#+OPTIONS: H:3 num:nil toc:2 \n:nil @:t ::t |:t ^:{} -:t f:t *:t TeX:t LaTeX:t skip:nil d:(HIDE) tags:not-in-toc #+STARTUP: align fold nodlcheck hidestars oddeven lognotestate hideblocks #+SEQ_TODO: TODO(t) INPROGRESS(i) WAITING(w@) | DONE(d) CANCELED(c@) #+TAGS: Write(w) Update(u) Fix(f) Check(c) #+TITLE: Org-babel #+AUTHOR: Eric Schulte, Dan Davison #+EMAIL: schulte.eric at gmail dot com, davison at stats dot ox dot ac dot uk #+LANGUAGE: en #+CATEGORY: worg # #+INFOJS_OPT: view:content #+begin_html

executable source code blocks in org-mode

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#+end_html * Introduction :PROPERTIES: :CUSTOM_ID: introduction :END: Org-babel is an extension to the very excellent [[http://orgmode.org/][Org-mode]], providing the ability to execute source code in many different languages within org-mode documents. The results of code execution --- text, tables and graphics --- can be integrated into the powerful publishing facilities of org-mode. Org-mode is an [[http://www.gnu.org/software/emacs/][Emacs]] major mode for doing almost anything with plain text. If you are not familiar with Org-mode please take a moment to read [[http://orgmode.org/][the Org-mode homepage]] before continuing. Org-babel provides the following modifications to [[http://orgmode.org/manual/Literal-examples.html][the existing support]] for blocks of source code examples in the org-mode core. 1. Interactive source code execution 2. Arguments to source code blocks 3. Exportation of source code blocks to files (literate programming) * Getting started :PROPERTIES: :CUSTOM_ID: getting-started :END: 1) Grab the latest code from the git repo at [[http://github.com/eschulte/org-babel/tree/master][github/org-babel]] #+begin_src sh git clone git://github.com/eschulte/org-babel.git #+end_src 2) Add the following lines to your .emacs, replacing the path as appropriate. A good place to check that things are up and running would the examples in [[* Basic org-babel functionality][Basic org-babel functionality]]. #+begin_src emacs-lisp (add-to-list 'load-path "/path/to/org-babel/lisp") (require 'org-babel-init) #+end_src 3) Finally, activate the subset of supported Org-babel languages which you want to be able to execute on your system. As an example, the following activates python, ruby and R. For a full list of languages, with notes on their dependencies see the [[#reference-and-documentation][Reference / Documentation]] section below. #+begin_src emacs-lisp (require 'org-babel-python) (require 'org-babel-ruby) (require 'org-babel-R) ;; ;; Once you've activated languages, load the library of babel to ;; make pre-built helper functions available in the languages you will be using. (org-babel-load-library-of-babel) #+end_src * Basic org-babel functionality :PROPERTIES: :CUSTOM_ID: basic-functionality :END: *** Source code blocks :PROPERTIES: :CUSTOM_ID: source-code-blocks :END: Org-babel is all about *source code blocks* in org mode. These are blocks of code (in whatever language), that can occur anywhere in an org-mode file. For example, the following is a source block containing [[http://www.ruby-lang.org/][ruby]] code: : #+begin_src ruby : "This file was last evaluated on #{Date.today}" : #+end_src If you are unfamiliar with the notion of a source code block in org-mode, please have a look at the [[http://orgmode.org/manual/Literal-examples.html][relevant manual section]] before proceding. Note that above is what the source block looks like in the org-mode file. We had to take [[http://orgmode.org/manual/Literal-examples.html#Literal-examples][special steps]] to make it look that way in the HTML output. Normally, when exported to HTML, source blocks are fontified according to their language, and the begin_src...end_src mark-up is omitted, like this: #+begin_src ruby "This file was last evaluated on #{Date.today}" #+end_src From now on, if you are viewing the HTML version, you will see the HTML output only. However, much of this document consists of interactive examples, and therefore in order to get a feeling for the mechanics of Org-babel it might make most sense to grab the plain text version of this file #+HTML: org-babel-worg.org and work through it in Emacs. Alternatively the htmlized version of the plain text of this file at #+HTML: org-babel-worg.html allows the plain text version to be viewed (non-interactively) in a web browser. *** Source code execution :PROPERTIES: :CUSTOM_ID: source-code-execution :END: For interpreted languages such as shell, python, R, etc, org-babel allows source blocks to be executed: the code is passed to the interpreter and you have control over what is done with the results