Mapl Error Handling

What I Do

Provide comprehensive guidance on MAPL's error handling system, including:

• Error handling macros and their usage
• Return code conventions
• Best practices for robust error handling
• Complete code examples
• Legacy patterns to avoid

When to Use Me

Use this skill when:

• Writing new Fortran procedures for MAPL
• Adding error handling to existing code
• Debugging error handling issues
• Code review (checking error handling compliance)
• Converting legacy error handling patterns

Required Include

All modules using MAPL error handling must include:

#include "MAPL_Generic.h"

module MyModule
   use MAPL
   implicit none
   private
   
   ! Your code here
   
end module MyModule

This provides access to all MAPL error handling macros.

Return Code Conventions

Standard convention:

• 0 = Success (use _SUCCESS macro)
• Non-zero = Failure (various error codes)

MAPL uses ESMF return code constants:

• ESMF_SUCCESS = 0
• ESMF_FAILURE = non-zero

MAPL macros:

• _SUCCESS → ESMF_SUCCESS (0)
• _FAILURE → ESMF_FAILURE (non-zero)

Core Error Handling Macros

_RC (Most Common)

Usage: Compact call + verify pattern

call some_procedure(arg1, arg2, _RC)

Expands to:

rc=status); _VERIFY(status)

What it does:

1. Passes status variable as rc argument
2. Checks if status is success
3. If failure: prints file/line, returns with rc set to status

Example:

subroutine process_data(input, output, rc)
   type(DataSet), intent(in) :: input
   type(DataSet), intent(out) :: output
   integer, optional, intent(out) :: rc
   
   integer :: status
   
   call read_configuration('config.yaml', _RC)
   call allocate_workspace(n_items, _RC)
   call transform_data(input, output, _RC)
   
   _RETURN(_SUCCESS)
end subroutine process_data

Benefits:

• Compact (one line for call + error check)
• Automatic file/line reporting
• Consistent error propagation

_VERIFY

Usage: Check return codes explicitly

call some_procedure(arg1, arg2, rc=status)
_VERIFY(status)

What it does:

• Checks if status == _SUCCESS
• If not: prints status, file, line number
• Returns from current procedure with rc=status

Use when:

• Need to separate call from verification
• Capturing return code for logging
• Custom logic between call and check

Example:

call read_file(filename, data, rc=status)
if (status /= _SUCCESS) then
   call log_error('Failed to read: ' // trim(filename))
end if
_VERIFY(status)

Usually _RC is preferred over this pattern.

_ASSERT

Usage: Check logical conditions

_ASSERT(condition, "Error message")

What it does:

• If condition is FALSE: prints message, file/line, returns with failure
• If condition is TRUE: continues execution

Examples:

_ASSERT(n_columns > 0, "Number of columns must be positive")
_ASSERT(allocated(workspace), "Workspace not allocated")
_ASSERT(size(input) == size(output), "Array size mismatch")

Use for:

• Precondition checking
• Invariant validation
• Argument validation
• State verification

Don't use for:

• Checking return codes (use _VERIFY or _RC)
• Conditions that should always use _FAIL instead

_FAIL

Usage: Force procedure to fail with message

_FAIL("Error message describing the problem")

What it does:

• Prints message, file, line
• Sets rc to _FAILURE
• Returns from procedure

Common use: SELECT CASE with invalid/unexpected values

Example:

select case(operation_type)
case(ADD)
   result = a + b
case(SUBTRACT)
   result = a - b
case(MULTIPLY)
   result = a * b
case default
   _FAIL("Unknown operation type")
end select

Also useful for:

• Error conditions without a logical test
• Explicit failure paths
• "This should never happen" scenarios

Note: If you find yourself doing _ASSERT(.FALSE., "message"), use _FAIL("message") instead - it's clearer.

_RETURN

Usage: Return from procedure with given status

_RETURN(status_value)

What it does:

• Sets rc to provided value (if rc present)
• Returns from procedure

Standard pattern at end of successful procedure:

subroutine my_procedure(arg, rc)
   integer, intent(in) :: arg
   integer, optional, intent(out) :: rc
   
   ! ... procedure implementation ...
   
   _RETURN(_SUCCESS)  ! Always end successful procedures this way
end subroutine my_procedure

Can return with any value:

if (early_exit_condition) then
   _RETURN(_SUCCESS)  ! Early successful return
end if

_SUCCESS

Usage: Represents successful return code (0)

_RETURN(_SUCCESS)

if (status == _SUCCESS) then
   ! Success path
end if

Expands to: ESMF_SUCCESS (which is 0)

Use instead of: Literal 0 in code

Benefits:

• Self-documenting
• Consistent with ESMF conventions
• Future-proof if convention ever changes

_FAILURE

Usage: Represents failure return code (non-zero)

if (status /= _SUCCESS) then
   ! This is a failure
end if

Expands to: ESMF_FAILURE

Note: Usually don't need this directly - macros handle it. Provided for completeness.

