Add a Factor tutorial

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+---
+language: factor
+contributors:
+    - ["hyphz", "http://github.com/hyphz/"]
+filename: learnfactor.factor
+---
+
+Factor is a modern stack-based language, based on Forth, created by Slava Pestov.
+
+Code in this file can be typed into Factor, but not directly imported because the vocabulary and import header would make the beginning thoroughly confusing.
+
+```
+! This is a comment
+
+! Like Forth, all programming is done by manipulating the stack.
+! Stating a literal value pushes it onto the stack.
+5 2 3 56 76 23 65    ! No output, but stack is printed out in interactive mode
+
+! Those numbers get added to the stack, from left to right.
+! .s prints out the stack non-destructively.
+.s     ! 5 2 3 56 76 23 65
+
+! Arithmetic works by manipulating data on the stack.
+5 4 +    ! No output
+
+! `.` pops the top result from the stack and prints it.
+.    ! 9 
+
+! More examples of arithmetic:
+6 7 * .        ! 42 
+1360 23 - .    ! 1337 
+12 12 / .      ! 1 
+13 2 mod .     ! 1 
+
+99 neg .       ! -99 
+-99 abs .      ! 99 
+52 23 max .    ! 52 
+52 23 min .    ! 23 
+
+! A number of words are provided to manipulate the stack, collectively known as shuffle words.
+
+3 dup -          ! duplicate the top item (1st now equals 2nd): 3 - 3
+2 5 swap /       ! swap the top with the second element:        5 / 2
+4 0 drop 2 /     ! remove the top item (dont print to screen):  4 / 2
+1 2 3 nip .s     ! remove the second item (similar to drop):    1 3
+1 2 clear .s     ! wipe out the entire stack
+1 2 3 4 over .s  ! duplicate the second item to the top: 1 2 3 4 3 
+1 2 3 4 2 pick .s ! duplicate the third item to the top: 1 2 3 4 2 3
+
+! Creating Words 
+! The `:` word sets Factor into compile mode until it sees the `;` word.
+: square ( n -- n ) dup * ;    ! No output
+5 square .                     ! 25 
+
+! We can view what a word does too.
+! \ suppresses evaluation of a word and pushes its identifier on the stack instead.
+\ square see    ! : square ( n -- n ) dup * ;
+
+! After the name of the word to create, the declaration between brackets gives the stack effect.
+! We can use whatever names we like inside the declaration:
+: weirdsquare ( camel -- llama ) dup * ;
+
+! Provided their count matches the word's stack effect:
+: doubledup ( a -- b ) dup dup ; ! Error: Stack effect declaration is wrong
+: doubledup ( a -- a a a ) dup dup ; ! Ok
+: weirddoubledup ( i -- am a fish ) dup dup ; ! Also Ok
+
+! Where Factor differs from Forth is in the use of quotations.
+! A quotation is a block of code that is pushed on the stack as a value.
+! [ starts quotation mode; ] ends it.
+[ 2 + ]       ! Quotation that adds 2 is left on the stack
+4 swap call . ! 6
+
+! And thus, higher order words. TONS of higher order words.
+2 3 [ 2 + ] dip .s      ! Pop top stack value, run quotation, push it back: 4 3
+3 4 [ + ] keep .s       ! Copy top stack value, run quotation, push the copy: 7 4
+1 [ 2 + ] [ 3 + ] bi .s ! Run each quotation on the top value, push both results: 3 4
+4 3 1 [ + ] [ + ] bi .s ! Quotations in a bi can pull values from deeper on the stack: 4 5 ( 1+3 1+4 )
+1 2 [ 2 + ] bi@ .s      ! Run the quotation on first and second values
+2 [ + ] curry           ! Inject the given value at the start of the quotation: [ 2 + ] is left on the stack
+
+! Conditionals
+! Any value is true except the built-in value f.
+! A built-in value t does exist, but its use isn't essential.
+! Conditionals are higher order words as with the combinators above.
+
+5 [ "Five is true" . ] when                     ! Five is true
+0 [ "Zero is true" . ] when                     ! Zero is true
+f [ "F is true" . ] when                        ! No output
+f [ "F is false" . ] unless                     ! F is false
+2 [ "Two is true" . ] [ "Two is false" . ] if   ! Two is true 
+
+! By default the conditionals consume the value under test, but starred variants 
+! leave it alone if it's true:
+
+5 [ . ] when*      ! 5
+f [ . ] when*      ! No output, empty stack, f is consumed because it's false
+
+
+! Loops
+! You've guessed it.. these are higher order words too.
