chickadee » fmt




A library of procedures for formatting Scheme objects to text in various ways, and for easily concatenating, composing and extending these formatters efficiently without resorting to capturing and manipulating intermediate strings.

Chicken eggs are provided for formatting of arbitrary objects (fmt), Unicode (fmt-unicode), C code (fmt-c) and ANSI color output (fmt-color).

This is a copy of the canonical documentation (off-site).


There are several approaches to text formatting. Building strings to display is not acceptable, since it doesn't scale to very large output. The simplest realistic idea, and what people resort to in typical portable Scheme, is to interleave display and write and manual loops, but this is both extremely verbose and doesn't compose well. A simple concept such as padding space can't be achieved directly without somehow capturing intermediate output.

The traditional approach is to use templates - typically strings, though in theory any object could be used and indeed Emacs' mode-line format templates allow arbitrary sexps. Templates can use either escape sequences (as in C's printf and CL's format) or pattern matching (as in Visual Basic's Format, Perl6's form, and SQL date formats). The primary disadvantage of templates is the relative difficulty (usually impossibility) of extending them, their opaqueness, and the unreadability that arises with complex formats. Templates are not without their advantages, but they are already addressed by other libraries such as SRFI-28 and SRFI-48.

This library takes a combinator approach. Formats are nested chains of closures, which are called to produce their output as needed. The primary goal of this library is to have, first and foremost, a maximally expressive and extensible formatting library. The next most important goal is scalability - to be able to handle arbitrarily large output and not build intermediate results except where necessary. The third goal is brevity and ease of use.


The primary interface is the fmt procedure:

fmt <output-dest> <format> ...procedure

where <output-dest> has the same semantics as with format - specifically it can be an output-port, #t to indicate the current output port, or #f to accumulate output into a string.

Each <format> should be a format closure as discussed below. As a convenience, non-procedure arguments are also allowed and are formatted similar to display, so that

(fmt #f "Result: " res nl)

would return the string "Result: 42n", assuming res is bound to 42.

nl is the newline format combinator.


The procedure names have gone through several variations, and I'm still open to new suggestions. The current approach is to use abbreviated forms of standard output procedures when defining an equivalent format combinator (thus display becomes dsp and write becomes wrt), and to use an fmt- prefix for utilities and less common combinators. Variants of the same formatter get a /<variant> suffix.

Formatting Objects

dsp <obj>procedure

Outputs <obj> using display semantics. Specifically, strings are output without surrounding quotes or escaping and characters are written as if by write-char. Other objects are written as with write (including nested strings and chars inside <obj>). This is the default behavior for top-level formats in fmt, cat and most other higher-order combinators.

wrt <obj>procedure

Outputs <obj> using write semantics. Handles shared structures as in SRFI-38.

wrt/unshared <obj>procedure

As above, but doesn't handle shared structures. Infinite loops can still be avoided if used inside a combinator that truncates data (see trim and fit below).

pretty <obj>procedure

Pretty-prints <obj>. Also handles shared structures. Unlike many other pretty printers, vectors and data lists (lists that don't begin with a (nested) symbol), are printed in tabular format when there's room, greatly saving vertical space.

pretty/unshared <obj>procedure

As above but without sharing.

(slashified <str> [<quote-ch> <esc-ch> <renamer>])procedure

Outputs the string <str>, escaping any quote or escape characters. If <esc-ch> is #f escapes only the <quote-ch> by doubling it, as in SQL strings and CSV values. If <renamer> is provided, it should be a procedure of one character which maps that character to its escape value, e.g. #\newline => #\n, or #f if there is no escape value.

(fmt #f (slashified "hi, "bob!""))

=> "hi, "bob!""
(maybe-slashified <str> <pred> [<quote-ch> <esc-ch> <renamer>])procedure

Like slashified, but first checks if any quoting is required (by the existence of either any quote or escape characters, or any character matching <pred>), and if so outputs the string in quotes and with escapes. Otherwise outputs the string as is.

(fmt #f (maybe-slashified "foo" char-whitespace? #\"))

=> "foo"

(fmt #f (maybe-slashified "foo bar" char-whitespace? #\"))

=> ""foo bar""

(fmt #f (maybe-slashified "foo"bar"baz" char-whitespace? #\"))

=> ""foo"bar"baz""

Formatting Numbers

(num <n> [<radix> <precision> <sign> <comma> <comma-sep> <decimal-sep>])procedure

Formats a single number <n>. You can optionally specify any <radix> from 2 to 36 (even if <n> isn't an integer). <precision> forces a fixed-point format.