_STAT

Usage: Check Fortran intrinsic STAT codes

allocate(array(n), _STAT)
deallocate(array, _STAT)

Expands to:

stat=status); _VERIFY(status)

Examples:

subroutine allocate_workspace(n, rc)
   integer, intent(in) :: n
   integer, optional, intent(out) :: rc
   
   integer :: status
   real, allocatable :: workspace(:)
   
   allocate(workspace(n), _STAT)
   
   ! Use workspace
   
   deallocate(workspace, _STAT)
   
   _RETURN(_SUCCESS)
end subroutine

_IOSTAT

Usage: Check Fortran I/O IOSTAT codes

open(unit, file=filename, _IOSTAT)
read(unit, *, _IOSTAT) values
close(unit, _IOSTAT)

Expands to:

iostat=status); _VERIFY(status)

Example:

subroutine read_data(filename, values, rc)
   character(len=*), intent(in) :: filename
   real, intent(out) :: values(:)
   integer, optional, intent(out) :: rc
   
   integer :: status, unit
   
   open(newunit=unit, file=filename, status='old', _IOSTAT)
   read(unit, *, _IOSTAT) values
   close(unit, _IOSTAT)
   
   _RETURN(_SUCCESS)
end subroutine read_data

_HERE

Usage: Debug print with file and line information

_HERE, "debug message", variables

Expands to:

print*, __FILE__, __LINE__, "debug message", variables

Examples:

_HERE, "n_columns:", n_columns
_HERE, "Entering iteration:", iteration_count
_HERE, "Checkpoint A"

Use for:

• Debugging prints with location info
• Tracking execution flow
• Quick diagnostics

Note: Remember to remove or comment out after debugging!

Complete Example

#include "MAPL_Generic.h"

module MAPL_DataProcessor
   use ESMF
   use MAPL_BaseMod
   implicit none
   private
   
   public :: process_dataset
   
contains

   subroutine process_dataset(input_file, output_file, n_iterations, rc)
      character(len=*), intent(in) :: input_file
      character(len=*), intent(in) :: output_file
      integer, intent(in) :: n_iterations
      integer, optional, intent(out) :: rc
      
      integer :: status
      real, allocatable :: data(:)
      integer :: i, n_points
      
      ! Validate inputs
      _ASSERT(n_iterations > 0, "Iterations must be positive")
      _ASSERT(len_trim(input_file) > 0, "Input filename cannot be empty")
      
      ! Read number of points from file
      call read_file_size(input_file, n_points, _RC)
      
      ! Allocate workspace
      allocate(data(n_points), _STAT)
      
      ! Read data
      call read_data(input_file, data, _RC)
      
      ! Process
      do i = 1, n_iterations
         call apply_filter(data, _RC)
      end do
      
      ! Write results
      call write_data(output_file, data, _RC)
      
      ! Cleanup
      deallocate(data, _STAT)
      
      _RETURN(_SUCCESS)
   end subroutine process_dataset
   
   subroutine read_file_size(filename, n_points, rc)
      character(len=*), intent(in) :: filename
      integer, intent(out) :: n_points
      integer, optional, intent(out) :: rc
      
      integer :: status, unit
      
      open(newunit=unit, file=filename, status='old', _IOSTAT)
      read(unit, *, _IOSTAT) n_points
      close(unit, _IOSTAT)
      
      _ASSERT(n_points > 0, "File contains invalid point count")
      
      _RETURN(_SUCCESS)
   end subroutine read_file_size
   
   subroutine apply_filter(data, rc)
      real, intent(inout) :: data(:)
      integer, optional, intent(out) :: rc
      
      integer :: status, filter_type
      
      ! Get filter type from configuration
      call get_filter_type(filter_type, _RC)
      
      ! Apply appropriate filter
      select case(filter_type)
      case(1)
         call apply_gaussian_filter(data, _RC)
      case(2)
         call apply_median_filter(data, _RC)
      case(3)
         call apply_moving_average(data, _RC)
      case default
         _FAIL("Unknown filter type")
      end select
      
      _RETURN(_SUCCESS)
   end subroutine apply_filter

end module MAPL_DataProcessor

Best Practices

1. Always Check Return Codes

DO:

call some_procedure(args, _RC)

DON'T:

call some_procedure(args)  ! Ignoring potential errors!

Why: If something fails, you want to know immediately, not debug mysterious crashes later.

2. End Procedures with _RETURN(_SUCCESS)

DO:

subroutine my_proc(rc)
   integer, optional, intent(out) :: rc
   
   ! ... successful execution ...
   
