User-Defined Functions in XMD

XMD now supports user-defined functions with shell-style syntax, following the design principles of mobile-first, minimal punctuation programming.

Overview

User-defined functions in XMD use a natural, shell-inspired syntax that avoids traditional programming punctuation like parentheses in function definitions. This makes XMD more accessible and easier to type on mobile devices.

Function Definition Syntax

Functions are defined using the function keyword followed by the function name and parameters:

Key Features:

  • Shell-style syntax: No parentheses around parameters
  • Natural language: Parameters separated by spaces
  • Mobile-friendly: Minimal punctuation required
  • Indented bodies: Function body follows indentation-based structure (future enhancement)

Function Call Syntax

Functions can be called using shell-style syntax within XMD directives:

The system automatically transforms these shell-style calls to proper function calls internally.

Examples

Simple Arithmetic Function




The double of 5 is:

Multi-Parameter Function







Calculate:  +  =

String Processing Function

Function Scope

Functions create their own local scope:

  • Parameters shadow global variables of the same name
  • Function parameters are only accessible within the function
  • Global variables remain unchanged after function execution





Global x: global
Function result: 
Global x after: global

Output:

Global x: global
Function result: local: parameter
Global x after: global

Return Statements

Functions can return values using the return keyword:

If no return statement is provided, functions return null.

Integration with Built-in Functions

User-defined functions work seamlessly with XMD’s built-in functions:





Numbers:

Current Limitations

  1. Indented Bodies: Currently functions must be defined with explicit return statements. Future versions will support indented function bodies.

  2. Recursion: Recursive function calls are supported but should be used carefully to avoid infinite loops.

  3. Closure Support: Functions currently have limited closure support - they can access parameters but not outer scope variables.

Implementation Details

The user-defined functions feature is built on XMD’s AST (Abstract Syntax Tree) system:

  1. Function Registration: Functions are stored in the evaluator context
  2. Shell Transformation: Shell-style calls are transformed to AST-compatible syntax
  3. Scope Management: Local scopes are created for parameter binding
  4. Return Handling: Return statements immediately exit function execution

Testing

The implementation includes comprehensive tests:

  • Basic function infrastructure tests
  • Shell-style transformation tests
  • Function evaluation and scope tests
  • Multi-parameter function tests

Run tests with:

make test_user_functions_basic
./test_shell_transformation

Future Enhancements

Planned improvements include:

  1. Indented Function Bodies: Support for multi-line, indented function definitions
  2. Enhanced Closure Support: Access to outer scope variables
  3. Function Composition: Better support for function composition and chaining
  4. Documentation Integration: Automatic function documentation generation

Design Philosophy

User-defined functions in XMD follow the core design principles:

  • Shell-style: Familiar to users of command-line interfaces
  • Mobile-first: Easy to type on mobile keyboards
  • Minimal punctuation: Reduces cognitive load
  • Natural language: Reads like English
  • Progressive enhancement: Works with existing XMD features

This implementation provides a solid foundation for user-defined functions while maintaining XMD’s commitment to simplicity and accessibility.