2.1.17 Inherited Indentation

The insertion and removal of ASTs in a software object often results in indentation that doesn’t match the surrounding code. The indentation mix-in can be used to address these situations and help prevent undesirable indentation by maintaining the expected indentation when an AST is inserted into a new position. This is accomplished by storing the current line’s indentation, and the amount of indentation that should be propagated to children and the difference between the indentation provided by all of the AST’s parents. To simplify the implementation, all tabs are treated as spaces.

Mix-ins

The ability to reinsert tabs is provided through the software-indentation mix-in. The following slots are provided:

• 'spaces-per-tab A number that indicates how many spaces each tab is worth. The initial form is 4.
• 'indent-with-tabs-p A boolean value indicating whether tabs should be used over spaces. The initial form is nil.

The ast-indentation mix-in is used to add indentation information to an AST. It provided the following slots:

• 'indent-children A number that indicates how many spaces should be added after newlines the precede a child of the AST. The value 't can be provided, and the slot’s value will be populated lazily with its parent’s value or a sane default based on the rest of the file. The initial form is nil.
• 'indent-adjustment A number (possibly negative that indicates the difference in indentation provided by an AST’s parents and the spaces on its current line. This value is also added to the indentation given to children. The value nil can be provided, and a value will instead be used that will match the indentation of the current line with that of its parent. The initial form is nil. Note that 0 and nil are not the same: zero indentation adjustment means the node will rigidly retain its original indentation.

Parsing

When using the indentation mix-in for software, the parser for the language needs to be modified to not insert whitespace after newlines, instead opting for storing this information in the indentation slots of an AST. This is accomplished by tracking the indentation that each AST provides to its children. When a newline is reached for a child, the current indentation provided by all of its parents is compared to the indentation provided for the child. If the indentation is different, the difference is used to to set either the parent’s indent-children slot when it hasn’t been changed from the default value or the indent-adjustment slot of the child otherwise.

Converting to Text

The source-text method also needs to be modified when using the indentation mix-in. While converting an AST to text, the interleaved text needs to be examined for newlines. When a newline is found in a string and nothing follows it, the next AST reached determines the amount of indentation needed. This will be the indentation provided by the indentation slots of all of its parents plus the indentation provided by its indent-adjustment slot. If a newline is found in a string and other text follows it, indentation is added that matches the current indentation of its parent.