base- Basic libraries

Copyright(c) Lennart Augustsson and Bart Massey 2013
LicenseBSD-style (see the file LICENSE in this distribution)
MaintainerBart Massey <>
Safe HaskellSafe




A C printf(3)-like formatter. This version has been extended by Bart Massey as per the recommendations of John Meacham and Simon Marlow <> to support extensible formatting for new datatypes. It has also been extended to support almost all C printf(3) syntax.


Printing Functions

printf :: PrintfType r => String -> r Source

Format a variable number of arguments with the C-style formatting string. The return value is either String or (IO a) (which should be (IO '()'), but Haskell's type system makes this hard).

The format string consists of ordinary characters and conversion specifications, which specify how to format one of the arguments to printf in the output string. A format specification is introduced by the % character; this character can be self-escaped into the format string using %%. A format specification ends with a /format character/ that provides the primary information about how to format the value. The rest of the conversion specification is optional. In order, one may have flag characters, a width specifier, a precision specifier, and type-specific modifier characters.

Unlike C printf(3), the formatting of this printf is driven by the argument type; formatting is type specific. The types formatted by printf "out of the box" are:

printf is also extensible to support other types: see below.

A conversion specification begins with the character %, followed by zero or more of the following flags:

   -      left adjust (default is right adjust)
   +      always use a sign (+ or -) for signed conversions
   space  leading space for positive numbers in signed conversions
   0      pad with zeros rather than spaces
   #      use an \"alternate form\": see below

When both flags are given, - overrides 0 and + overrides space. A negative width specifier in a * conversion is treated as positive but implies the left adjust flag.

The "alternate form" for unsigned radix conversions is as in C printf(3):

   %o           prefix with a leading 0 if needed
   %x           prefix with a leading 0x if nonzero
   %X           prefix with a leading 0X if nonzero
   %b           prefix with a leading 0b if nonzero
   %[eEfFgG]    ensure that the number contains a decimal point

Any flags are followed optionally by a field width:

   num    field width
   *      as num, but taken from argument list

The field width is a minimum, not a maximum: it will be expanded as needed to avoid mutilating a value.

Any field width is followed optionally by a precision:

   .num   precision
   .      same as .0
   .*     as num, but taken from argument list

Negative precision is taken as 0. The meaning of the precision depends on the conversion type.

   Integral    minimum number of digits to show
   RealFloat   number of digits after the decimal point
   String      maximum number of characters

The precision for Integral types is accomplished by zero-padding. If both precision and zero-pad are given for an Integral field, the zero-pad is ignored.

Any precision is followed optionally for Integral types by a width modifier; the only use of this modifier being to set the implicit size of the operand for conversion of a negative operand to unsigned:

   hh     Int8
   h      Int16
   l      Int32
   ll     Int64
   L      Int64

The specification ends with a format character:

   c      character               Integral
   d      decimal                 Integral
   o      octal                   Integral
   x      hexadecimal             Integral
   X      hexadecimal             Integral
   b      binary                  Integral
   u      unsigned decimal        Integral
   f      floating point          RealFloat
   F      floating point          RealFloat
   g      general format float    RealFloat
   G      general format float    RealFloat
   e      exponent format float   RealFloat
   E      exponent format float   RealFloat
   s      string                  String
   v      default format          any type

The "%v" specifier is provided for all built-in types, and should be provided for user-defined type formatters as well. It picks a "best" representation for the given type. For the built-in types the "%v" specifier is converted as follows:

   c      Char
   u      other unsigned Integral
   d      other signed Integral
   g      RealFloat
   s      String

Mismatch between the argument types and the format string, as well as any other syntactic or semantic errors in the format string, will cause an exception to be thrown at runtime.

Note that the formatting for RealFloat types is currently a bit different from that of C printf(3), conforming instead to showEFloat, showFFloat and showGFloat (and their alternate versions showFFloatAlt and showGFloatAlt). This is hard to fix: the fixed versions would format in a backward-incompatible way. In any case the Haskell behavior is generally more sensible than the C behavior. A brief summary of some key differences:

  • Haskell printf never uses the default "6-digit" precision used by C printf.
  • Haskell printf treats the "precision" specifier as indicating the number of digits after the decimal point.
  • Haskell printf prints the exponent of e-format numbers without a gratuitous plus sign, and with the minimum possible number of digits.
  • Haskell printf will place a zero after a decimal point when possible.


  > printf "%d\n" (23::Int)
  > printf "%s %s\n" "Hello" "World"
  Hello World
  > printf "%.2f\n" pi

hPrintf :: HPrintfType r => Handle -> String -> r Source

Similar to printf, except that output is via the specified Handle. The return type is restricted to (IO a).

Extending To New Types

This printf can be extended to format types other than those provided for by default. This is done by instancing PrintfArg and providing a formatArg for the type. It is possible to provide a parseFormat to process type-specific modifiers, but the default instance is usually the best choice.

