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33 lines
3.8 KiB
Markdown
33 lines
3.8 KiB
Markdown
#### Just a few remarks upfront:
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* There is the (gradle/IDEA) module `parser`: that's the parser generated by ANTLR4, in Java. The only file to be edited here is the grammar, `prog8.g4`.
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* Then we have the module `compilerAst` - in Kotlin - which uses `parser` and adds AST nodes. Here we put our additions to the generated thing, *including any tests of the parsing stage*.
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- the name is a bit misleading, as this module isn't (or, resp. shouldn't be; see below) about *compiling*, only the parsing stage
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- also, the tree that comes out isn't much of an *abstraction*, but rather still more or less a parse tree (this might very well change).
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- **However, let's not *yet* rename the module.** We'll find a good name during refactoring.
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#### Problems with `compilerAst`:
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* `ModuleImporter.kt`, doing (Prog8-) module resolution. That's not the parser's job.
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* `ParsingFailedError` (in `ModuleParsing.kt`): this exception (it is actually *not* a `java.lang.Error`...) is thrown in a number of places, where other exceptions would make more sense. For example: not finding a file should just yield a `NoSuchFileException`, not this one. The other problem with it is that it does not provide any additional information about the source of parsing error, in particular a `Position`.
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* During parsing, character literals are turned into UBYTEs (since there is no basic type e.g. CHAR). That's bad because it depends on a specific character encoding (`IStringEncoding` in `compilerAst/src/prog8/ast/AstToplevel.kt`) of/for some target platform. Note that *strings* are indeed encoded later, in the `compiler` module.
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* The same argument applies to `IMemSizer`, and - not entirely sure about that - `IBuiltinFunctions`.
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#### Steps to take, in conceptual (!) order:
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1. introduce an abstraction `SourceCode` that encapsulates the origin and actual loading of Prog8 source code
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- from the local file system (use case: user programs)
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- from resources (prog8lib)
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- from plain strings (for testing)
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2. add subclass `ParseError : ParsingFailedError` which adds information about the *source of parsing error* (`SourceCode` and `Position`). We cannot just replace `ParsingFailedError` right away because it is so widely used (even in the `compiler` module). Therefore we'll just subclass for the time being, add more and more tests requiring the new one to be thrown (or, resp., NOT to be thrown), and gradually transition.
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3. introduce a minimal interface to the outside, input: `SourceCode`, output: a tree with a `Module` node as the root
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- this will be the Kotlin singleton `Prog8Parser` with the main method `parseModule`
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- plus, optionally, method's for registering/unregistering a listener with the parser
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- the *only* exception ever thrown / reported to listeners (TBD) will be `ParseError`
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- anything related to the lexer, error strategies, character/token streams is hidden from the outside
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- to make a clear distinction between the *generated* parser (and lexer) vs. `Prog8Parser`, and to discourage directly using the generated stuff, we'll rename the existing `prog8Parser`/`prog8Lexer` to `Prog8ANTLRParser` and `Prog8ANTLRLexer` and move them to package `prog8.parser.generated`
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4. introduce AST node `CharLiteral` and keep them until after identifier resolution and type checking; insert there an AST transformation step that turns them in UBYTE constants (literals)
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5. remove uses of `IStringEncoding` from module `compilerAst` - none should be necessary anymore
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6. move `IStringEncoding` to module `compiler`
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7. same with `ModuleImporter`, then rewrite that (addressing #46)
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8. refactor AST nodes and grammar: less generated parse tree nodes (`XyzContext`), less intermediary stuff (private classes in `Antrl2Kotlin.kt`), more compact code. Also: nicer names such as simply `StringLiteral` instead of `StringLiteralValue`
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9. re-think `IStringEncoding` to address #38
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