of execution. Here are three examples of code blocks in three different languages, followed by their output. If you are viewing the plain text version of this document in emacs, place point anywhere inside the blocks and use =C-c C-c= to run the code[fn:1] (and feel free to alter it!). **** Ruby #+begin_src ruby "This file was last evaluated on #{Date.today}" #+end_src #+resname: : This file was last evaluated on 2009-08-09 **** [[http://www.r-project.org/][R]] #+begin_src R :results value matrix(rnorm(6), nrow=2) #+end_src #+resname: | -0.138279734486552 | -2.2476234005706 | -0.0839549402407832 | | 0.0730510956002737 | 0.0634015508602321 | 0.174013159381603 | **** [[http://ditaa.sourceforge.net/][ditaa]] #+begin_src ditaa :file images/blue.png :cmdline -r +---------+ | cBLU | | | | +----+ | |cPNK| | | | +----+----+ #+end_src #+resname: [[file:images/blue.png]] *** Source code block syntax The basic syntax of source-code blocks in Org-babel is as follows: : #+srcname: name(arguments) : #+begin_src language header-arguments : body : #+end_src - name :: This name is associated with the source-code block. This is similar to the =#+tblname= lines which can be used to name tables in org-mode files. By referencing the srcname of a source-code block it is possible to evaluate the block from other places, files, or from inside tables. - arguments :: Code blocks can have arguments (see [[#arguments-to-source-code-blocks][below]]) which are provided using a familiar function-call syntax similar to (e.g.) python or R. - language :: The language of the code in the source-code block. Valid values must be members of `org-babel-interpreters'. - header-arguments :: Header arguments control many facets of the evaluation and output of source-code blocks. See the [[* Header Arguments][Header Arguments]] section for a complete review of available header arguments. - body :: The actual source code which will be evaluated. An important key-binding to become familiar with is =C-c '=. This calls `org-edit-special' which brings up an edit buffer containing the code using the emacs major mode appropriate to the language. *** What happens to the results? :PROPERTIES: :CUSTOM_ID: results :END: Org-babel provides two fundamentally different modes for capturing the results of code evaluation, specified by the =:results= header argument. **** =:results value= (functional mode) This means that the 'result' of code evaluation is defined to be the *value* of the last statement in the block. Thus with this setting, one can view the code block as a function with a return value. And not only can you view it that way, but you can actually use the return value of one source block as input for another (see [[meta-programming-language]]). This setting is the default. As an example, consider the following block of python code and its output. #+begin_src python :results value import time print("Hello, today's date is %s" % time.ctime()) print('Two plus two is') 2 + 2 #+end_src #+resname: : 4 Notice that in functional mode, the output consists of the value of the last statement, and nothing else. **** =:results output= (scripting mode) With this setting, org-babel captures all the text output of the code block and places it in the org buffer. One can think of this as a 'scripting' mode: the code block contains a series of commands, and you get the output of all the commands. Unlike in the 'functional' mode, the code block has no return value. (This mode will be more familiar to Sweave users). Now consider the result of evaluating the same source block as before, but under scripting mode. #+srcname: name #+begin_src python :results output import time print("Hello, today's date is %s" % time.ctime()) print('Two plus two is') 2 + 2 #+end_src #+resname: name : Hello, today's date is Fri Sep 4 19:49:06 2009 : Two plus two is Again, we got what we asked for: all the text output (stdout) from python. Since we didn't print the last value (2 + 2), we didn't get it in our output. *** Arguments to source code blocks :PROPERTIES: :CUSTOM_ID: arguments-to-source-code-blocks :END: In addition to evaluation of code blocks, org-babel allows them to be parameterised (i.e. have arguments). Thus source code blocks now have the status of *functions*. Arguments to code blocks can be used in both functional and scripting mode. **** Simple example of using a source block as a function First let's look at a very simple example. The following source block defines an org-babel function that will square its input. #+srcname: square(x) #+begin_src python x*x #+end_src In the org-mode file that looks like this: : #+srcname: square(x) : #+begin_src python : x*x : #+end_src Now we use the source block: : #+lob: square(x=6) (/for information on the/ =lob= /syntax see [[library-of-babel]]/) #+lob: square(x=6) #+resname: square(x=6) : 36 **** A more complex example: using an org-table as input In this example we're going to define a function to compute a Fibonacci sequence, and we're going to make it take its input from a table in the org-mode buffer. Here are the inputs for fibonacci-seq: #+tblname: fibonacci-inputs | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | in the Org-mode buffer this looks like : #+tblname: fibonacci-inputs : | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | : | 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | [[http://www.gnu.org/software/emacs/manual/elisp.html][Emacs Lisp]] source code #+srcname: fibonacci-seq(fib-inputs=fibonacci-inputs) #+begin_src emacs-lisp (defun fibonacci (n) (if (or (= n 0) (= n 1)) n (+ (fibonacci (- n 1)) (fibonacci (- n 2))))) (mapcar (lambda (row) (mapcar #'fibonacci row)) fib-inputs) #+end_src in the Org-mode buffer this looks like : #+srcname: fibonacci-seq(fib-inputs=fibonacci-inputs) : #+begin_src emacs-lisp : (defun fibonacci (n) : (if (or (= n 0) (= n 1)) : n : (+ (fibonacci (- n 1)) (fibonacci (- n 2))))) : : (mapcar (lambda (row) : (mapcar #'fibonacci row)) fib-inputs) : #+end_src Results of Emacs Lisp code evaluation #+resname: | 1 | 1 | 2 | 3 | 5 | 8 | 13 | 21 | 34 | 55 | | 1 | 3 | 8 | 21 | 55 | 144 | 377 | 987 | 2584 | 6765 | * A meta-programming language for org-mode :PROPERTIES: :CUSTOM_ID: meta-programming-language :END: Since information can pass freely between source-code blocks and org-mode tables you can mix and match languages using each language for those tasks to which it is suited. This makes Org-mode files with Org-babel into a kind of meta-functional programming language in which functions from many languages can work together. As an example, lets take some system diagnostics in the shell, and then graph them with R. 1. First we create a code block containing shell code creating a list of the directories in our home directory, together with their sizes. Org-babel automatically converts the output into an org table. #+srcname: directories #+begin_src bash :results replace cd ~ && du -sc * |grep -v total #+end_src #+resname: directories | 72 | "Desktop" | | 12156104 | "Documents" | | 3482440 | "Downloads" | | 2901720 | "Library" | | 57344 | "Movies" | | 16548024 | "Music" | | 120 | "News" | | 7649472 | "Pictures" | | 0 | "Public" | | 152224 | "Sites" | | 8 | "System" | | 56 | "bin" | | 3821872 | "mail" | | 10605392 | "src" | | 1264 | "tools" | 2. Now we use a single line of R code to plot the data as a pie-chart. Note the way that this source block uses the =srcname= of the previous source block to obtain the data. #+srcname: directory-pie-chart(dirs = directories) #+begin_src R :session R-pie-example pie(dirs[,1], labels = dirs[,2]) #+end_src [[file:images/dirs.png]] * Multilingual spreadsheet plugins for org-mode :PROPERTIES: :CUSTOM_ID: spreadsheet :END: *NOTE*: Maybe in-addition-to/in-stead-of this example we should do a more traditional "spreadsheet" example with R [Eric] Not only can Org-babel pass entire tables of data to source code blocks (see [[arguments-to-source-code-blocks]]), Org-babel can also be used to call source code blocks from *within* tables using the Org-mode's [[http://orgmode.org/manual/The-spreadsheet.html#The-spreadsheet][existing spreadsheet functionality]]. In fact the functional test suite for Org-babel is implemented as a large Org-mode table. To run the entire test suite you simple evaluate the table =C-u C-c C-c=, and all of the tests are run updating the table with pass/fail statistics. Here's a sample of our test suite. #+TBLNAME: org-babel-tests | functionality | block | arg | expected | results | pass | |------------------+--------------+-----+-------------+-------------+------| | basic evaluation | | | | | pass | |------------------+--------------+-----+-------------+-------------+------| | emacs lisp | basic-elisp | 2 | 4 | 4 | pass | | shell | basic-shell | | 6 | 6 | pass | | ruby | basic-ruby | | org-babel | org-babel | pass | | python | basic-python | | hello world | hello world | pass | | R | basic-R | | 13 | 13 | pass | #+TBLFM: $5='(if (= (length $3) 1) (progn (message (format "running %S" '(sbe $2 (n $3)))) (sbe $2 (n $3))) (sbe $2))::$6='(if (string= $4 $5) "pass" (format "expected %S but was %S" $4 $5)) #+TBLFM: $5=""::$6="" *** code blocks for tests #+srcname: basic-elisp #+begin_src emacs-lisp :var n=7 (* 2 n) #+end_src #+srcname: basic-shell #+begin_src sh :results silent expr 1 + 5 #+end_src #+srcname: date-simple #+begin_src sh :results silent date #+end_src #+srcname: basic-ruby #+begin_src ruby :results silent "org-babel" #+end_src #+srcname: basic-python #+begin_src python :results silent 'hello world' #+end_src #+srcname: basic-R #+begin_src R :results silent b <- 9 b + 4 #+end_src * The Library of Babel :PROPERTIES: :CUSTOM_ID: library-of-babel :END: #+begin_html #+end_html As we saw above with the [[*Simple%20example%20of%20using%20a%20source%20block%20as%20a%20function][=square=]] example, once a source block function has been defined it can be called using the following short =lob= notation: : #+lob: square(x=6) But what about those source code blocks which are so useful you want to have them available in every org-mode buffer? In addition to the current buffer, Org-babel searches for pre-defined source block functions in the Library of Babel. This is a user-extensible collection of ready-made source-code blocks for handling common tasks. One use for the LoB (not yet done!) will be to provide a choice of data graphing procedures for data held in org-mode tables, using languages such as R, gnuplot, asymptote, etc. If you implement something that might be of use to other org users, please consider adding it to the LoB; similarly, feel free to request help solving a problem using external code via org-babel -- there's always a chance that other org users will be able to contribute some helpful code. Org-mode demonstrates that an enormous amount can be achieved using plain text and emacs lisp; the LoB is intended to fill in the gaps. Org-babel comes pre-populated with the source-code blocks located in the [[file:library-of-babel.org][library-of-babel.org]] file. It is possible to add source-code blocks from any org-mode file to the library by calling #+srcname: add-file-to-lob #+begin_src emacs-lisp (org-babel-lob-ingest "path/to/file.org") #+end_src Note that it is also possible to pass table values or the output of a source-code block to lob functions, and it is possible to reference lob functions in source block arguments. * Reproducible Research :PROPERTIES: :CUSTOM_ID: reproducable-research :END: #+begin_quote An article about computational science in a scientific publication is not the scholarship itself, it is merely advertising of the scholarship. The actual scholarship is the complete software development environment and the complete set of instructions which generated the figures. -- D. Donoho #+end_quote [[http://reproducibleresearch.net/index.php/Main_Page][Reproducible Research]] (RR) is the practice of distributing along with an article of research all data, code, and tools required to reproduce the results discussed in the paper. As such the paper becomes not only a document describing the research but a complete laboratory in which the research can be reproduced and extended. Org-mode already has exceptional support for [[http://orgmode.org/manual/Exporting.html#Exporting][exporting to html and LaTeX]]. Org-babel makes Org-mode a tool for RR by *activating* the data and source code embedded into Org-mode documents making the entire document executable. This makes it not only possible, but natural to distribute research in a format that encourages readers to recreate your results, and perform their own analysis. One notable existing RR tool is [[http://en.wikipedia.org/wiki/Sweave][Sweave]] which provides for the embedding of [[http://www.r-project.org/][R]] code into LaTeX documents. While Sweave is a mature and very useful tool, we believe that Org-babel has several advantages: - It supports multiple languages (we're not aware of other RR tools that do this) - The [[http://orgmode.org/manual/Exporting.html#Exporting][export process]] is flexible and powerful, including HTML as a target in addition to LaTeX - The document can make native use of all the features of Org-mode, such as those for [[http://orgmode.org/manual/Agenda-Views.html#Agenda-Views][project planning]] and [[http://orgmode.org/manual/TODO-Items.html#TODO-Items][task management]] * Literate programming :PROPERTIES: :CUSTOM_ID: literate-programming :END: #+begin_quote Let us change our traditional attitude to the construction of programs: Instead of imagining that our main task is to instruct a /computer/ what to do, let us concentrate rather on explaining to /human beings/ what we want a computer to do. The practitioner of literate programming can be regarded as an essayist, whose main concern is with exposition and excellence of style. Such an author, with thesaurus in hand, chooses the names of variables carefully and explains what each variable means. He or she strives for a program that is comprehensible because its concepts