lfe_io_pretty.erl - Pretty Printer

Purpose: Write LFE terms in formatted, indented, multi-line layout for readability.

Location: src/lfe_io_pretty.erl Size: 405 LOC, 15KB

Module Classification: I/O support, pretty formatting

Public API

term(Term) -> [char()]
term(Term, Depth) -> [char()]
term(Term, Depth, Indentation, LineLength) -> [char()]

Pretty print term with specified formatting parameters. Located at lfe_io_pretty.erl:47-49.

Formatting Strategy

Layout Modes:

1. Horizontal - Single line: (a b c)

2. Vertical - Multi-line with indentation:

   (defun foo (x)
     (let ((y (* x 2)))
       (+ y 1)))
   

Line Length Tracking:

The pretty printer tries to fit output within LineLength (default 80) by:

1. Attempting horizontal layout
2. Falling back to vertical if too long
3. Intelligent indentation based on form type

Special Form Formatting (format_form/4 at lines 178-267):

Different forms get custom layouts:

; Function definitions - indent body
(defun name (args)
  body)

; Let bindings - align bindings
(let ((x 1)
      (y 2))
  body)

; Conditionals - indent branches
(if test
    then-branch
    else-branch)

; Case - indent clauses
(case expr
  (pattern1 result1)
  (pattern2 result2))

Indentation Rules

By Form Type:

• defun, defmacro - Indent 2 after function head
• let, let*, letrec - Align binding pairs
• if, cond - Indent branches
• lambda - Indent body
• Default lists - Indent 1 from opening paren

Column Tracking (write_tail/4 at lines 298-356):

Maintains current column position to:

• Decide horizontal vs vertical layout
• Calculate indentation
• Enforce line length limits

Dependencies

• lists, io_lib, string (Erlang stdlib)

Used By

• lfe_io - Via prettyprint1/*
• lfe_shell - Value display
• lfe_error - Error formatting

Special Considerations

Performance: Slower than compact writing due to layout calculations.

Configurable: LineLength and initial indentation customizable.

Readable Output: Prioritizes human readability over machine parsing.