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diff --git a/factor.html.markdown b/factor.html.markdown new file mode 100644 index 00000000..a0726420 --- /dev/null +++ b/factor.html.markdown @@ -0,0 +1,182 @@ +--- +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) |