have been introduced in an order that is best for human understanding, using a mixture of formal and informal methods that reinforce each other. -- Donald Knuth #+end_quote Org-babel supports [[http://en.wikipedia.org/wiki/Literate_programming][Literate Programming]] (LP) by allowing the act of programming to take place inside of Org-mode documents. The Org-mode file can then be exported (*woven* in LP speak) to html or LaTeX for consumption by a human, and the embedded source code can be extracted (*tangled* in LP speak) into structured source code files for consumption by a computer. To support these operations Org-babel relies on Org-mode's [[http://orgmode.org/manual/Exporting.html#Exporting][existing exporting functionality]] for *weaving* of documentation, and on the =org-babel-tangle= function which makes use of [[http://www.cs.tufts.edu/~nr/noweb/][Noweb]] [[reference-expansion][reference syntax]] for *tangling* of code files. The [[literate-programming-example][following example]] demonstrates the process of *tangling* in Org-babel. *** Simple Literate Programming Example (Noweb syntax) :PROPERTIES: :CUSTOM_ID: literate-programming-example :END: Tangling functionality is controlled by the =tangle= family of [[header-arguments]]. These arguments can be used to turn tangling on or off (the default) on the source code block, or the outline heading level. The following demonstrates the combination of three source code blocks into a single source code file using =org-babel-tangle=. The following two blocks will not be tangled by default since they have no =tangle= header arguments. #+srcname: hello-world-prefix #+begin_src sh :exports none echo "/-----------------------------------------------------------\\" #+end_src : #+srcname: hello-world-prefix : #+begin_src sh :exports none : echo "/-----------------------------------------------------------\\" : #+end_src #+srcname: hello-world-postfix #+begin_src sh :exports none echo "\-----------------------------------------------------------/" #+end_src : #+srcname: hello-world-postfix : #+begin_src sh :exports none : echo "\-----------------------------------------------------------/" : #+end_src The third block does have a =tangle= header argument indicating the name of the file to which it should be written. It also has [[http://www.cs.tufts.edu/~nr/noweb/][Noweb]] style references to the two previous source code blocks which will be expanded during tangling to include them in the output file as well. #+srcname: hello-world #+begin_src sh :tangle hello :exports none # <> echo "| hello world |" # <> #+end_src : #+srcname: hello-world : #+begin_src sh :tangle hello :exports none : # <> : echo "| hello world |" : # <> : #+end_src Calling =org-babel-tangle= will result in the following being written to the =hello.sh= file. #+srcname: hello-world-output #+begin_src sh #!/usr/bin/env sh # generated by org-babel-tangle # [[file:~/src/org-babel/org-babel-worg.org::#literate-programming-example][block-16]] # <> echo "/-----------------------------------------------------------\\" echo "| hello world |" # <> echo "\-----------------------------------------------------------/" # block-16 ends here #+end_src *** Emacs Initialization with Org-babel Org-babel has special support for embedding your emacs initialization into Org-mode files. The =org-babel-load-file= function can be used to load the emacs lisp embedded in a literate Org-mode file in the same way that you might load a regular elisp file. This allows you to have all the niceness of Org-mode (folding, tags, notes, html export, etc...) available in your emacs initialization. To try this out either see the simple [[literate-emacs-init][Literate Emacs Initialization]] example directly below, or check out the Org-babel Literate Programming version of Phil Hagelberg's excellent [[http://github.com/technomancy/emacs-starter-kit/tree/master][emacs-starter-kit]] available at [[http://github.com/eschulte/emacs-starter-kit/tree/master][Org-babel-emacs-starter-kit]]. ***** Literate Emacs Initialization :PROPERTIES: :CUSTOM_ID: literate-emacs-init :END: For a simple example of usage follow these 4 steps. 