Michael Schimmel
3b063dc2c9
Implement strict block scoping and distinguish between statements and expressions. This change introduces hierarchical scopes for functions and blocks, enforces that `def` is a statement with no return value, and prevents statements from appearing in expression positions or as the last element of a block. A new example `design-flaw.myc` is added to demonstrate the uninitialized variable issue solved by these changes.
46 lines
2.2 KiB
Markdown
46 lines
2.2 KiB
Markdown
# Development Log: Binder Refactoring & Block Scoping
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## Status Quo
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- Myc currently uses a flat scope per function.
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- `def` is treated as a side-effect expression that "leaks" into the function scope regardless of nesting (e.g., inside `if` or `do`).
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- This leads to uninitialized variables at runtime if the definition is skipped by control flow.
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## Goal
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Implement strict **Block Scoping** and distinguish between **Statements** and **Expressions** to eliminate undefined variable states.
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### Rules
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1. **Block Scoping:** Every `(do ...)` and `if` branch creates a new lexical scope.
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2. **Statements vs. Expressions:**
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- `def` is a **Statement**.
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- Statements have no return value (Void).
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- Statements **cannot** be used as expressions (e.g., as arguments, RHS of assignments, or conditions).
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- Statements **cannot** be the last element of a block (since the block's value is determined by its last expression).
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3. **No Shadowing:** Redefining a symbol within the **same** scope level is an error. Shadowing from outer scopes is allowed (standard lexical scoping).
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4. **Self-Reference:** A variable is only available in its scope **after** its definition is complete (RHS of `def` sees outer scope).
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## Implementation Plan
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### Phase 1: Binder Infrastructure (In Progress)
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1. [ ] Update `CompilerScope` to support hierarchical nesting via a stack in `FunctionCompiler`.
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2. [ ] Implement `push_scope` and `pop_scope` in `Binder`.
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3. [ ] Refactor `resolve_variable` to traverse the scope stack (inner-to-outer).
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4. [ ] Update `define_variable` to enforce "No Shadowing" at the current scope level.
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### Phase 2: Statement/Expression Validation
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1. [ ] Introduce a mechanism to track "Context" (Expression vs. Statement) during binding.
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2. [ ] Validate that `def` is only used in statement positions.
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3. [ ] Enforce that the last node in a `BoundKind::Block` is an expression.
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### Phase 3: VM Compatibility
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1. [ ] Ensure the VM handles "Void" results from statements correctly.
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2. [ ] (Optional) Optimize stack allocation based on scope depth.
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---
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## Log Entries
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### 2024-05-22: Initialization
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- Analyzed `design-flaw.myc`.
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- Defined the "Statement" concept to solve the uninitialized variable problem.
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- Initialized this log.
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