+
+5 [ . ] each-integer               ! 0 1 2 3 4 
+4 3 2 1 0 5 [ + . ] each-integer   ! 0 2 4 6 8
+5 [ "Hello" . ] times              ! Hello Hello Hello Hello Hello
+
+! Here's a list:
+{ 2 4 6 8 }                        ! Goes on the stack as one item
+
+! Loop through the list:
+{ 2 4 6 8 } [ 1 + . ] each          ! Prints 3 5 7 9
+{ 2 4 6 8 } [ 1 + ] map             ! Leaves { 3 5 7 9 } on stack
+
+! Loop reducing or building lists:
+{ 1 2 3 4 5 } [ 2 mod 0 = ] filter  ! Keeps only list members for which quotation yields true: { 2 4 }  
+{ 2 4 6 8 } 0 [ + ] reduce .        ! Like "fold" in functional languages: prints 20 (0+2+4+6+8)
+{ 2 4 6 8 } 0 [ + ] accumulate . .  ! Like reduce but keeps the intermediate values in a list: prints { 0 2 6 12 } then 20 
+1 5 [ 2 * dup ] replicate .         ! Loops the quotation 5 times and collects the results in a list: { 2 4 8 16 32 }
+1 [ dup 100 < ] [ 2 * dup ] produce ! Loops the second quotation until the first returns false and collects the results: { 2 4 8 16 32 64 128 }
+
+! If all else fails, a general purpose while loop:
+1 [ dup 10 < ] [ "Hello" . 1 + ] while  ! Prints "Hello" 10 times
+                                        ! Yes, it's hard to read
+                                        ! That's what all those variant loops are for
+
+! Variables
+! Usually Factor programs are expected to keep all data on the stack.
+! Using named variables makes refactoring harder (and it's called Factor for a reason)
+! Global variables, if you must:
+
+SYMBOL: name            ! Creates name as an identifying word
+"Bob" name set-global   ! No output
+name get-global .       ! "Bob"                     
+
+! Named local variables are considered an extension but are available
+! In a quotation..
+[| m n                  ! Quotation captures top two stack values into m and n
+ | m n + ]              ! Read them
+
+! Or in a word..
+:: lword ( -- )           ! Note double colon to invoke lexical variable extension
+   2 :> c                 ! Declares immutable variable c to hold 2
+   c . ;                  ! Print it out
+
+! In a word declared this way,  the input side of the stack declaration 
+! becomes meaningful and gives the variable names stack values are captured into
+:: double ( a -- result ) a 2 * ;
+
+! Variables are declared mutable by ending their name with a shriek
+:: mword2 ( a! -- x y )   ! Capture top of stack in mutable variable a
+   a                      ! Push a
+   a 2 * a!               ! Multiply a by 2 and store result back in a
+   a ;                    ! Push new value of a 
+5 mword2                  ! Stack: 5 10
+
+! Lists and Sequences
+! We saw above how to push a list onto the stack
+
+0 { 1 2 3 4 } nth         ! Access a particular member of a list: 1
+10 { 1 2 3 4 } nth        ! Error: sequence index out of bounds
+1 { 1 2 3 4 } ?nth        ! Same as nth if index is in bounds: 2
+10 { 1 2 3 4 } ?nth       ! No error if out of bounds: f
+
+{ "at" "the" "beginning" } "Append" prefix    ! { "Append" "at" "the" "beginning" }
+{ "Append" "at" "the" } "end" suffix          ! { "Append" "at" "the" "end" }
+"in" 1 { "Insert" "the" "middle" } insert-nth ! { "Insert" "in" "the" "middle" }
+"Concat" "enate" append                       ! "Concatenate" - strings are sequences too
+"Concatenate" "Reverse " prepend              ! "Reverse Concatenate"
+{ "Concatenate " "seq " "of " "seqs" } concat ! "Concatenate seq of seqs"
+{ "Connect" "subseqs" "with" "separators" } " " join  ! "Connect subseqs with separators" 
+
+! And if you want to get meta, quotations are sequences and can be dismantled..
+0 [ 2 + ] nth                              ! 2
+1 [ 2 + ] nth                              ! +
+[ 2 + ] \ - suffix                         ! Quotation [ 2 + - ]
+
+
+```
+
+##Ready For More?
+
+* [Factor Documentation](http://docs.factorcode.org/content/article-help.home.html)