Start with the idea
A strong answer defines input ownership, source spans, recovery strategy, and whether the parser returns a tree or events. Separate lexing from grammar only when that boundary improves errors or complexity.
A parser design starts with the language and its error contract. Is input trusted? Must the parser preserve exact source positions? Should it return one tree, a stream of events, or partial results with diagnostics? Define a small grammar and one ambiguous case before choosing an algorithm. A tokenizer can simplify grammar code by turning characters into identifiers, numbers, and punctuation with spans. For a tiny format, a separate lexer may be unnecessary; choose the boundary that improves clarity and error messages.
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