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7.11 Formatted Output

The format function is a powerful way to print numbers, strings and other objects together with literal text under the control of a format string. This function is available from

(use-modules (ice-9 format))

A format string is generally more compact and easier than using just the standard procedures like display, write and newline. Parameters in the output string allow various output styles, and parameters can be taken from the arguments for runtime flexibility.

format is similar to the Common Lisp procedure of the same name, but it’s not identical and doesn’t have quite all the features found in Common Lisp.

C programmers will note the similarity between format and printf, though escape sequences are marked with ~ instead of %, and are more powerful.

Scheme Procedure: format dest fmt arg …

Write output specified by the fmt string to dest. dest can be an output port, #t for current-output-port (see Default Ports), or #f to return the output as a string.

fmt can contain literal text to be output, and ~ escapes. Each escape has the form

~ [param [, param…] [:] [@] code

code is a character determining the escape sequence. The : and @ characters are optional modifiers, one or both of which change the way various codes operate. Optional parameters are accepted by some codes too. Parameters have the following forms,


An integer, with optional + or -.

' (apostrophe)

The following character in the format string, for instance 'z for z.


The next function argument as the parameter. v stands for “variable”, a parameter can be calculated at runtime and included in the arguments. Upper case V can be used too.


The number of arguments remaining. (See ~* below for some usages.)

Parameters are separated by commas (,). A parameter can be left empty to keep its default value when supplying later parameters.

The following escapes are available. The code letters are not case-sensitive, upper and lower case are the same.


Object output. Parameters: minwidth, padinc, minpad, padchar.

~a outputs an argument like display, ~s outputs an argument like write (see Scheme Write).

(format #t "~a" "foo") -| foo
(format #t "~s" "foo") -| "foo"

~:a and ~:s put objects that don’t have an external representation in quotes like a string.

(format #t "~:a" car) -| "#<primitive-procedure car>"

If the output is less than minwidth characters (default 0), it’s padded on the right with padchar (default space). ~@a and ~@s put the padding on the left instead.

(format #f "~5a" 'abc)       ⇒ "abc  "
(format #f "~5,,,'-@a" 'abc) ⇒ "--abc"

minpad is a minimum for the padding then plus a multiple of padinc. Ie. the padding is minpad + N * padinc, where n is the smallest integer making the total object plus padding greater than or equal to minwidth. The default minpad is 0 and the default padinc is 1 (imposing no minimum or multiple).

(format #f "~5,1,4a" 'abc) ⇒ "abc    "

Character. Parameter: charnum.

Output a character. The default is to simply output, as per write-char (see Venerable Port Interfaces). ~@c prints in write style. ~:c prints control characters (ASCII 0 to 31) in ^X form.

(format #t "~c" #\z)        -| z
(format #t "~@c" #\z)       -| #\z
(format #t "~:c" #\newline) -| ^J

If the charnum parameter is given then an argument is not taken but instead the character is (integer->char charnum) (see Characters). This can be used for instance to output characters given by their ASCII code.

(format #t "~65c")  -| A

Integer. Parameters: minwidth, padchar, commachar, commawidth.

Output an integer argument as a decimal, hexadecimal, octal or binary integer (respectively), in a locale-independent way.

(format #t "~d" 123) -| 123

~@d etc shows a + sign is shown on positive numbers.

(format #t "~@b" 12) -| +1100

If the output is less than the minwidth parameter (default no minimum), it’s padded on the left with the padchar parameter (default space).

(format #t "~5,'*d" 12)   -| ***12
(format #t "~5,'0d" 12)   -| 00012
(format #t "~3d"    1234) -| 1234

~:d adds commas (or the commachar parameter) every three digits (or the commawidth parameter many). However, when your intent is to write numbers in a way that follows typographical conventions, using ~h is recommended.

