Parsing to AST

After tokenization, build an AST representing the template structure.

Node types:

• Text: literal text.
• Var: variable reference + optional filters.
• If: condition + then-block + optional else-block.
• For: var + iterable + block.
• Block: list of nodes (root, then-arm, else-arm, for-body).
• Include: reference to another template.
• Block (definition for inheritance).

Parser:

parse_if consumes the {% if cond %} token, then calls parse_block(tokens, until=['else','endif']). If next tag is else, it consumes and parses the else-block.

Errors:

• Unclosed {% if %} without {% endif %} → SyntaxError.
• {% endif %} without matching {% if %} → SyntaxError.
• Unknown tag → SyntaxError.

Inside expressions (in {{ ... }}): mini-parser for the language. Could be:

• Just identifier (Jinja2 default).
• Full expressions: user.name | default("guest").
• Method calls (some engines).
• Function calls.

Most template engines have a restricted expression sub-language. Avoids "templates are full programs" risk.