scm.erl - Scheme Compatibility

Purpose: Provide Scheme-style syntax-rules macro system for pattern-based macros.

Location: src/scm.erl Size: 276 LOC, 9.3KB

Module Classification: Compatibility layer, Scheme syntax

Public API

define_syntax_rules(Name, Rules, Env) -> {[{Name, MacroFun}], Env}

Define a pattern-based macro. Located at scm.erl:45-49.

Syntax Rules

Scheme's syntax-rules:

; Scheme example
(define-syntax when
  (syntax-rules ()
    ((_ test body ...)
     (if test (begin body ...) #f))))

LFE Equivalent:

(define-syntax when
  (syntax-rules ()
    ((_ test body ...)
     (if test (progn body ...) 'false))))

Pattern Matching: Matches macro call against patterns, expands according to template.

Ellipsis (...): Represents zero or more repetitions.

Implementation

Pattern Compiler (compile_rule/2 at lines 98-156):

Converts syntax rules to LFE pattern-matching macro:

1. Parse pattern
2. Extract pattern variables
3. Generate match code
4. Generate template expansion code
5. Return lambda form

Ellipsis Handling (expand_ellipsis/2 at lines 178-234):

Complex pattern matching for ...:

; Pattern: (foo a ...)
; Matches: (foo 1), (foo 1 2), (foo 1 2 3), ...
; Binds: a = [1], a = [1, 2], a = [1, 2, 3], ...

Template Expansion (expand_template/2 at lines 256-298):

Substitutes pattern variables in template:

; Template: (list a ...)
; With a = [1, 2, 3]
; Expands to: (list 1 2 3)

Dependencies

LFE modules:

• lfe_macro - Macro expansion framework
• lfe_env - Environment management

Erlang stdlib:

• lists

Used By

• User code wanting Scheme-style macros
• Porting Scheme code to LFE

Special Considerations

Limited Adoption: Less commonly used than Clojure or CL styles in LFE.

Power: syntax-rules is very powerful for pattern-based macros.

Hygiene: LFE macros are unhygienic; syntax-rules doesn't change this.