(format #t "~:d" 1234567)         -| 1,234,567
(format #t "~10,'*,'/,2:d" 12345) -| ***1/23/45

Hexadecimal ~x output is in lower case, but the ~( and ~) case conversion directives described below can be used to get upper case.

(format #t "~x"       65261) -| feed
(format #t "~:@(~x~)" 65261) -| FEED

Integer in words, roman numerals, or a specified radix. Parameters: radix, minwidth, padchar, commachar, commawidth.

With no parameters output is in words as a cardinal like “ten”, or ~:r prints an ordinal like “tenth”.

(format #t "~r" 9)  -| nine        ;; cardinal
(format #t "~r" -9) -| minus nine  ;; cardinal
(format #t "~:r" 9) -| ninth       ;; ordinal

And also with no parameters, ~@r gives roman numerals and ~:@r gives old roman numerals. In old roman numerals there’s no “subtraction”, so 9 is VIIII instead of IX. In both cases only positive numbers can be output.

(format #t "~@r" 89)  -| LXXXIX     ;; roman
(format #t "~:@r" 89) -| LXXXVIIII  ;; old roman

When a parameter is given it means numeric output in the specified radix. The modifiers and parameters following the radix are the same as described for ~d etc above.

(format #f "~3r" 27)   ⇒ "1000"    ;; base 3
(format #f "~3,5r" 26) ⇒ "  222"   ;; base 3 width 5

Fixed-point float. Parameters: width, decimals, scale, overflowchar, padchar.

Output a number or number string in fixed-point format, ie. with a decimal point.

(format #t "~f" 5)      -| 5.0
(format #t "~f" "123")  -| 123.0
(format #t "~f" "1e-1") -| 0.1

~@f prints a + sign on positive numbers (including zero).

(format #t "~@f" 0) -| +0.0

If the output is less than width characters it’s padded on the left with padchar (space by default). If the output equals or exceeds width then there’s no padding. The default for width is no padding.

(format #f "~6f" -1.5)      ⇒ "  -1.5"
(format #f "~6,,,,'*f" 23)  ⇒ "**23.0"
(format #f "~6f" 1234567.0) ⇒ "1234567.0"

decimals is how many digits to print after the decimal point, with the value rounded or padded with zeros as necessary. (The default is to output as many decimals as required.)

(format #t "~1,2f" 3.125) -| 3.13
(format #t "~1,2f" 1.5)   -| 1.50

scale is a power of 10 applied to the value, moving the decimal point that many places. A positive scale increases the value shown, a negative decreases it.

(format #t "~,,2f" 1234)  -| 123400.0
(format #t "~,,-2f" 1234) -| 12.34

If overflowchar and width are both given and if the output would exceed width, then that many overflowchars are printed instead of the value.

(format #t "~6,,,'xf" 12345) -| 12345.
(format #t "~5,,,'xf" 12345) -| xxxxx

Localized number31. Parameters: width, decimals, padchar.

Like ~f, output an exact or floating point number, but do so according to the current locale, or according to the given locale object when the : modifier is used (see number->locale-string).

(format #t "~h" 12345.5678)  ; with "C" as the current locale
-| 12345.5678

(format #t "~14,,'*:h" 12345.5678
        (make-locale LC_ALL "en_US"))
-| ***12,345.5678

(format #t "~,2:h" 12345.5678
        (make-locale LC_NUMERIC "fr_FR"))
-| 12 345,56

Exponential float. Parameters: width, mantdigits, expdigits, intdigits, overflowchar, padchar, expchar.

Output a number or number string in exponential notation.

(format #t "~e" 5000.25) -| 5.00025E+3
(format #t "~e" "123.4") -| 1.234E+2
(format #t "~e" "1e4")   -| 1.0E+4

~@e prints a + sign on positive numbers (including zero). (This is for the mantissa, a + or - sign is always shown on the exponent.)

(format #t "~@e" 5000.0) -| +5.0E+3

If the output is less than width characters it’s padded on the left with padchar (space by default). The default for width is to output with no padding.

