for loops implementation

[ba11b0y]

TODO: nested for loops don't work right now.

---

Important

Implement for loop support in BAML, including parsing, AST, bytecode, and VM execution, with tests.

• Behavior:
  • Implements for loop support in BAML, including parsing, AST representation, bytecode generation, and VM execution.
  • for loops iterate over arrays, pushing elements to the stack.
  • Nested for loops are not yet supported.
• AST and Parsing:
  • Adds ForLoop variant to Stmt enum in stmt.rs.
  • Updates parse.rs and parse_expr.rs to handle for loops.
  • Modifies datamodel.pest to include for_loop rule.
• Bytecode and VM:
  • Adds CreateIterator and IterNext instructions to bytecode.rs.
  • Implements CreateIterator and IterNext in vm.rs to handle array iteration.
• Tests:
  • Adds tests for for loop execution in vm.rs.

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Name	Status	Preview	Comments	Updated (UTC)
baml	❌ Failed (Inspect)			Jul 9, 2025 11:01pm

[ellipsis-dev]

Important

Looks good to me! 👍

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• Reviewed 1019 lines of code in 17 files
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• Skipped posting 6 draft comments. View those below.
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1. engine/baml-lib/ast/src/ast/stmt.rs:7

• Draft comment:
  The new ForLoop variant looks clear. Remember that nested for loops may need careful handling of shadowing and scope cleanup – verify that the loop variable is removed correctly when exiting nested loops.
• Reason this comment was not posted:
  Comment did not seem useful. Confidence is useful = 0% <= threshold 50% The comment is informative and suggests verifying the handling of loop variables in nested for loops. It doesn't provide a specific suggestion or point out a specific issue with the code. It also asks the author to verify something, which is against the rules.

2. engine/baml-lib/ast/src/parser/parse_expr.rs:67

• Draft comment:
  The parse_for_loop function correctly extracts the identifier, iterator, and body. However, since nested for loops are known to have issues, please add tests for nested loops and review how local scopes are handled to avoid variable clobbering.
• Reason this comment was not posted:
  Decided after close inspection that this draft comment was likely wrong and/or not actionable: usefulness confidence = 10% vs. threshold = 50% The comment is speculative - it warns about potential issues with nested loops without evidence that there are actual problems. The parse_for_loop function just handles the syntax parsing into an AST node. Scope handling would be handled by the interpreter/evaluator, not the parser. The comment goes beyond the scope of the parsing changes being made. Maybe there are actual scope handling issues that the reviewer is aware of from experience with the codebase. Maybe the AST structure itself needs to handle scopes differently. Without concrete evidence of scope issues in the current implementation, this is speculative. The parser's job is just to build the AST correctly, which this code appears to do. The comment should be deleted as it makes speculative suggestions about potential issues without evidence, and discusses concerns beyond the scope of the parsing changes.

3. engine/baml-lib/src/validate/validation_pipeline/validations/expr_fns.rs:105

• Draft comment:
  In the validation of a for loop, a new scope is created by cloning the parent's scope and adding the loop variable. For deeply nested loops, ensure that inner-loop variables do not improperly shadow or persist. Consider testing edge cases.
• Reason this comment was not posted:
  Comment was not on a location in the diff, so it can't be submitted as a review comment.

4. engine/baml-vm/src/vm.rs:757

• Draft comment:
  The ITER_NEXT instruction pops an iterator and pushes the next element and has_next flag. Verify that nested loop iterations do not share or overwrite the iterator state.
• Reason this comment was not posted:
  Comment did not seem useful. Confidence is useful = 0% <= threshold 50% The comment is asking the PR author to verify that nested loop iterations do not share or overwrite the iterator state. This falls under asking the author to double-check or ensure behavior, which is against the rules. Therefore, this comment should be removed.

5. engine/baml-vm/tests/vm.rs:250

• Draft comment:
  While tests for for loops (for_loop_simple and for_loop_with_expressions) exist, there is no explicit test for nested for loops. Adding a nested loop test would help identify and fix the known issue.
• Reason this comment was not posted:
  Comment was on unchanged code.

6. engine/baml-vm/src/bytecode.rs:137

• Draft comment:
  Typo in the TODO comment: the phrase "if this insn is complex than needed" seems off. Consider rephrasing it to "if this insn is more complex than needed."
• Reason this comment was not posted:
  Decided after close inspection that this draft comment was likely wrong and/or not actionable: usefulness confidence = 10% vs. threshold = 50% While the grammar suggestion is technically correct, TODO comments are temporary by nature and will be removed when the actual work is done. The meaning is clear despite the minor grammatical error. This kind of nitpicking on grammar in temporary comments doesn't add value and could be seen as noise. The grammar is indeed incorrect, and clear communication is important in code comments. Maybe improving clarity in comments helps future developers. While clear communication is important, this is a temporary TODO comment that will be removed soon. The meaning is already clear enough, and focusing on grammar in TODOs is excessive. Delete this comment as it's an unnecessary nitpick about grammar in a temporary TODO comment that will be removed anyway.

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