A <sign> of #t indicates to output a plus sign (+) for positive integers. However, if <sign> is a character, it means to wrap the number with that character and its mirror opposite if the number is negative. For example, #\( prints negative numbers in parenthesis, financial style: -3.14 => (3.14).

<comma> is an integer specifying the number of digits between commas. Variable length, as in subcontinental-style, is not yet supported.

<comma-sep> is the character to use for commas, defaulting to #\,.

<decimal-sep> is the character to use for decimals, defaulting to #\., or to #\, (European style) if <comma-sep> is already #\..

These parameters may seem unwieldy, but they can also take their defaults from state variables, described below.

(num/comma <n> [<base> <precision> <sign>])procedure

Shortcut for num to print with commas.

(fmt #f (num/comma 1234567))

=> "1,234,567"
(num/si <n> [<base> <suffix>])procedure

Abbreviates <n> with an SI suffix as in the -h or --si option to many GNU commands. The base defaults to 1024, using suffix names like Ki, Mi, Gi, etc. Other bases (e.g. the standard 1000) have the suffixes k, M, G, etc.

The <suffix> argument is appended only if an abbreviation is used.

(fmt #f (num/si 608))

=> "608"

(fmt #f (num/si 3986))

=> "3.9Ki"

(fmt #f (num/si 3986 1000 "B"))

=> "4kB"


num/fit <width> <n> #!rest <ARGS>procedure

Like num, but if the result doesn't fit in <width>, output instead a string of hashes (with the current <precision>) rather than showing an incorrectly truncated number. For example

(fmt #f (fix 2 (num/fit 4 12.345))) => "#.##"
num/roman <n>procedure

Formats the number as a Roman numeral:

(fmt #f (num/roman 1989)) => "MCMLXXXIX"
num/old-roman <n>procedure

Formats the number as an old-style Roman numeral, without the subtraction abbreviation rule:

(fmt #f (num/old-roman 1989)) => "MDCCCCLXXXVIIII"

Formatting Space


Outputs a newline.


Short for "fresh line," outputs a newline only if we're not already at the start of a line.

space-to <column>procedure

Outputs spaces up to the given <column>. If the current column is already >= <column>, does nothing.

tab-to #!optional <tab-width>procedure

Outputs spaces up to the next tab stop, using tab stops of width <tab-width>, which defaults to 8. If already on a tab stop, does nothing. If you want to ensure you always tab at least one space, you can use (cat " " (tab-to width)).


Outputs nothing (useful in combinators and as a default noop in conditionals).


cat <format> ...procedure

Concatenates the output of each <format>.

apply-cat <list>procedure

Equivalent to (apply cat <list>) but may be more efficient.

fmt-join <formatter> <list> #!optional <sep>procedure

Formats each element <elt> of <list> with (<formatter> <elt>), inserting <sep> in between. <sep> defaults to the empty string, but can be any format.

(fmt #f (fmt-join dsp '(a b c) ", "))

=> "a, b, c"
fmt-join/prefix, fmt-join-suffix
fmt-join/prefix <formatter> <list> #!optional <sep>procedure
fmt-join/suffix <formatter> <list> #!optional <sep>procedure
(fmt #f (fmt-join/prefix dsp '(usr local bin) "/"))

=> "/usr/local/bin"

As fmt-join, but inserts <sep> before/after every element.

fmt-join/last <formatter> <last-formatter> <list> #!optional <sep>procedure

As fmt-join, but the last element of the list is formatted with <last-formatter> instead.

fmt-join/dot <formatter> <dot-formatter> <list> #!optional <sep>procedure

As fmt-join, but if the list is a dotted list, then formats the dotted value with <dot-formatter> instead.

Padding and Trimming

pad, pad/left, pad/both
pad <width> <format> ...procedure
pad/left <width> <format> ...procedure
pad/both <width> <format> ...procedure

Analogs of SRFI-13 string-pad, these add extra space to the left, right or both sides of the output generated by the <format>s to pad it to <width>. If <width> is exceeded has no effect. pad/both will include an extra space on the right side of the output if the difference is odd.

pad does not accumulate any intermediate data.