(format #f "~10e" 1234.0)     ⇒ "  1.234E+3"
(format #f "~10,,,,,'*e" 0.5) ⇒ "****5.0E-1"

mantdigits is the number of digits shown in the mantissa after the decimal point. The value is rounded or trailing zeros are added as necessary. The default mantdigits is to show as much as needed by the value.

(format #f "~,3e" 11111.0) ⇒ "1.111E+4"
(format #f "~,8e" 123.0)   ⇒ "1.23000000E+2"

expdigits is the minimum number of digits shown for the exponent, with leading zeros added if necessary. The default for expdigits is to show only as many digits as required. At least 1 digit is always shown.

(format #f "~,,1e" 1.0e99) ⇒ "1.0E+99"
(format #f "~,,6e" 1.0e99) ⇒ "1.0E+000099"

intdigits (default 1) is the number of digits to show before the decimal point in the mantissa. intdigits can be zero, in which case the integer part is a single 0, or it can be negative, in which case leading zeros are shown after the decimal point.

(format #t "~,,,3e" 12345.0)  -| 123.45E+2
(format #t "~,,,0e" 12345.0)  -| 0.12345E+5
(format #t "~,,,-3e" 12345.0) -| 0.00012345E+8

If overflowchar is given then width is a hard limit. If the output would exceed width then instead that many overflowchars are printed.

(format #f "~6,,,,'xe" 100.0) ⇒ "1.0E+2"
(format #f "~3,,,,'xe" 100.0) ⇒ "xxx"

expchar is the exponent marker character (default E).

(format #t "~,,,,,,'ee" 100.0) -| 1.0e+2

General float. Parameters: width, mantdigits, expdigits, intdigits, overflowchar, padchar, expchar.

Output a number or number string in either exponential format the same as ~e, or fixed-point format like ~f but aligned where the mantissa would have been and followed by padding where the exponent would have been.

Fixed-point is used when the absolute value is 0.1 or more and it takes no more space than the mantissa in exponential format, ie. basically up to mantdigits digits.

(format #f "~12,4,2g" 999.0)    ⇒ "   999.0    "
(format #f "~12,4,2g" "100000") ⇒ "  1.0000E+05"

The parameters are interpreted as per ~e above. When fixed-point is used, the decimals parameter to ~f is established from mantdigits, so as to give a total mantdigits+1 figures.


Monetary style fixed-point float. Parameters: decimals, intdigits, width, padchar.

Output a number or number string in fixed-point format, ie. with a decimal point. decimals is the number of decimal places to show, default 2.

(format #t "~$" 5)       -| 5.00
(format #t "~4$" "2.25") -| 2.2500
(format #t "~4$" "1e-2") -| 0.0100

~@$ prints a + sign on positive numbers (including zero).

(format #t "~@$" 0) -| +0.00

intdigits is a minimum number of digits to show in the integer part of the value (default 1).

(format #t "~,3$" 9.5)   -| 009.50
(format #t "~,0$" 0.125) -| .13

If the output is less than width characters (default 0), it’s padded on the left with padchar (default space). ~:$ puts the padding after the sign.

(format #f "~,,8$" -1.5)   ⇒ "   -1.50"
(format #f "~,,8:$" -1.5)  ⇒ "-   1.50"
(format #f "~,,8,'.:@$" 3) ⇒ "+...3.00"

Note that floating point for dollar amounts is generally not a good idea, because a cent 0.01 cannot be represented exactly in the binary floating point Guile uses, which leads to slowly accumulating rounding errors. Keeping values as cents (or fractions of a cent) in integers then printing with the scale option in ~f may be a better approach.


Complex fixed-point float. Parameters: width, decimals, scale, overflowchar, padchar.

Output the argument as a complex number, with both real and imaginary part shown (even if one or both are zero).