1) create a directory named =.emacs.d= in the base of your home directory. #+begin_src sh mkdir ~/.emacs.d #+end_src 2) checkout the latest versions of Org-mode and Org-babel into the src subdirectory of this new directory #+begin_src sh cd ~/.emacs.d mkdir src cd src git clone git://repo.or.cz/org-mode.git git clone git://github.com/eschulte/org-babel.git #+end_src 3) place the following in a file called =init.el= in your emacs initialization directory (=~/.emacs.d=). #+srcname: emacs-init #+begin_src emacs-lisp ;;; init.el --- Where all the magic begins ;; ;; This file loads both ;; - Org-mode : http://orgmode.org/ and ;; - Org-babel: http://eschulte.github.com/org-babel/ ;; ;; It then loads the rest of our Emacs initialization from Emacs lisp ;; embedded in literate Org-mode files. ;; Load up Org Mode and Org Babel for elisp embedded in Org Mode files (setq dotfiles-dir (file-name-directory (or (buffer-file-name) load-file-name))) (add-to-list 'load-path (expand-file-name "lisp" (expand-file-name "org" (expand-file-name "src" dotfiles-dir)))) (add-to-list 'load-path (expand-file-name "lisp" (expand-file-name "org-babel" (expand-file-name "src" dotfiles-dir)))) (require 'org-babel-init) ;; load up all literate org-mode files in this directory (mapc #'org-babel-load-file (directory-files dotfiles-dir t "\\.org$")) ;;; init.el ends here #+end_src 4) Implement all of your emacs customizations inside of elisp source-code blocks located in Org-mode files in this directory. They will be loaded by emacs on startup. * Reference / Documentation :PROPERTIES: :CUSTOM_ID: reference-and-documentation :END: *** Languages The following can be added to your .emacs and used to activate languages. It includes a brief list of the requirements for each language. *Note*: this also serves as the list of languages currently supported by Org-babel. #+begin_src emacs-lisp ;; Uncomment each of the following require lines if you want org-babel ;; to support that language. Each language has a comment explaining ;; it's dependencies. See the related files in lisp/langs for more ;; detailed explanations of requirements. ;; (require 'org-babel-R) ;; R and ess-mode ;; (require 'org-babel-asymptote) ;; asymptote ;; (require 'org-babel-css) ;; none ;; (require 'org-babel-ditaa) ;; ditaa ;; (require 'org-babel-dot) ;; dot ;; (require 'org-babel-gnuplot) ;; gnuplot, and gnuplot-mode ;; (require 'org-babel-haskell) ;; haskell, haskell-mode, inf-haskell ;; (require 'org-babel-ocaml) ;; ocaml, and tuareg-mode ;; (require 'org-babel-python) ;; python, and python-mode ;; (require 'org-babel-ruby) ;; ruby, irb, ruby-mode, and inf-ruby ;; (require 'org-babel-sass) ;; sass, sass-mode ;; (require 'org-babel-sql) ;; none #+end_src *** Header Arguments :PROPERTIES: :CUSTOM_ID: header-arguments :END: - results :: results arguments specify what should be done with the output of source-code blocks - The following options are mutually exclusive, and specify how the results should be collected from the source-code block - value :: - output :: - The following options are mutually exclusive and specify what type of results the code block will return - vector :: specifies that the results should be interpreted as a multidimensional vector (even if the vector is trivial), and will be inserted into the org-mode file as a table - scalar :: specifies that the results should be interpreted as a scalar value, and will be inserted into the org-mode file as quoted text - file :: specifies that the results should be interpreted as the path to a file, and will be inserted into the org-mode file as a link - The following options specify how the results should be inserted into the org-mode file - replace :: the current results replace any previously inserted results from the code block - silent :: rather than being inserted into the org-mode file the results are echoed into the message bar - exports :: exports arguments specify what should be included in html or latex exports of the org-mode file - code :: the body of code is included into the exported file - results :: the results of evaluating the code is included in the exported file - both :: both the code and results are included in the exported file - none :: nothing is included in the exported file - tangle :: tangle arguments specify whether or not the source-code block should be included in tangled extraction of source-code files - yes :: the source-code block is exported to a source-code file named after the basename (name w/o extension) of the org-mode file - no :: (default) the source-code block is not exported to a source-code file - other :: any other string passed to the =tangle= header argument is interpreted as a file basename to which the block will be exported *** Noweb reference syntax The [[http://www.cs.tufts.edu/~nr/noweb/][Noweb]] Literate Programming system allows named blocks of code to be referenced by using a =<>= syntax. When a document is tangled these references are replaced with the named code. An example is provided in the [[literate-programming-example]] in this document. * Footnotes [fn:1] Calling =C-c C-o= on a source-code block will open the block's results in a separate buffer.