Note these are column-oriented padders, so won't necessarily work with multi-line output (padding doesn't seem a likely operation for multi-line output).

trim, trim/left, trim/both
trim <width> <format> ...procedure
trim/left <width> <format> ...procedure
trim/both <width> <format> ...procedure

Analogs of SRFI-13 string-trim, truncates the output of the <format>s to force it in under <width> columns. As soon as any of the <format>s exceed <width>, stop formatting and truncate the result, returning control to whoever called trim. If <width> is not exceeded has no effect.

If a truncation ellipse is set (e.g. with the ellipses procedure below), then when any truncation occurs trim and trim/left will append and prepend the ellipse, respectively. trim/both will both prepend and append. The length of the ellipse will be considered when truncating the original string, so that the total width will never be longer than <width>.

(fmt #f (ellipses "..." (trim 5 "abcde")))

=> "abcde"

(fmt #f (ellipses "..." (trim 5 "abcdef")))

=> "ab..."
trim/length <width> <format> ...procedure

A variant of trim which acts on the actual character count rather than columns, useful for truncating potentially cyclic data.

fit, fit/left, fit/both
fit <width> <format> ...procedure
fit/left <width> <format> ...procedure
fit/both <width> <format> ...procedure

A combination of pad and trunc, ensures the output width is exactly <width>, truncating if it goes over and padding if it goes under.

Format Variables

You may have noticed many of the formatters are aware of the current column. This is because each combinator is actually a procedure of one argument, the current format state, which holds basic information such as the row, column, and any other information that a format combinator may want to keep track of. The basic interface is:

fmt-let, fmt-bind
fmt-let <name> <value> <format> ...procedure
fmt-bind <name> <value> <format> ...procedure

fmt-let sets the name for the duration of the <format>s, and restores it on return. fmt-bind sets it without restoring it.

A convenience control structure can be useful in combination with these states:

fmt-if <pred> <pass> #!optional <fail>procedure

<pred> takes one argument (the format state) and returns a boolean result. If true, the <pass> format is applied to the state, otherwise <fail> (defaulting to the identity) is applied.

Many of the previously mentioned combinators have behavior which can be altered with state variables. Although fmt-let and fmt-bind could be used, these common variables have shortcuts:

radix, fix
radix <k> <format> ...procedure
fix <k> <format> ...procedure

These alter the radix and fixed point precision of numbers output with dsp, wrt, pretty or num. These settings apply recursively to all output data structures, so that

(fmt #f (radix 16 '(70 80 90)))

will return the string "(#x46 #x50 #x5a)". Note that read/write invariance is essential, so for dsp, wrt and pretty the radix prefix is always included when not decimal. Use num if you want to format numbers in alternate bases without this prefix. For example,

(fmt #f (radix 16 "(" (fmt-join num '(70 80 90) " ") ")"))

would return "(46 50 5a)", the same output as above without the "#x" radix prefix.

Note that fixed point formatting supports arbitrary precision in implementations with exact non-integral rationals. When trying to print inexact numbers more than the machine precision you will typically get results like

(fmt #f (fix 30 #i2/3))

=> "0.666666666666666600000000000000"

but with an exact rational it will give you as many digits as you request:

(fmt #f (fix 30 2/3))

=> "0.666666666666666666666666666667"
decimal-align <k> <format> ...procedure

Specifies an alignment for the decimal place when formatting numbers, useful for outputting tables of numbers.

  (define (print-angles x)
     (fmt-join num (list x (sin x) (cos x) (tan x)) " "))

  (fmt #t (decimal-align 5 (fix 3 (fmt-join/suffix print-angles (iota 5) nl))))

would output

  0.000    0.000    1.000    0.000
  1.000    0.842    0.540    1.557
  2.000    0.909   -0.416   -2.185
  3.000    0.141   -0.990   -0.142
  4.000   -0.757   -0.654    1.158
comma-char, decimal-char
comma-char <k> <format> ...procedure
decimal-char <k> <format> ...procedure

comma-char and decimal-char set the defaults for number formatting.

pad-char <k> <format> ...procedure

The pad-char sets the character used by space-to, tab-to, pad/*, and fit/*, and defaults to #\space.

