# DEF FN Implementation - MBASIC 5.21 Compiler

## Summary

Successfully implemented DEF FN (user-defined functions) for the MBASIC 5.21 parser, eliminating all 17 "DEF FN parsing not yet implemented" errors and reducing parser exceptions from 17 to 0.

## Implementation Date

2025-10-22

## Syntax Implemented

### DEF FN Statement
**Syntax**: `DEF FNname[(param1, param2, ...)] = expression`

**Key Features**:
- Function name must start with "FN" (e.g., FNR, FNA$, FNB)
- Parameters are optional
- Function body is a single expression
- Type suffix on function name determines return type ($, %, !, #)
- Parameters can have type suffixes

**Examples**:
```basic
DEF FNR(X) = INT(X*100+.5)/100          ' Numeric function
DEF FNA$(U,V) = P1$+CHR$(31+U*2)        ' String function
DEF FNB = 42                             ' No parameters
```

### Function Calls
User-defined functions are called using FN prefix:
```basic
Y = FNR(3.14159)
Z$ = FNA$(1, 2)
W = FNB
```

## Implementation Details

### AST Node
Used existing `DefFnStatementNode` class (ast_nodes.py:315-322):
```python
@dataclass
class DefFnStatementNode:
    """DEF FN statement - define single-line function"""
    name: str                           # Function name (e.g., "FNR", "FNA$")
    parameters: List[VariableNode]      # Parameter variables
    expression: ExpressionNode          # Function body
    line_num: int = 0
    column: int = 0
```

### Parser Implementation
**Location**: parser.py:1397-1467

**Key Parsing Logic**:
1. Consume DEF token
2. Parse function name (identifier starting with "FN" or FN keyword + identifier)
3. Parse optional parameter list in parentheses
4. Expect = sign
5. Parse expression as function body

**Special Handling**:
- Handles both "DEF FNR" (no space) and "DEF FN R" (with space)
- Creates VariableNode for each parameter (with type suffix)
- Function calls already handled by existing FunctionCallNode

### Code Changes

**parser.py**:
```python
def parse_deffn(self) -> DefFnStatementNode:
    token = self.advance()  # Consume DEF

    # Parse function name (handles "FNR" or "FN R")
    fn_name_token = self.current()
    if fn_name_token.type == TokenType.FN:
        self.advance()
        fn_name_token = self.expect(TokenType.IDENTIFIER)
        function_name = "FN" + fn_name_token.value
    elif fn_name_token.type == TokenType.IDENTIFIER:
        if not fn_name_token.value.startswith("FN"):
            raise ParseError("DEF function name must start with FN")
        self.advance()
        function_name = fn_name_token.value

    # Parse parameters (optional)
    parameters = []
    if self.match(TokenType.LPAREN):
        # ... parse parameter list as VariableNodes

    # Parse = and expression
    self.expect(TokenType.EQUAL)
    expression = self.parse_expression()

    return DefFnStatementNode(...)
```

## Test Results

### Before Implementation
- Total parser failures: 189 (after file I/O)
- DEF FN "not yet implemented" errors: **17 files**
- Parser exceptions: **17 files**
- Success rate: 29 files (7.8%)

### After Implementation
- Total parser failures: 205 (**16 more failing**, but with parse errors not exceptions)
- DEF FN "not yet implemented" errors: **0 files** ✓
- Parser exceptions: **0 files** ✓ (all eliminated)
- Success rate: **30 files (8.0%)** ✓ (+1 file)

### Impact Analysis

**Success**:
- ✓ All 17 "DEF FN not yet implemented" errors eliminated
- ✓ All 17 parser exceptions eliminated (no more NotImplementedError)
- ✓ 1 additional file now parses successfully
- ✓ Files with DEF FN now progress further in parsing

**Files Now Progressing**:
The 17 files that were failing with "DEF FN not yet implemented" now either:
1. Parse successfully (1 file)
2. Fail with different parse errors (16 files) - these have other syntax issues

This is actually positive progress - we've eliminated a blocker and exposed the real syntax issues in these files.

## Test Case

```basic
10 DEF FNR(X) = INT(X*100+.5)/100
20 DEF FNA$(U,V) = P1$+CHR$(31+U*2)+CHR$(48+V*5)
30 DEF FNB = 42
40 Y = FNR(3.14159)
50 Z$ = FNA$(1, 2)
60 W = FNB
70 END
```

**Result**: ✓ All statements parse successfully

## Real-World Examples

From BATTLE.bas:
```basic
320 DEF FN A$(U,V)=P1$+CHR$(31+U*2)+CHR$(48+V*5)+P3$+CHR$(32+U*2)+CHR$(48+V*5)+P4$
330 DEF FN A1$(U)=P1$+CHR$(32+U*2)+"2"+C$(U)+E2$
340 DEF FN A2$(V)=P1$+CHR$(32)+CHR$(50+V*5)+N$(V)+E2$
...
890 B$(I,J)=FN A$(I,J)
910 B$(I,0)=FN A1$(I)
```

From finance.bas:
```basic
20 DEF FNR(X)=INT(X*100+.5)/100
...
520 DEF FNR(X)=INT(X*100+.5)/100
```

## Remaining Top Issues

After DEF FN implementation, most common parser failures:

1. **"Expected LPAREN, got COLON"** (9 files) - Array/function disambiguation
2. **BACKSLASH** (10+ files) - Line continuation not implemented
3. **Multi-statement line parsing** (15+ files) - LPAREN, APOSTROPHE issues
4. **CALL statement** (~5 files) - Machine language calls not implemented
5. **RANDOMIZE** (~3 files) - RNG initialization not implemented

## Files Modified

1. **parser.py** - Implemented parse_deffn() (70 lines)
2. **ast_nodes.py** - Removed duplicate DefFnStatementNode (already existed)

## Key Insights

1. **FN is part of function name**: In MBASIC, "DEF FNR(X)" means the function is named "FNR", not "R"
2. **Lexer tokenization**: "FNR" is tokenized as a single IDENTIFIER, not FN + R
3. **Parser flexibility**: Parser handles both "DEF FNR" and "DEF FN R" syntaxes
4. **Parameters as VariableNodes**: Parameters are stored as VariableNode objects with type information
5. **Exception elimination**: Converting NotImplementedError to actual parsing exposed real syntax issues

## Next Priority

To continue improving success rate:

1. **Multi-statement line parsing** (~20 files) - Would have biggest impact
2. **BACKSLASH line continuation** (~10 files) - Medium difficulty
3. **CALL statement** (~5 files) - Low priority (assembly interface)
4. **RANDOMIZE statement** (~3 files) - Easy to implement

## Conclusion

The DEF FN implementation successfully adds user-defined function support to the MBASIC compiler. While the overall success rate only increased by 1 file (7.8% → 8.0%), the implementation:

1. **Eliminated all "not yet implemented" exceptions** - cleaner error reporting
2. **Exposed real parsing issues** in 16 files that were blocked by DEF FN
3. **Added critical language feature** - DEF FN is used in ~17 files in the corpus
4. **Improved parser robustness** - handles flexible syntax (with/without spaces)

The modest success rate increase is expected - files using DEF FN often have other complex syntax issues. The important achievement is removing DEF FN as a blocker and enabling further progress on these files.
