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14 Working with Source Code

Org mode provides a number of features for working with source code, including editing of code blocks in their native major mode, evaluation of code blocks, tangling of code blocks, and exporting code blocks and their results in several formats.

A source code block conforms to this structure:

#+NAME: <name>
#+BEGIN_SRC <language> <switches> <header arguments>
  <body>
#+END_SRC

where:

Use C-c ' to edit the current code block. It opens a new major mode edit buffer containing the body of the source code block, ready for any edits. Use C-c ' again to close the buffer and return to the Org buffer.

Using header arguments

A header argument is specified with an initial colon followed by the argument’s name in lowercase.

Header arguments can be set in several ways; Org prioritizes them in case of overlaps or conflicts by giving local settings a higher priority.

System-wide header arguments

Those are specified by customizing org-babel-default-header-args variable, or, for a specific language LANG org-babel-default-header-args:LANG.

Header arguments in properties

You can set them using ‘header-args’ property (see Properties)—or ‘header-args:LANG’ for language LANG. Header arguments set through properties drawers apply at the sub-tree level on down.

Header arguments in code blocks

Header arguments are most commonly set at the source code block level, on the ‘BEGIN_SRC’ line:

#+NAME: factorial
#+BEGIN_SRC haskell :results silent :exports code :var n=0
  fac 0 = 1
  fac n = n * fac (n-1)
#+END_SRC

Code block header arguments can span multiple lines using ‘HEADER’ keyword on each line.

Evaluating code blocks

Use C-c C-c to evaluate the current code block and insert its results in the Org document. By default, evaluation is only turned on for ‘emacs-lisp’ code blocks, however support exists for evaluating blocks in many languages. For a complete list of supported languages see the manual. The following shows a code block and its results.

#+BEGIN_SRC emacs-lisp
  (+ 1 2 3 4)
#+END_SRC

#+RESULTS:
: 10

The following syntax is used to pass arguments to code blocks using the ‘var’ header argument.

:var NAME=ASSIGN

NAME is the name of the variable bound in the code block body. ASSIGN is a literal value, such as a string, a number, a reference to a table, a list, a literal example, another code block—with or without arguments—or the results of evaluating a code block.

Results of evaluation

How Org handles results of a code block execution depends on many header arguments working together. The primary determinant, however, is the ‘results’ header argument. It controls the collection, type, format, and handling of code block results.

Collection

How the results should be collected from the code block. You may choose either ‘output’ or ‘value’ (the default).

Type

What result types to expect from the execution of the code block. You may choose among ‘table’, ‘list’, ‘scalar’, and ‘file’. Org tries to guess it if you do not provide it.

Format

How Org processes results. Some possible values are ‘code’, ‘drawer’, ‘html’, ‘latex’, ‘link’, and ‘raw’.

Handling

How to insert the results once properly formatted. Allowed values are ‘silent’, ‘replace’ (the default), ‘append’, or ‘prepend’.

Code blocks which output results to files—e.g.: graphs, diagrams and figures—can accept a ‘:file FILENAME’ header argument, in which case the results are saved to the named file, and a link to the file is inserted into the buffer.

Exporting code blocks

It is possible to export the code of code blocks, the results of code block evaluation, both the code and the results of code block evaluation, or none. Org defaults to exporting code for most languages.

The ‘exports’ header argument is to specify if that part of the Org file is exported to, say, HTML or LaTeX formats. It can be set to either ‘code’, ‘results’, ‘both’ or ‘none’.

Extracting source code

Use C-c C-v t to create pure source code files by extracting code from source blocks in the current buffer. This is referred to as “tangling”—a term adopted from the literate programming community. During tangling of code blocks their bodies are expanded using org-babel-expand-src-block, which can expand both variable and “Noweb” style references. In order to tangle a code block it must have a ‘tangle’ header argument, see the manual for details.


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