  (define (print-table-of-contents alist)
    (define (print-line x)
      (cat (car x) (space-to 72) (pad/left 3 (cdr x))))
    (fmt #t (pad-char #\. (fmt-join/suffix print-line alist nl))))

   '(("An Unexpected Party" . 29)
     ("Roast Mutton" . 60)
     ("A Short Rest" . 87)
     ("Over Hill and Under Hill" . 100)
     ("Riddles in the Dark" . 115)))

would output

 An Unexpected Party.....................................................29
 Roast Mutton............................................................60
 A Short Rest............................................................87
 Over Hill and Under Hill...............................................100
 Riddles in the Dark....................................................115
ellipse <ell> <format> ...procedure

Sets the truncation ellipse to <ell>, would should be a string or character.

with-width <width> <format> ...procedure

Sets the maximum column width used by some formatters. The default is 78.

Columnar Formatting

Although tab-to, space-to and padding can be used to manually align columns to produce table-like output, these can be awkward to use. The optional extensions in this section make this easier.

columnar <column> ...procedure

Formats each <column> side-by-side, i.e. as though each were formatted separately and then the individual lines concatenated together. The current column width is divided evenly among the columns, and all but the last column are right-padded. For example

(fmt #t (columnar (dsp "abc\ndef\n") (dsp "123\n456\n")))


    abc     123
    def     456

assuming a 16-char width (the left side gets half the width, or 8 spaces, and is left aligned). Note that we explicitly use DSP instead of the strings directly. This is because columnar treats raw strings as literals inserted into the given location on every line, to be used as borders, for example:

  (fmt #t (columnar "/* " (dsp "abc\ndef\n")
                    " | " (dsp "123\n456\n")
                    " */"))

would output

 /* abc | 123 */
 /* def | 456 */

You may also prefix any column with any of the symbols 'left, 'right or 'center to control the justification. The symbol 'infinite can be used to indicate the column generates an infinite stream of output.

You can further prefix any column with a width modifier. Any positive integer is treated as a fixed width, ignoring the available width. Any real number between 0 and 1 indicates a fraction of the available width (after subtracting out any fixed widths). Columns with unspecified width divide up the remaining width evenly.

Note that columnar builds its output incrementally, interleaving calls to the generators until each has produced a line, then concatenating that line together and outputting it. This is important because as noted above, some columns may produce an infinite stream of output, and in general you may want to format data larger than can fit into memory. Thus columnar would be suitable for line numbering a file of arbitrary size, or implementing the Unix yes(1) command, etc.

As an implementation detail, columnar uses first-class continuations to interleave the column output. The core fmt itself has no knowledge of or special support for columnar, which could complicate and potentially slow down simpler fmt operations. This is a testament to the power of call/cc - it can be used to implement coroutines or arbitrary control structures even where they were not planned for.

tabular <column> ...procedure

Equivalent to columnar except that each column is padded at least to the minimum width required on any of its lines. Thus

(fmt #t (tabular "|" (dsp "a\nbc\ndef\n") "|" (dsp "123\n45\n6\n") "|"))


|a  |123|
|bc |45 |
|def|6  |

This makes it easier to generate tables without knowing widths in advance. However, because it requires generating the entire output in advance to determine the correct column widths, tabular cannot format a table larger than would fit in memory.

fmt-columns <column> ...procedure

The low-level formatter on which columnar is based. Each <column> must be a list of 2-3 elements:

(<line-formatter> <line-generator> [<infinite?>])

where <line-generator> is the column generator as above, and the <line-formatter> is how each line is formatted. Raw concatenation of each line is performed, without any spacing or width adjustment. <infinite?>, if true, indicates this generator produces an infinite number of lines and termination should be determined without it.

wrap-lines <format> ...procedure

Behaves like cat, except text is accumulated and lines are optimally wrapped to fit in the current width as in the Unix fmt(1) command.

justify <format> ...procedure

Like wrap-lines except the lines are full-justified.

  (define func
    '(define (fold kons knil ls)
       (let lp ((ls ls) (acc knil))
         (if (null? ls) acc (lp (cdr ls) (kons (car ls) acc))))))

  (define doc
      "The fundamental list iterator.  Applies KONS to each element "
      "of LS and the result of the previous application, beginning "
      "with KNIL.  With KONS as CONS and KNIL as '(), equivalent to REVERSE."))