The parameters and modifiers are the same as for fixed-point ~f described above. The real and imaginary parts are both output with the same given parameters and modifiers, except that for the imaginary part the @ modifier is always enabled, so as to print a + sign between the real and imaginary parts.

(format #t "~i" 1)  -| 1.0+0.0i

Plural. No parameters.

Output nothing if the argument is 1, or ‘s’ for any other value.

(format #t "enter name~p" 1) -| enter name
(format #t "enter name~p" 2) -| enter names

~@p prints ‘y’ for 1 or ‘ies’ otherwise.

(format #t "pupp~@p" 1) -| puppy
(format #t "pupp~@p" 2) -| puppies

~:p re-uses the preceding argument instead of taking a new one, which can be convenient when printing some sort of count.

(format #t "~d cat~:p" 9)   -| 9 cats
(format #t "~d pupp~:@p" 5) -| 5 puppies

~p is designed for English plurals and there’s no attempt to support other languages. ~[ conditionals (below) may be able to help. When using gettext to translate messages ngettext is probably best though (see Internationalization).


Structured printing. Parameters: width.

~y outputs an argument using pretty-print (see Pretty Printing). The result will be formatted to fit within width columns (79 by default), consuming multiple lines if necessary.

~@y outputs an argument using truncated-print (see Pretty Printing). The resulting code will be formatted to fit within width columns (79 by default), on a single line. The output will be truncated if necessary.

~:@y is like ~@y, except the width parameter is interpreted to be the maximum column to which to output. That is to say, if you are at column 10, and ~60:@y is seen, the datum will be truncated to 50 columns.


Sub-format. No parameters.

Take a format string argument and a second argument which is a list of arguments for that string, and output the result.

(format #t "~?" "~d ~d" '(1 2))    -| 1 2

~@? takes arguments for the sub-format directly rather than in a list.

(format #t "~@? ~s" "~d ~d" 1 2 "foo") -| 1 2 "foo"

~? and ~k are the same, ~k is provided for T-Scheme compatibility.


Argument jumping. Parameter: N.

Move forward N arguments (default 1) in the argument list. ~:* moves backwards. (N cannot be negative.)

(format #f "~d ~2*~d" 1 2 3 4) ⇒ "1 4"
(format #f "~d ~:*~d" 6)       ⇒ "6 6"

~@* moves to argument number N. The first argument is number 0 (and that’s the default for N).

(format #f "~d~d again ~@*~d~d" 1 2) ⇒ "12 again 12"
(format #f "~d~d~d ~1@*~d~d" 1 2 3)  ⇒ "123 23"

A # move to the end followed by a : modifier move back can be used for an absolute position relative to the end of the argument list, a reverse of what the @ modifier does.

(format #t "~#*~2:*~a" 'a 'b 'c 'd)   -| c

At the end of the format string the current argument position doesn’t matter, any further arguments are ignored.


Advance to a column position. Parameters: colnum, colinc, padchar.

Output padchar (space by default) to move to the given colnum column. The start of the line is column 0, the default for colnum is 1.

(format #f "~tX")  ⇒ " X"
(format #f "~3tX") ⇒ "   X"

If the current column is already past colnum, then the move is to there plus a multiple of colinc, ie. column colnum + N * colinc for the smallest N which makes that value greater than or equal to the current column. The default colinc is 1 (which means no further move).

(format #f "abcd~2,5,'.tx") ⇒ "abcd...x"

~@t takes colnum as an offset from the current column. colnum many pad characters are output, then further padding to make the current column a multiple of colinc, if it isn’t already so.

(format #f "a~3,5'*@tx") ⇒ "a****x"

~t is implemented using port-column (see Textual I/O), so it works even there has been other output before format.


Tilde character. Parameter: n.

Output a tilde character ~, or n many if a parameter is given. Normally ~ introduces an escape sequence, ~~ is the way to output a literal tilde.


Newline. Parameter: n.

Output a newline character, or n many if a parameter is given. A newline (or a few newlines) can of course be output just by including them in the format string.


