Mito looks interesting! Regarding the sin, I don't think it entirely applies here, unless I'm misunderstanding you - I'm not super familiar with transpilers.

In this case the template is also ruby code, but with placeholders for the values. So both the template and the input get parsed into ASTs, then both of them are recursively walked and compared on the fly. During that walk it builds up the data structure with initial values.

In the code generation direction it walks one AST and does lookups and rewrites with values from the values.

Does that make sense?