https://janet-lang.org/api/index.html) for more info.