Start a new line. Parameter: n.

Output a newline if not already at the start of a line. With a parameter, output that many newlines, but with the first only if not already at the start of a line. So for instance 3 would be a newline if not already at the start of a line, and 2 further newlines.


Space character. Parameter: n.

Output a space character, or n many if a parameter is given.

With a variable parameter this is one way to insert runtime calculated padding (~t or the various field widths can do similar things).

(format #f "~v_foo" 4) ⇒ "    foo"

Tab character. Parameter: n.

Output a tab character, or n many if a parameter is given.


Formfeed character. Parameter: n.

Output a formfeed character, or n many if a parameter is given.


Force output. No parameters.

At the end of output, call force-output to flush any buffers on the destination (see Buffering). ~! can occur anywhere in the format string, but the force is done at the end of output.

When output is to a string (destination #f), ~! does nothing.

~newline (ie. newline character)

Continuation line. No parameters.

Skip this newline and any following whitespace in the format string, ie. don’t send it to the output. This can be used to break up a long format string for readability, but not print the extra whitespace.

(format #f "abc~
            ~d def~
            ~d" 1 2) ⇒ "abc1 def2"

~:newline skips the newline but leaves any further whitespace to be printed normally.

~@newline prints the newline then skips following whitespace.

~( ~)

Case conversion. No parameters.

Between ~( and ~) the case of all output is changed. The modifiers on ~( control the conversion.

  • ~( — lower case.
  • ~:@( — upper case.

For example,

(format #t "~(Hello~)")   -| hello
(format #t "~:@(Hello~)") -| HELLO

In the future it’s intended the modifiers : and @ alone will capitalize the first letters of words, as per Common Lisp format, but the current implementation of this is flawed and not recommended for use.

Case conversions do not nest, currently. This might change in the future, but if it does then it will be to Common Lisp style where the outermost conversion has priority, overriding inner ones (making those fairly pointless).

~{ ~}

Iteration. Parameter: maxreps (for ~{).

The format between ~{ and ~} is iterated. The modifiers to ~{ determine how arguments are taken. The default is a list argument with each iteration successively consuming elements from it. This is a convenient way to output a whole list.

(format #t "~{~d~}"     '(1 2 3))       -| 123
(format #t "~{~s=~d ~}" '("x" 1 "y" 2)) -| "x"=1 "y"=2

~:{ takes a single argument which is a list of lists, each of those contained lists gives the arguments for the iterated format.

(format #t "~:{~dx~d ~}" '((1 2) (3 4) (5 6)))
-| 1x2 3x4 5x6

~@{ takes arguments directly, with each iteration successively consuming arguments.

(format #t "~@{~d~}"     1 2 3)       -| 123
(format #t "~@{~s=~d ~}" "x" 1 "y" 2) -| "x"=1 "y"=2

~:@{ takes list arguments, one argument for each iteration, using that list for the format.

(format #t "~:@{~dx~d ~}" '(1 2) '(3 4) '(5 6))
-| 1x2 3x4 5x6

Iterating stops when there are no more arguments or when the maxreps parameter to ~{ is reached (default no maximum).

(format #t "~2{~d~}" '(1 2 3 4)) -| 12

If the format between ~{ and ~} is empty, then a format string argument is taken (before iteration argument(s)) and used instead. This allows a sub-format (like ~? above) to be iterated.

(format #t "~{~}" "~d" '(1 2 3)) -| 123

Iterations can be nested, an inner iteration operates in the same way as described, but of course on the arguments the outer iteration provides it. This can be used to work into nested list structures. For example in the following the inner ~{~d~}x is applied to (1 2) then (3 4 5) etc.

(format #t "~{~{~d~}x~}" '((1 2) (3 4 5))) -| 12x345x

See also ~^ below for escaping from iteration.

~[ ~; ~]

Conditional. Parameter: selector.

