Fixing GAMS Parser: Multiple Option Assignments Supported

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Fixing GAMS Parser: Multiple Option Assignments Supported

Hey guys! Today, we're diving into a fascinating issue we tackled: how our parser choked when it met GAMS option statements with multiple assignments on a single line. This was a real blocker, stopping pool.gms from being parsed completely. Let's break down the problem, the solution, and how we made sure it's all working smoothly.

Problem Statement

So, what was the big deal? The parser was stumbling when it encountered GAMS code like this:

option clear = q,          clear = y,       clear = z  // start from same default point
       clear = obj,        clear = clower,  clear = cupper
       clear = pszrlt,     clear = plower,  clear = pupper
       clear = pqlower,    clear = pqupper, clear = fraction
       clear = extensions;

Basically, the parser couldn't handle multiple clear = variable_name assignments separated by commas on a single line, especially when they spanned multiple lines.

Current Behavior

When the parser hit one of these multi-assignments, it threw a parse error, like this:

Error: Parse error at line 708, column 16: Unexpected character: 'q'
  option clear = q,          clear = y,       clear = z
                 ^

Not very helpful, right? It just didn't know what to do with all those commas and assignments.

Expected Behavior

What we wanted was for the parser to calmly and effectively parse these option statements, no matter how they were formatted. This included:

  • Multiple assignments on a single line, separated by commas.
  • Continuation across multiple lines.
  • Mixed whitespace formatting (because, you know, people format code differently).
  • Inline comments after the statement (because comments are our friends).

Basically, we needed the parser to be more flexible and understanding of the GAMS language specification.

GAMS Language Specification

According to the GAMS documentation, the option statement should support multiple assignments. Here's the syntax:

option option_name = value [, option_name = value]* ;

And here are some examples:

* Single assignment
option limrow = 10;

* Multiple assignments on one line
option limrow = 10, limcol = 5;

* Multiple assignments across lines
option limrow = 10,
       limcol = 5,
       optcr = 0.01;

* The clear option specifically
option clear = x, clear = y, clear = z;

The clear option, in particular, is used to reset variables to their default values, which is pretty important before resolving a model.

Technical Analysis

To understand the fix, we needed to dive into the parser's grammar.

Current Grammar State

The existing option_stmt grammar rule looked something like this:

option_stmt: "option" ID "=" option_value ";"

This rule only expected a single assignment, which is why it choked on multiple ones.

Required Changes

To fix this, we needed to update the grammar to support a comma-separated list of assignments. The new rule looks like this:

option_stmt: "option" option_assignment ("," option_assignment)* ";"
option_assignment: ID "=" option_value

Here, option_value can be an identifier (like variable names for clear), a number (like limrow = 10), a string (for some options), or special keywords like on/off or yes/no.

Specific Patterns to Support

We needed to make sure the parser could handle these patterns:

  1. Simple multiple assignments:

    option limrow = 10, limcol = 5;

  2. Multiple clear assignments:

    option clear = x, clear = y, clear = z;

  3. Multi-line with continuation:

    option clear = q,
       clear = y,
       clear = z;

  4. Mixed whitespace formatting:

    option clear = q,          clear = y,       clear = z
       clear = obj,        clear = clower,  clear = cupper;

Implementation Plan

Here's how we implemented the fix:

1. Update Grammar (src/gams/gams_grammar.lark)

We updated the option_stmt rule in the grammar file to support comma-separated assignments:

option_stmt: "option" option_assignment ("," option_assignment)* ";"
option_assignment: ID "=" option_value
option_value: ID | NUMBER | STRING | "on" | "off" | "yes" | "no"

2. Update Parser (src/ir/parser.py)

Next, we updated the _parse_option_stmt method to handle multiple assignments:

  • Extract all option assignments from the statement.
  • Process each assignment individually.
  • Handle the special semantics of common options like clear, limrow, etc.

For the IR (Intermediate Representation), we decided to create separate OptionStmt nodes for each assignment. This was the simplest approach, although a more compact representation (a single node with a list of assignments) could be considered in the future.

3. Testing Strategy

Testing was crucial to make sure we didn't break anything and that the fix actually worked. We used a combination of unit and integration tests.

Unit Tests:

  • Single option assignment (to ensure existing behavior wasn't broken).
  • Two assignments on one line.
  • Multiple assignments across lines.
  • Various option types (clear, limrow, limcol, etc.).
  • Mixed whitespace and formatting.

Integration Tests:

  • Verified that pool.gms parses successfully past line 708.
  • Tested other GAMS library files with option statements.

Test Examples:

* Test 1: Simple multiple
option limrow = 10, limcol = 5;

* Test 2: Multiple clear
option clear = x, clear = y;

* Test 3: Multi-line
option clear = a,
       clear = b,
       clear = c;

* Test 4: Pool.gms pattern
option clear = q,          clear = y,       clear = z
       clear = obj,        clear = clower,  clear = cupper;

Acceptance Criteria

To consider the fix complete, we needed to meet these criteria:

  1. ✅ Grammar supports comma-separated option assignments.
  2. ✅ Parser correctly processes multiple assignments.
  3. ✅ pool.gms parses successfully past line 708.
  4. ✅ All existing tests continue to pass.
  5. ✅ New tests cover multiple assignment patterns.
  6. ✅ Quality gates pass (typecheck, lint, format, test).

Related Issues

This issue was closely related to these previous issues:

  • Issue #409: Pool.gms missing include file (completed).
  • Issue #412: Conditional sum syntax (completed).

This was the next blocker preventing pool.gms from parsing completely. With this fix in place, we're one step closer to full GAMS compatibility.

References

  • GAMS Documentation: Option Statement
  • File: tests/fixtures/tier2_candidates/pool.gms (line 708)
  • GAMS Library: pool.gms is model 237 in GAMSLib

So, there you have it! We successfully tackled a tricky parsing issue, making our system more robust and capable of handling complex GAMS code. Keep an eye out for more updates as we continue to improve and expand our parser's capabilities. Thanks for following along, and happy coding!