Design notes on kratz

Compilation occurs in the following steps:

• Lexing & parsing: After this stage, each file is represented as one big list of Nodes. The only statements we’ve already singled out at this point are those with a special syntax, i.e., add, del, if/else-if/else, repeat, and until statements. Boundaries between individual scripts haven’t been identified yet.

• Transforming: In this stage, we convert the abstract Nodes into more specific Blocks, each representing an actual Scratch block. To this end, we also convert built-in functions (lerp, max, …) and statements (swap, …) into their Scratch block expansions. We also desugar custom function calls used in expressions.

• Evaluating: This stage necessarily includes constant folding, followed by validation of field parameters as well as variables and custom blocks. At this point we can finally cut the Block list into individual scripts. We validate that the file starts with a hat block, and that cap blocks are always followed by hat blocks.