Indentation Parsing in PetitParser

The PetitParser supports for indentation grammar definitions for languages such as Haskel, F#, Python, YAML, MarkDown and many others.

Nested Lists Example

Let us define a PPListsParser that can parse documents such as this one:

 Cool Cake Recipe:
 - Add flour and baking powder into the bowl
 - Mix ingredients:
   - Butter
   - Suggar
   - Eggs
 - Put into the owen

The document is list of paragraphs and another lists. Of course, lists can be nested into each other.

The result of the example above has two elements: i) paragraph: Cool Cake Recipe: and ii) list with 3 instructions. Furthermore, the second instruction contains one paragraph ’Mix ingredients:’ and one nested list with three ingredients: Butter, Suggar and Eggs.

As we see, there are two basic elements in our documents, list and paragraph. Therefore, we define contentElement as this:

	^ (list / paragraph)

For simplicity, paragraph is a single line:

	^ #newline asParser negate star, #newline asParser

List is a bit more complex, since it is recursive. List is a sequence of items, which may contain another lists. Let us focus on item definition first. Item has to start with ’-’. The item content is any valid document if aligned properly. Last but not least, item should end, when content no longer aligns.

Therefore, we define item as the sequence:

	^ itemStarts, content, itemEnds 
	^ bullet, blanks, #setIl asParser
	^ #restoreIl asParser

	^ #blank asParser star

In itemStarts we define the new indentation level. The indentation level defines a column where the item content starts. In itemEnds we restore the previous indentation level.

Now the question is, how to force content to align its contentElements (i.e paragraphs and another lists) to the indentation level. Simply by using trimAlign message. The trimAlign expression consumes whitespace before an expression up until the column as set by #setIl asParser is reached. Note, that trimAlign does not consume any whitespace after an expression.

	^ contentElement, 
	 (contentElement trimAlign) star

Once we have a listItem and content properly defined, we can close the circle or list recursive definition and define the list:

	^ item,
	 (item trimAlign) star

Try this out, download the PetitParser and see the class PPListsParser for the whole grammar. If you are interested, there are many more grammars, such as Python or MarkDown!


The indentation in PetitParser is implemented using the indentation stack. The indentation stack is a stack of parsing expressions. The expressions on the indentation stack can be added, removed and evaluated at will.

Fore example, #setIl asParser pushes special parsing expression that compares current column with the specified column. The #restoreIl asParser expression pops from the stack. The trimAlign consumes the whitespace until the expression on top of the indentation stack is evaluated to true.

Last changed by admin on 21 April 2009