  (fmt #t (columnar (pretty func) " ; " (justify doc)))


 (define (fold kons knil ls)          ; The   fundamental   list   iterator.
   (let lp ((ls ls) (acc knil))       ; Applies  KONS  to  each  element  of
     (if (null? ls)                   ; LS  and  the  result of the previous
         acc                          ; application,  beginning  with  KNIL.
         (lp (cdr ls)                 ; With  KONS  as CONS and KNIL as '(),
             (kons (car ls) acc)))))  ; equivalent to REVERSE.
fmt-file <pathname>procedure

Simply displays the contents of the file <pathname> a line at a time, so that in typical formatters such as columnar only constant memory is consumed, making this suitable for formatting files of arbitrary size.

line-numbers #!optional <start>procedure

A convenience utility, just formats an infinite stream of numbers (in the current radix) beginning with <start>, which defaults to 1.

The Unix nl(1) utility could be implemented as:

  (fmt #t (columnar 6 'right 'infinite (line-numbers)
                    " " (fmt-file "read-line.scm")))
    2 (define (read-line . o)
    3   (let ((port (if (pair? o) (car o) (current-input-port))))
    4     (let lp ((res '()))
    5       (let ((c (read-char port)))
    6         (if (or (eof-object? c) (eqv? c #\newline))
    7             (list->string (reverse res))
    8             (lp (cons c res)))))))

C Formatting

C Formatting Basics

For purposes such as writing wrappers, code-generators, compilers or other language tools, people often need to generate or emit C code. Without a decent library framework it's difficult to maintain proper indentation. In addition, for the Scheme programmer it's tedious to work with all the context sensitivities of C, such as the expression vs. statement distinction, special rules for writing preprocessor macros, and when precedence rules require parenthesis. Fortunately, context is one thing this formatting library is good at keeping track of. The C formatting interface tries to make it as easy as possible to generate C code without getting in your way.

There are two approaches to using the C formatting extensions - procedural and sexp-oriented (described in "C as S-Expressions" bellow). In the procedural interface, C operators are made available as formatters with a "c-" prefix, literals are converted to their C equivalents and symbols are output as-is (you're responsible for making sure they are valid C identifiers). Indentation is handled automatically.

(fmt #t (c-if 1 2 3))


 if (1) {
 } else {

In addition, the formatter knows when you're in an expression and when you're in a statement, and behaves accordingly, so that

(fmt #t (c-if (c-if 1 2 3) 4 5))


 if (1 ? 2 : 3) {
 } else {

Similary, c-begin, used for sequencing, will separate with semi-colons in a statement and commas in an expression.

Moreover, we also keep track of the final expression in a function and insert returns for you:

(fmt #t (c-fun 'int 'foo '() (c-if (c-if 1 2 3) 4 5)))


 int foo () {
     if (1 ? 2 : 3) {
         return 4;
     } else {
         return 5;

although it knows that void functions don't return.

Switch statements insert breaks by default if they don't return:

  (fmt #t (c-switch 'y
            (c-case 1 (c+= 'x 1))
            (c-default (c+= 'x 2))))
 switch (y) {
     case 1:
         x += 1;
         x += 2;

though you can explicitly fallthrough if you want:

  (fmt #t (c-switch 'y
            (c-case/fallthrough 1 (c+= 'x 1))
            (c-default (c+= 'x 2))))
 switch (y) {
     case 1:
         x += 1;
         x += 2;

Operators are available with just a "c" prefix, e.g. c+, c-, c*, c/, etc. c++ is a prefix operator, c++/post is postfix. ||, | and |= are written as c-or, c-bit-or and c-bit-or= respectively.

Function applications are written with c-apply. Other control structures such as c-for and c-while work as expected. The full list is in the procedure index below.

When a C formatter encounters an object it doesn't know how to write (including lists and records), it outputs them according to the format state's current 'gen variable. This allows you to specify generators for your own types, e.g. if you are using your own AST records in a compiler.

If the 'gen variable isn't set it defaults to the c-expr/sexp procedure, which formats an s-expression as if it were C code. Thus instead of c-apply you can just use a list. The full API is available via normal s-expressions - formatters that aren't keywords in C are prefixed with a % or otherwise made invalid C identifiers so that they can't be confused with function application.