A conditional block is delimited by ~[ and ~], and ~; separates clauses within the block. ~[ takes an integer argument and that number clause is used. The first clause is number 0.

(format #f "~[peach~;banana~;mango~]" 1)  ⇒ "banana"

The selector parameter can be used for the clause number, instead of taking an argument.

(format #f "~2[peach~;banana~;mango~]") ⇒ "mango"

If the clause number is out of range then nothing is output. Or the last clause can be ~:; to use that for a number out of range.

(format #f "~[banana~;mango~]"         99) ⇒ ""
(format #f "~[banana~;mango~:;fruit~]" 99) ⇒ "fruit"

~:[ treats the argument as a flag, and expects two clauses. The first is used if the argument is #f or the second otherwise.

(format #f "~:[false~;not false~]" #f)   ⇒ "false"
(format #f "~:[false~;not false~]" 'abc) ⇒ "not false"

(let ((n 3))
  (format #t "~d gnu~:[s are~; is~] here" n (= 1 n)))
-| 3 gnus are here

~@[ also treats the argument as a flag, and expects one clause. If the argument is #f then no output is produced and the argument is consumed, otherwise the clause is used and the argument is not consumed, it’s left for the clause. This can be used for instance to suppress output if #f means something not available.

(format #f "~@[temperature=~d~]" 27) ⇒ "temperature=27"
(format #f "~@[temperature=~d~]" #f) ⇒ ""

Escape. Parameters: val1, val2, val3.

Stop formatting if there are no more arguments. This can be used for instance to have a format string adapt to a variable number of arguments.

(format #t "~d~^ ~d" 1)   -| 1
(format #t "~d~^ ~d" 1 2) -| 1 2

Within a ~{ ~} iteration, ~^ stops the current iteration step if there are no more arguments to that step, but continuing with possible further steps and the rest of the format. This can be used for instance to avoid a separator on the last iteration, or to adapt to variable length argument lists.

(format #f "~{~d~^/~} go"    '(1 2 3))     ⇒ "1/2/3 go"
(format #f "~:{ ~d~^~d~} go" '((1) (2 3))) ⇒ " 1 23 go"

Within a ~? sub-format, ~^ operates just on that sub-format. If it terminates the sub-format then the originating format will still continue.

(format #t "~? items" "~d~^ ~d" '(1))   -| 1 items
(format #t "~? items" "~d~^ ~d" '(1 2)) -| 1 2 items

The parameters to ~^ (which are numbers) change the condition used to terminate. For a single parameter, termination is when that value is zero (notice this makes plain ~^ equivalent to ~#^). For two parameters, termination is when those two are equal. For three parameters, termination is when val1 <= val2 and val2 <= val3.


Inquiry message. Insert a copyright message into the output.

~:q inserts the format implementation version.

It’s an error if there are not enough arguments for the escapes in the format string, but any excess arguments are ignored.

Iterations ~{ ~} and conditionals ~[ ~; ~] can be nested, but must be properly nested, meaning the inner form must be entirely within the outer form. So it’s not possible, for instance, to try to conditionalize the endpoint of an iteration.

(format #t "~{ ~[ ... ~] ~}" ...)       ;; good
(format #t "~{ ~[ ... ~} ... ~]" ...)   ;; bad

The same applies to case conversions ~( ~), they must properly nest with respect to iterations and conditionals (though currently a case conversion cannot nest within another case conversion).

When a sub-format (~?) is used, that sub-format string must be self-contained. It cannot for instance give a ~{ to begin an iteration form and have the ~} up in the originating format, or similar.

Guile contains a format procedure even when the module (ice-9 format) is not loaded. The default format is simple-format (see Simple Output), it doesn’t support all escape sequences documented in this section, and will signal an error if you try to use one of them. The reason for two versions is that the full format is fairly large and requires some time to load. simple-format is often adequate too.



The ~h format specifier first appeared in Guile version 2.0.6.

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