   _RETURN(_SUCCESS)
end subroutine

DON'T:

subroutine my_proc(rc)
   integer, optional, intent(out) :: rc
   
   ! ... successful execution ...
   
   ! Missing return status!
end subroutine

3. Validate Inputs Early

subroutine process(n_items, tolerance, rc)
   integer, intent(in) :: n_items
   real, intent(in) :: tolerance
   integer, optional, intent(out) :: rc
   
   ! Validate first
   _ASSERT(n_items > 0, "n_items must be positive")
   _ASSERT(tolerance > 0.0, "tolerance must be positive")
   _ASSERT(tolerance < 1.0, "tolerance must be less than 1")
   
   ! Now proceed with validated inputs
   ! ...
   
   _RETURN(_SUCCESS)
end subroutine

4. Check All Allocations

allocate(workspace(n), _STAT)
allocate(temporary(m), _STAT)

Never:

allocate(workspace(n))  ! Unchecked!

5. Check All File Operations

open(newunit=unit, file=filename, _IOSTAT)
read(unit, *, _IOSTAT) data
close(unit, _IOSTAT)

6. Provide Informative Error Messages

Good:

_ASSERT(n_rows == n_cols, "Matrix must be square for this operation")
_FAIL("Configuration file missing required 'output_dir' parameter")

Poor:

_ASSERT(n_rows == n_cols, "Error")
_FAIL("Bad input")

Error Handling Patterns

Pattern: Cleanup on Error

subroutine process_with_cleanup(rc)
   integer, optional, intent(out) :: rc
   integer :: status
   real, allocatable :: temp(:)
   logical :: file_open
   
   file_open = .false.
   
   allocate(temp(100), _STAT)
   
   open(newunit=unit, file='data.txt', _IOSTAT)
   file_open = .true.
   
   call risky_operation(_RC)
   
   ! Cleanup
   if (file_open) close(unit)
   deallocate(temp)
   
   _RETURN(_SUCCESS)
end subroutine

Note: Error macros return immediately, so cleanup after error is tricky. Consider Fortran block scope or finalization for robust cleanup.

Pattern: Error Message Context

call read_configuration(config_file, config, rc=status)
if (status /= _SUCCESS) then
   call log_error("Failed reading config: " // trim(config_file))
end if
_VERIFY(status)

Pattern: Continue on Error (Rare)

! Unusual case: want to continue despite errors
call optional_operation(rc=status)
if (status /= _SUCCESS) then
   call log_warning("Optional operation failed, continuing")
   ! Don't call _VERIFY - continue execution
end if

Legacy Macros (DO NOT USE)

Old macros ending with underscore: VERIFY_, ASSERT_, __RC__

Problem: Required IAM character variable with procedure name

! OLD STYLE - Don't do this!
character(len=*), parameter :: IAM = 'my_procedure'
call something(args, __RC__)

Issues:

• Easily copied incorrectly (IAM doesn't match actual procedure)
• Manual maintenance burden
• Less informative than FILE and LINE

Migration:

! OLD
character(len=*), parameter :: IAM = 'my_procedure'
call foo(args, __RC__)

! NEW
call foo(args, _RC)

If you see these in code review: Request update to new macros

Common Mistakes

Forgetting Include

Error: Macros not defined Solution:

#include "MAPL_Generic.h"

Must come before module statement.

No RC Parameter

Error: Macros try to set rc but procedure doesn't have it Solution: Add optional rc parameter

subroutine my_proc(arg, rc)  ! Add rc parameter
   integer, intent(in) :: arg
   integer, optional, intent(out) :: rc

Using == for Logical

! WRONG - not standard conforming
if (flag == .true.) then

! CORRECT - but redundant
if (flag .eqv. .true.) then

! BEST - direct use
if (flag) then

Code Review Checklist

• #include "MAPL_Generic.h" present
• All procedures with potential errors have optional rc parameter
• All procedure calls with rc checked (use _RC)
• All allocations checked (use _STAT)
• All file I/O checked (use _IOSTAT)
• Procedures end with _RETURN(_SUCCESS)
• Assertions used for precondition validation
• Informative error messages provided
• No legacy macros (VERIFY_, __RC__, etc.)
• No unchecked operations

Related Skills

• fortran-style - General Fortran coding standards
• github-workflow - Code review process
• Error Handling Macros wiki - Detailed macro documentation

Summary

Key Points:

1. Include MAPL_Generic.h in all modules
2. Use _RC for compact call + check pattern
3. Use _ASSERT for precondition validation
4. Use _STAT and _IOSTAT for intrinsics
5. End successful procedures with _RETURN(_SUCCESS)
6. Always check return codes - no exceptions
7. Provide informative error messages
8. Avoid legacy macros (ending with _)

Philosophy: Error handling everywhere in calling chain. If something can fail, check it. Your future self (and teammates) will thank you during debugging.