C Preprocessor Formatting

C preprocessor formatters also properly handle their surrounding context, so you can safely intermix them in the normal flow of C code.

  (fmt #t (c-switch 'y
            (c-case 1 (c= 'x 1))
            (cpp-ifdef 'H_TWO (c-case 2 (c= 'x 4)))
            (c-default (c= 'x 5))))
 switch (y) {
     case 1:
         x = 1;
 #ifdef H_TWO
     case 2:
         x = 4;
 #endif /* H_TWO */
         x = 5;

Macros can be handled with cpp-define, which knows to wrap individual variable references in parenthesis:

(fmt #t (cpp-define '(min x y) (c-if (c< 'x 'y) 'x 'y)))
 #define min(x, y) (((x) < (y)) ? (x) : (y))

As with all C formatters, the CPP output is pretty printed as needed, and if it wraps over several lines the lines are terminated with a backslash.

To write a C header file that is included at most once, you can wrap the entire body in cpp-wrap-header:

  (fmt #t (cpp-wrap-header "FOO_H"
            (c-extern (c-prototype 'int 'foo '()))))
 #ifndef FOO_H
 #define FOO_H
 extern int foo ();
 #endif /* ! FOO_H */

Customizing C Style

The output uses a simplified K&R style with 4 spaces for indentation by default. The following state variables let you override the style:


how many spaces to indent bodies, default 4


how many spaces to indent switch clauses, also defaults to 4


insert a newline before an open brace (non-K&R), defaults to #f


omit braces when we can prove they aren't needed


omit spaces between operators and operands for groups of variables and literals (e.g. "a+b+3" instead of "a + b + 3")


Don't wrap function calls and long operator groups over mulitple lines. Functions and control structures will still use multiple lines.

The C formatters also respect the 'radix and 'precision settings.

C Formatter Index

(c-if <condition> <pass> [<fail> [<condition2> <pass2> ...]])procedure

Print a chain of if/else conditions. Use a final condition of 'else for a final else clause.

c-for, c-while
c-for <init> <condition> <update> <body> ...procedure
c-while <condition> <body> ...procedure

Basic loop constructs.

c-fun, c-prototype
c-fun <type> <name> <params> <body> ...procedure
c-prototype <type> <name> <params>procedure

Output a function or function prototype. The parameters should be a list 2-element lists of the form (<param-type> <param-name>), which are output with DSP. A parameter can be abbreviated as just the symbol name, or #f can be passed as the type, in which case the 'default-type state variable is used. The parameters may be a dotted list, in which case ellipses for a C variadic are inserted - the actual name of the dotted value is ignored.

Types are just typically just symbols, or lists of symbols such as '(const char). A complete description is given below in section "C Types".

These can also accessed as %fun and %prototype at the head of a list.

c-var <type> <name> #!optional <init-value>procedure

Declares and optionally initializes a variable. Also accessed as %var at the head of a list.

c-begin <expr> ...procedure

Outputs each of the <expr>s, separated by semi-colons if in a statement or commas if in an expression.

c-switch, c-case, c-case/fallthrough, c-default
c-switch <clause> ...procedure
c-case <values> <body> ...procedure
c-case/fallthrough <values> <body> ...procedure
c-default <body> ...procedure

Switch statements. In addition to using the clause formatters, clauses inside a switch may be handled with a Scheme CASE-like list, with the car a list of case values and the cdr the body.

c-label, c-goto, c-return, c-break, c-continue
c-label <name>procedure
c-goto <name>procedure
c-return #!optional <result>procedure

Manual labels and jumps. Labels can also be accessed as a list beginning with a colon, e.g. '(: label1).

c-const, c-static, c-volatile, c-restrict, c-register, c-auto, c-inline, c-extern
c-const <expr>procedure
c-static <expr>procedure
c-volatile <expr>procedure
c-restrict <expr>procedure
c-register <expr>procedure
c-auto <expr>procedure
c-inline <expr>procedure
c-extern <expr>procedure

Declaration modifiers. May be nested.

c-extern/C <body> ...procedure

Wraps body in an

 extern "C" { ... }

for use with C++.

c-cast <type> <expr>procedure

Casts an expression to a type. Also %cast at the head of a list.

c-typedef <type> <new-name> ...procedure

Creates a new type definition with one or more names.

c-struct, c-union, c-class, c-attribute
(c-struct [<name>] <field-list> [<attributes>])procedure
(c-union [<name>] <field-list> [<attributes>])procedure
(c-class [<name>] <field-list> [<attributes>])procedure
c-attribute <values> ...procedure

Composite type constructors. Attributes may be accessed as %attribute at the head of a list.

