Critical Fortran Formatter Bug: Fix Program Structure Loss

by Felix Dubois 59 views

Hey guys! We've got a serious issue on our hands with the Fortran formatter. It's mangling code structure during transformations, and honestly, it's a major headache. Let's dive into the nitty-gritty details and figure out what's going on.

Critical Bug Description

The core problem? This formatter is systematically destroying the basic Fortran program structure when it's supposed to be, you know, formatting it. The output we're getting is either flat-out invalid or significantly different from the original code. This isn't just a minor inconvenience; it's a showstopper.

Primary Errors

We're seeing two main error messages pop up:

ERROR STOP Should preserve program declaration
ERROR STOP Test failed

These aren't exactly confidence-builders, are they?

Complete Stack Trace

For the tech-savvy folks, here's the complete stack trace:

Error termination. Backtrace:
#3  0x56381b0ef47c in format_and_check
    at test/test_formatter_advanced.f90:438
#4  0x56381b0f4e6c in test_complex_expression_formatting
    at test/test_formatter_advanced.f90:59
#5  0x56381b0d82d9 in test_formatter_advanced
    at test/test_formatter_advanced.f90:15

This helps pinpoint where the issues are originating in the code.

Failing Test Cases

To really understand the scope of the problem, let's look at some specific test cases that are failing. These examples will give you a clear picture of what's going wrong.

Test 1: Basic Program Structure Preservation

This test is about as fundamental as it gets. We're checking if the formatter can handle a simple program structure.

Source Code:

program test
    implicit none  
    integer :: x
    x = 42
end program test

Expected: The program structure should be perfectly preserved.

Actual: The formatter output is missing the program test declaration. Seriously?

Error: Should preserve program declaration at test_formatter_framework.f90:62. This error message pretty much sums it up.

This example highlights a critical flaw: the formatter is failing to maintain the most basic program structure.

Test 2: Complex Expression Formatting

This test digs into how the formatter handles more complex scenarios, specifically mathematical expressions.

Location: test_formatter_advanced.f90:59

Issue: Complex mathematical expressions are getting mangled during formatting. It's like the formatter is having a mathematical meltdown.

Error: Test failed at format_and_check (line 438). This indicates a more general failure in the formatting process.

This second test broadens the scope of the problem, suggesting that the formatter struggles not just with overall structure but also with specific code constructs.

Specific Formatter Issues

Let's break down the specific issues we're encountering. This will help us pinpoint the root causes and develop effective solutions. We need to get down to the brass tacks of what's causing these failures.

Issue 1: Missing Program Declaration

! Formatter check at test_formatter_framework.f90:61-63
if (index(formatted_code, "program test") == 0) then
    error stop "Should preserve program declaration"  ! ← FAILING HERE
end if

Analysis: The formatted output completely lacks the original program test declaration. This indicates a major problem in how the formatter handles program structure. There are several possible culprits:

  1. Stripping program declarations entirely: The formatter might be intentionally removing these declarations, which is obviously not the desired behavior.
  2. Renaming them incorrectly: Perhaps the formatter is trying to rename the declarations but doing it wrong, leading to their disappearance.
  3. Not emitting them in the output: The formatter might be processing the declarations correctly but failing to include them in the final output.

This issue strikes at the heart of Fortran program structure. Without proper program declarations, the code simply won't work.

Issue 2: Missing Implicit None

! Formatter check at test_formatter_framework.f90:65-67
if (index(formatted_code, "implicit none") == 0) then
    error stop "Should preserve implicit none"
end if

Analysis: Just like the program test declaration, implicit none statements are vanishing during formatting. This is another critical issue, as implicit none is crucial for maintaining code clarity and preventing implicit typing errors in Fortran.

The disappearance of implicit none suggests a pattern: the formatter is struggling with essential Fortran declarations and constructs. This goes beyond mere formatting preferences; it's altering the fundamental meaning of the code.

Issue 3: Complex Expression Mangling

Location: test_formatter_advanced.f90

Test: "Long expression breaking" and nested expressions

Issue: Complex mathematical expressions are not being formatted correctly. This means the formatter is garbling the logic of the code, making it difficult to read and potentially introducing errors.

This issue demonstrates that the formatter's problems extend beyond basic program structure. It's also struggling with the intricacies of Fortran syntax, particularly when it comes to complex expressions.

Root Cause Analysis

Let's try to diagnose the underlying causes of these issues. By understanding the patterns and potential problems, we can develop targeted solutions.

Pattern: The formatter appears to be:

  1. Parsing the AST correctly (no parse errors): This is good news! It means the formatter can understand the basic structure of the code.
  2. Losing essential structure during code generation: This is where things go wrong. The formatter seems to be forgetting or misinterpreting key elements during the code generation phase.
  3. Not preserving fundamental Fortran constructs: This is a direct consequence of the previous point. Essential elements like program declarations and implicit none are being lost in translation.

Likely Issues:

  • Code generation phase not emitting program declarations: The code generator might simply be skipping these declarations, leading to their absence in the output.
  • AST → source transformation losing structural elements: The process of converting the Abstract Syntax Tree (AST) back into source code might be flawed, causing the loss of structural information.
  • Formatter configuration stripping essential statements: It's possible that the formatter is configured to remove certain statements, although this seems unlikely given the severity of the issue.
  • Standardization process removing or renaming constructs: A more subtle issue might be that the formatter's standardization process is inadvertently altering or removing constructs.

Impact Assessment

The impact of these bugs is severe. This isn't just a matter of code aesthetics; it's a matter of code functionality.

  • Makes formatter unusable for real code: The formatter, in its current state, is simply not reliable for formatting real-world Fortran code.
  • Breaks basic Fortran program structure: As we've seen, the formatter is failing to maintain the fundamental structure of Fortran programs.
  • Prevents roundtrip formatting (format → format → same result): A good formatter should produce the same output when run multiple times on the same code. This formatter is failing this basic test.
  • Critical blocker for production use: These issues are a major obstacle to using the formatter in a production environment.

Test Environment Context

It's important to understand the context in which these failures are occurring. This helps us narrow down the potential causes and replicate the issues.

These failures occur in:

  • test_formatter_framework.f90 (basic structure tests)
  • test_formatter_advanced.f90 (complex expression tests)
  • Both during standard formatting operations using transform_lazy_fortran_string_with_format

This indicates that the problems are widespread and not limited to specific code constructs or test cases.

Expected Behavior

Let's be clear about what we expect from a Fortran formatter. It's not just about making the code look pretty; it's about maintaining its integrity.

Formatting should be structure-preserving:

  • Program declarations should remain intact: This is non-negotiable.
  • Implicit none statements should be preserved: Another essential requirement.
  • Variable declarations should be maintained: The formatter shouldn't be altering variable declarations.
  • Only whitespace/indentation should change: The core logic of the code should remain untouched.

Anything less than this is a failure.

Environment

Here's the environment in which these issues are occurring:

  • fortfront commit: 5b3f9a1 (latest)
  • Operation: transform_lazy_fortran_string_with_format
  • Platform: Linux

This information helps ensure that we're all on the same page and can reproduce the issues consistently.

Request

This is a critical formatter bug that makes fortfront unusable for basic Fortran code. We need to get this fixed ASAP. The request is simple:

Please investigate why the code generation phase is not preserving essential program structure elements. This is a top priority, and we need to get to the bottom of it.

Let's work together to get this sorted out and make fortfront a reliable tool for Fortran development!