For example:

instance PrintfArg () where
  formatArg x fmt | fmtChar (vFmt 'U' fmt) == 'U' =
    formatString "()" (fmt { fmtChar = 's', fmtPrecision = Nothing })
  formatArg _ fmt = errorBadFormat $ fmtChar fmt

main :: IO ()
main = printf "[%-3.1U]\n" ()

prints "[() ]". Note the use of formatString to take care of field formatting specifications in a convenient way.

class PrintfArg a where Source

Typeclass of printf-formattable values. The formatArg method takes a value and a field format descriptor and either fails due to a bad descriptor or produces a ShowS as the result. The default parseFormat expects no modifiers: this is the normal case. Minimal instance: formatArg.

Minimal complete definition



formatArg :: a -> FieldFormatter Source


parseFormat :: a -> ModifierParser Source


type FieldFormatter = FieldFormat -> ShowS Source

This is the type of a field formatter reified over its argument.


data FieldFormat Source

Description of field formatting for formatArg. See UNIX printf(3) for a description of how field formatting works.





fmtWidth :: Maybe Int

Total width of the field.

fmtPrecision :: Maybe Int

Secondary field width specifier.

fmtAdjust :: Maybe FormatAdjustment

Kind of filling or padding to be done.

fmtSign :: Maybe FormatSign

Whether to insist on a plus sign for positive numbers.

fmtAlternate :: Bool

Indicates an "alternate format". See printf(3) for the details, which vary by argument spec.

fmtModifiers :: String

Characters that appeared immediately to the left of fmtChar in the format and were accepted by the type's parseFormat. Normally the empty string.

fmtChar :: Char

The format character printf was invoked with. formatArg should fail unless this character matches the type. It is normal to handle many different format characters for a single type.

data FormatAdjustment Source

Whether to left-adjust or zero-pad a field. These are mutually exclusive, with LeftAdjust taking precedence.




data FormatSign Source

How to handle the sign of a numeric field. These are mutually exclusive, with SignPlus taking precedence.




vFmt :: Char -> FieldFormat -> FieldFormat Source

Substitute a 'v' format character with the given default format character in the FieldFormat. A convenience for user-implemented types, which should support "%v".


Handling Type-specific Modifiers

In the unlikely case that modifier characters of some kind are desirable for a user-provided type, a ModifierParser can be provided to process these characters. The resulting modifiers will appear in the FieldFormat for use by the type-specific formatter.

type ModifierParser = String -> FormatParse Source

Type of a function that will parse modifier characters from the format string.


data FormatParse Source

The "format parser" walks over argument-type-specific modifier characters to find the primary format character. This is the type of its result.





fpModifiers :: String

Any modifiers found.

fpChar :: Char

Primary format character.

fpRest :: String

Rest of the format string.

Standard Formatters

These formatters for standard types are provided for convenience in writting new type-specific formatters: a common pattern is to throw to formatString or formatInteger to do most of the format handling for a new type.

formatString :: IsChar a => [a] -> FieldFormatter Source

Formatter for String values.


formatChar :: Char -> FieldFormatter Source

Formatter for Char values.


formatInt :: (Integral a, Bounded a) => a -> FieldFormatter Source

Formatter for Int values.


formatInteger :: Integer -> FieldFormatter Source

Formatter for Integer values.


formatRealFloat :: RealFloat a => a -> FieldFormatter Source

Formatter for RealFloat values.


Raising Errors

These functions are used internally to raise various errors, and are exported for use by new type-specific formatters.

errorBadFormat :: Char -> a Source

Calls perror to indicate an unknown format letter for a given type.


errorShortFormat :: a Source

Calls perror to indicate that the format string ended early.


errorMissingArgument :: a Source

Calls perror to indicate that there is a missing argument in the argument list.


errorBadArgument :: a Source

Calls perror to indicate that there is a type error or similar in the given argument.


perror :: String -> a Source

Raises an error with a printf-specific prefix on the message string.


Implementation Internals

These types are needed for implementing processing variable numbers of arguments to printf and hPrintf. Their implementation is intentionally not visible from this module. If you attempt to pass an argument of a type which is not an instance of the appropriate class to printf or hPrintf, then the compiler will report it as a missing instance of PrintfArg. (All PrintfArg instances are PrintfType instances.)

class PrintfType t Source

The PrintfType class provides the variable argument magic for printf. Its implementation is intentionally not visible from this module. If you attempt to pass an argument of a type which is not an instance of this class to printf or hPrintf, then the compiler will report it as a missing instance of PrintfArg.

Minimal complete definition



IsChar c => PrintfType [c] 
(~) * a () => PrintfType (IO a) 
(PrintfArg a, PrintfType r) => PrintfType (a -> r) 

class HPrintfType t Source

The HPrintfType class provides the variable argument magic for hPrintf. Its implementation is intentionally not visible from this module.

Minimal complete definition



(~) * a () => HPrintfType (IO a) 
(PrintfArg a, HPrintfType r) => HPrintfType (a -> r) 

This class is needed as a Haskell98 compatibility workaround for the lack of FlexibleInstances.

class IsChar c where Source

This class, with only the one instance, is used as a workaround for the fact that String, as a concrete type, is not allowable as a typeclass instance. IsChar is exported for backward-compatibility.


toChar :: c -> Char Source


fromChar :: Char -> c Source