  (fmt #f (c-struct 'employee
                      '((short age)
                        ((char *) name)
                        ((struct (year month day)) dob))
                      (c-attribute 'packed)))
 struct employee {
     short age;
     char* name;
     struct {
         int year;
         int month;
         int day;
     } dob;
 } __attribute__ ((packed));
(c-enum [<name>] <enum-list>)procedure

Enumerated types. <enum-list> may be strings, symbols, or lists of string or symbol followed by the enum's value.

c-comment <formatter> ...procedure

Outputs the <formatter>s wrapped in C's /* ... */ comment. Properly escapes nested comments inside in an Emacs-friendly style.

C Preprocessor Formatter Index

cpp-include <file>procedure

If file is a string, outputs in it "quotes", otherwise (as a symbol or arbitrary formatter) it outputs it in brackets.

(fmt #f (cpp-include 'stdio.h))

=> "#include <stdio.h>\n"

(fmt #f (cpp-include "config.h"))

=> "#include "config.h\n"
cpp-define <macro> #!optional <value>procedure

Defines a preprocessor macro, which may be just a name or a list of name and parameters. Properly wraps the value in parenthesis and escapes newlines. A dotted parameter list will use the C99 variadic macro syntax, and will also substitute any references to the dotted name with __VA_ARGS__:

(fmt #t (cpp-define '(eprintf . args) '(fprintf stderr args)))
 #define eprintf(...) (fprintf(stderr, __VA_ARGS__))
cpp-if,cpp-ifdef, cpp-ifndef, cpp-elif, cpp-else
(cpp-if <condition> <pass> [<fail> ...])procedure
(cpp-ifdef <condition> <pass> [<fail> ...])procedure
(cpp-ifndef <condition> <pass> [<fail> ...])procedure
(cpp-elif <condition> <pass> [<fail> ...])procedure
cpp-else <body> ...procedure

Conditional compilation.

cpp-line <num> #!optional <file>procedure

Line number information.

cpp-pragma, cpp-error, cpp-warning
cpp-pragma <args> ...procedure
cpp-error <args> ...procedure
cpp-warning <args> ...procedure

Additional preprocessor directives.

cpp-stringify <expr>procedure

Stringifies <expr> by prefixing the # operator.

cpp-sym-cat <args> ...procedure

Joins the <args> into a single preprocessor token with the ## operator.

cpp-wrap-header <name> <body> ...procedure

Wrap an entire header to only be included once.


 c++ c-- c+ c- c* c/ c% c& c^ c~ c! c&& c<< c>> c== c!=
 c< c> c<= c>= c= c+= c-= c*= c/= c%= c&= c^= c<<= c>>=
 c++/post c--/post c-or c-bit-or c-bit-or=

C Types

Typically a type is just a symbol such as 'char or 'int. You can wrap types with modifiers such as c-const, but as a convenience you can just use a list such as in '(const unsignedchar *). You can also nest these lists, so the previous example is equivalent to '(* (const (unsigned char))).

Pointers may be written as '(%pointer <type>) for readability - %pointer is exactly equivalent to * in types.

Unamed structs, classes, unions and enums may be used directly as types, using their respective keywords at the head of a list.

Two special types are the %array type and function pointer type. An array is written:

(%array <type> [<size>])

where <type> is any other type (including another array or function pointer), and <size>, if given, will print the array size. For example:

(c-var '(%array (unsigned long) SIZE) 'table '#(1 2 3 4))
unsigned long table[SIZE] = {1, 2, 3, 4};

A function pointer is written:

(%fun <return-type> (<param-types> ...)) 

For example:

(c-typedef '(%fun double (double double int)) 'f)
typedef double (*f)(double, double, int);

Wherever a type is expected but not given, the value of the 'default-type formatting state variable is used. By default this is just 'int.

Type declarations work uniformly for variables and parameters, as well for casts and typedefs.

C as S-Expressions

Rather than building formatting closures by hand, it can be more convenient to just build a normal s-expression and ask for it to be formatted as C code. This can be thought of as a simple Scheme->C compiler without any runtime support.

