Split Assign into local variable and field assignment

[keyboardDrummer-bot]

Closes #19

Summary

Adds a new FieldAssign constructor to StmtExpr for field assignments, separating them from local variable assignments, and strengthens the Assign constructor to only accept Identifier targets:

| Assign (targets : List (AstNode Identifier)) (value : AstNode StmtExpr)
| FieldAssign (target : AstNode StmtExpr) (member : Identifier) (value : AstNode StmtExpr)

Previously, field assignments were represented as Assign [FieldSelect target member] value, requiring pattern matching on the target list to distinguish field writes from local variable assignments. The new FieldAssign constructor makes this distinction explicit in the type, and Assign targets are now List (AstNode Identifier) instead of List (AstNode StmtExpr), enforcing at the type level that assign targets are identifiers.

Grammar

• Single-target assign: a := value
• Multi-target assign: (a, b) := value (new multiAssign rule)
• Field assign: obj#field := value (parsed via assign + fieldAccess, translated to FieldAssign)

Changes

Core type change (Laurel.lean):

• Assign now takes List (AstNode Identifier) targets (was List (AstNode StmtExpr))
• New FieldAssign takes a target: AstNode StmtExpr and member: Identifier

Grammar (LaurelGrammar.st):

• Added multiAssign rule for (targets) := value syntax

Parser (ConcreteToAbstractTreeTranslator.lean):

• assign extracts the identifier name from the target, or produces FieldAssign for field access targets
• multiAssign parses comma-separated identifiers

Printer (AbstractToConcreteTreeTranslator.lean):

• Single-target Assign prints as assign(identifier, value)
• Multi-target Assign prints as multiAssign(targets, value)
• FieldAssign prints as assign(fieldAccess(target, member), value)

All Laurel passes: Updated to work with AstNode Identifier targets. Passes that previously recursed into targets (MapStmtExpr, FilterPrelude, Resolution, CoreGroupingAndOrdering, TypeHierarchy, HeapParameterization) no longer need to since targets are just identifiers.

Python frontend (PythonToLaurel.lean):

• Updated to construct AstNode Identifier targets for Assign
• Field assignments produce FieldAssign

Testing

All existing tests pass (the only failure is the pre-existing StrataTest.DDM.Integration.Java.TestGen which requires a missing jar file).

[keyboardDrummer]

@keyboardDrummer-bot can the assign constructor be Assign (targets : List AstNode Identifier) (value : AstNode StmtExpr) ?

And once you have that, can we also enable this multi assign in the grammar?