lfe_io_format.erl - Formatted Output

Purpose: Printf-style formatted output for LFE with support for LFE-specific format directives.

Location: src/lfe_io_format.erl Size: 494 LOC, 17KB

Module Classification: I/O support, formatted output

Public API

fwrite1(Format, Args) -> [char()]

Format string with arguments. Located at lfe_io_format.erl:52-56.

Format Directives

From Erlang's io:format:

~w  - Write term
~p  - Pretty print term
~s  - String
~c  - Character
~d  - Decimal integer
~f  - Float
~e  - Exponential float
~g  - General float
~b  - Integer in arbitrary base
~x  - Hexadecimal
~n  - Newline
~~  - Literal tilde

LFE Extensions:

~a  - Write atom (without quotes)
~l  - Write list (without parens)

Modifiers:

~Nw    - Width N
~N.Mf  - Width N, precision M
~-Nw   - Left-align in width N
~0Nd   - Pad with zeros

Examples

(lfe_io:format "Hello ~s!~n" '("world"))
→ "Hello world!\n"

(lfe_io:format "Number: ~d, Float: ~.2f~n" '(42 3.14159))
→ "Number: 42, Float: 3.14\n"

(lfe_io:format "~w ~p~n" '((a b c) (x y z)))
→ "(a b c) (x y z)\n"

(lfe_io:format "Atom: ~a, List: ~l~n" '(foo (1 2 3)))
→ "Atom: foo, List: 1 2 3\n"

Implementation

Parser (parse_format/1 at lines 89-156):

Parses format string into directive list:

parse_format("~d ~s") →
    [{int, [], []}, {string, " "}, {string, [], []}]

Formatter (format_directive/2 at lines 178-312):

Applies each directive:

format_directive({int, Width, Prec}, [Arg|Args]) ->
    S = integer_to_list(Arg),
    Padded = pad_string(S, Width, Prec),
    {Padded, Args}.

LFE-Specific (write_atom/1 at lines 387-401, write_list/1 at lines 403-419):

Special handling for LFE atoms and lists.

Dependencies

• lfe_io_write - Term writing
• lists, io_lib, string (Erlang stdlib)

Used By

• lfe_io - Via format1/2
• LFE user code - Formatted output

Special Considerations

Error Handling: Throws badarg if format/args mismatch.

Performance: Parsing format string has overhead; cache results for repeated formats.

Compatibility: Most Erlang ~ directives work identically.