In a s-expression, strings and characters are printed as C strings and characters, booleans are printed as 0 or 1, symbols are displayed as-is, and numbers are printed as C numbers (using the current formatting radix if specified). Vectors are printed as comma-separated lists wrapped in braces, which can be used for initializing arrays or structs.

A list indicates a C expression or statement. Any of the existing C keywords can be used to pretty-print the expression as described with the c-keyword formatters above. Thus, the example above

(fmt #t (c-if (c-if 1 2 3) 4 5))

could also be written

(fmt #t (c-expr '(if (if 1 2 3) 4 5)))

C constructs that are dependent on the underlying syntax and have no keyword are written with a % prefix (%fun, %var, %pointer, %array, %cast), including C preprocessor constructs (%include, %define, %pragma, %error, %warning, %if, %ifdef, %ifndef, %elif). Labels are written as (: <label-name>). You can write a sequence as (%begin <expr> ...).

For example, the following definition of the fibonacci sequence, which apart from the return type of #f looks like a Lisp definition:

(fmt #t (c-expr '(%fun #f fib (n) (if (<= n 1) 1 (+ (fib (- n 1)) (fib (- n 2)))))))

prints the working C definition:

int fib (int n) {
    if (n <= 1) {
        return 1;
    } else {
        return fib((n - 1)) + fib((n - 2));

Formatting with Color

The fmt-color library provides the following utilities:

 (fmt-red <formatter> ...)
 (fmt-blue <formatter> ...)
 (fmt-green <formatter> ...)
 (fmt-cyan <formatter> ...)
 (fmt-yellow <formatter> ...)
 (fmt-magenta <formatter> ...)
 (fmt-white <formatter> ...)
 (fmt-black <formatter> ...)
 (fmt-bold <formatter> ...)
 (fmt-underline <formatter> ...)

and more generally

fmt-color <color> <formatter> ...procedure

where <color> can be a symbol name or #xRRGGBB numeric value. Outputs the formatters colored with ANSI escapes. In addition

fmt-in-html <formatter> ...procedure

can be used to mark the format state as being inside HTML, which the above color formats will understand and output HTML <span> tags with the appropriate style colors, instead of ANSI escapes.


The fmt-unicode library provides the fmt-unicode formatter, which just takes a list of formatters and overrides the string-length for padding and trimming, such that Unicode double or full width characters are considered 2 characters wide (as they typically are in fixed-width terminals), while treating combining and non-spacing characters as 0 characters wide.

It also recognizes and ignores ANSI escapes, in particular useful if you want to combine this with the fmt-color utilities.


The library is designed for scalability and flexibility, not speed, and I'm not going to think about any fine tuning until it's more stabilised. One aspect of the design, however, was influenced for the sake of future optimizations, which is that none of the default format variables are initialized by global parameters, which leaves room for inlining and subsequent simplification of format calls.

If you don't have an aggressively optimizing compiler, you can easily achieve large speedups on common cases with CL-style compiler macros.

Common Lisp Format Cheat Sheet

A quick reference for those of you switching over from Common Lisp's format.

format 	fmt
~a 	dsp
~c 	dsp
~s 	wrt/unshared
~w 	wrt
~y 	pretty
~x 	(radix 16 ...) or (num <n> 16)
~o 	(radix 8 ...) or (num <n> 8)
~b 	(radix 2 ...) or (num <n> 2)
~f 	(fix <digits> ...) or (num <n> <radix> <digits>)
~% 	nl
~& 	fl
~[...~] 	normal if or fmt-if (delayed test)
~{...~} 	(fmt-join ... <list> [<sep>])


Alex Shinn

Version history

fix tests [Mario Domenech Goulart]
fix error in wrap-lines for certain combinations of wrap widths and long lines [Jim Ursetto]
prevent string-tokenize from usurping srfi-13 binding [noticed by Taylor Venable]
patch wrap-lines to not fail on whitespace-only strings; disable last column padding skip optimization; 100% width no longer treated as fixed length 1; override string-tokenize to properly split strings containing utf8 sequences [Jim Ursetto]
Sync to upstream fmt 0.7 [Jim Ursetto]
initial fmt 0.5 port to Chicken 4 by Alex Shinn

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