Quasiquotations for a python like interpolated string formater

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BSD-3-Clause licensed by Guillaume Bouchard
Maintained by [email protected]
This version can be pinned in stack with:PyF-,2567

Module documentation for



PyF is a Haskell library for string interpolation and formatting.

PyF exposes a quasiquoter f which introduces string interpolation and formatting with a mini language inspired from printf and Python.

Quick Start

>>> import PyF

>>> name = "Dave"
>>> age = 54

>>> [fmt|Person's name is {name}, age is {age}|]
"Person's name is Dave, age is 54"

The formatting mini language can represent:

  • Numbers with different representations (fixed point, general representation, binary, hexadecimal, octal)
  • Padding, with the choice of padding char, alignment (left, right, around, between sign and number)
  • Sign handling, to display or not the + for positive number
  • Number grouping
  • Floating point representation
  • The interpolated value can be any Haskell expression.

You will need the extension QuasiQuotes, enable it with {-# LANGUAGE QuasiQuotes #-} in top of your source file or with :set -XQuasiQuotes in your ghci session. ExtendedDefaultRules and OverloadedStrings may be more convenient.

Expression to be formatted are referenced by {expression:formatingOptions} where formatingOptions follows the Python format mini-language. It is recommended to read the python documentation, but the Test file as well as this readme contain many examples.

More Examples


Left < / Right > / Around ^ padding:

>>> name = "Guillaume"
>>> [fmt|{name:<11}|]
"Guillaume  "
>>> [fmt|{name:>11}|]
"  Guillaume"
>>> [fmt|{name:|^13}|]

Padding inside = the sign

>>> [fmt|{-3:=6}|]
"-    3"

Float rounding

>>> [fmt|{pi:.2}|]

Binary / Octal / Hex representation (with or without prefix using #)

>>> v = 31
>>> [fmt|Binary: {v:#b}|]
"Binary: 0b11111"
>>> [fmt|Octal: {v:#o}|]
"Octal: 0o37"
>>> [fmt|Octal (no prefix): {v:o}|]
"Octal (no prefix): 37"
>>> [fmt|Hexa (caps and prefix): {v:#X}|]
"Hexa (caps and prefix): 0x1F"


Using , or _.

>>> [fmt|{10 ^ 9 - 1:,}|]
>>> [fmt|{2 ^ 32  -1:_b}|]

Sign handling

Using + to display the positive sign (if any) or to display a space instead:

>>> [fmt|{pi:+.3}|]
>>> [fmt|{-pi:+.3} (Negative number)|]
"-3.142 (Negative number)"
>>> [fmt|{pi: .3}|]
" 3.142"
>>> [fmt|{-pi: .3} (Negative number)|]
"-3.142 (Negative number)"


Preceding the width with a 0 enables sign-aware zero-padding, this is equivalent to inside = padding with a fill char of 0.

>>> [f{10:010}|]
>>> [f{-10:010}|]


First argument inside the curly braces can be a valid Haskell expression, for example:

>>> [fmt|2pi = {2* pi:.2}|]
2pi = 6.28
>>> [fmt|tail "hello" = {tail "hello":->6}|]
"tail \"hello\" = --ello"

However the expression must not contain } or : characters.


Most options can be combined. This generally leads to totally unreadable format string ;)

>>> [fmt|{pi:~>5.2}|]

Multi-line strings

You can ignore a line break with \ if needed. For example:

- a
- b\

Will returns -a\n-b. Note how the first and last line breaks are ignored.

Arbitrary value for precision

The precision field can be any haskell expression instead of a fixed number:

>>> [fmt|{pi:.{1+2}}|]

Output type

PyF aims at extending the string literal syntax. As such, it default to String type. However, if the OverloadedString is enabled, PyF will happilly generate IsString t => t instead. This means that you can use PyF to generate String, but also Text and why not ByteString, with all the caveats known to this extension.

>>> [fmt|hello {pi.2}|] :: String
"hello 3.14"

Custom types

PyF can format three categories of input types:

  • Floating. Using the f, g, e, … type specifiers. Any type instance of RealFloat can be formated as such.
  • Integral. Using the d, b, x, o, … type specifiers. Any type instance of Integral can be formated as such.
  • String. Using the s type specifier. Any type instance of PyFToString can be formated as such.

See PyF.Class if you want to create new instances for the PyFToString class.

By default, if you do not provide any type specifier, PyF uses the PyFClassify type class to decide if your type must be formated as a Floating, Integral or String.


Type inference

Type inference with numeric literals can be unreliable if your variables are too polymorphic. A type annotation or the extension ExtendedDefaultRules will help.

>>> v = 10 :: Double
>>> [fmt|A float: {v}|]
A float: 10

Error reporting

Template haskell is generally known to give developers a lot of frustration when it comes to error message, dumping an unreadable piece of generated code.

However, in PyF, we took great care to provide clear error reporting, this means that:

  • Any parsing error on the mini language results in a clear indication of the error, for example:
>>> [fmt|{age:.3d}|]

<interactive>:77:4: error:
    • <interactive>:1:8:
1 | {age:.3d}
  |        ^
Type incompatible with precision (.3), use any of {'e', 'E', 'f', 'F', 'g', 'G', 'n', 's', '%'} or remove the precision field.
  • Error in variable name are also readable:
>>> [fmt|{toto}|]
<interactive>:78:4: error: Variable not in scope: toto
  • However, if the interpolated name is not of a compatible type (or too polymorphic), you will get an awful error:
>>*> [fmt|{True:d}|]

<interactive>:80:10: error:
    • No instance for (Integral Bool)
        arising from a use of ‘PyF.Internal.QQ.formatAnyIntegral’
  • There is also one class of error related to alignement which can be triggered, when using alignement inside sign (i.e. =) with string:
*PyF PyF.Internal.QQ> [fmt|{"hello":=10}|]

<interactive>:89:10: error:
    • String Cannot be aligned with the inside `=` mode
  • Finally, if you make any type error inside the expression field, you are on your own:
>>> [fmt|{3 + pi + "hello":10}|]

<interactive>:99:10: error:
    • No instance for (Floating [Char]) arising from a use of ‘pi’

Custom Delimiters

If { and } does not fit your needs, for example if you are formatting a lot of json, you can use custom delimiters. All quasi quoters have a parametric form which accepts custom delimiters. Due to template haskell stage restriction, you must define your custom quasi quoter in an other module.

For example, in MyCustomDelimiter.hs:

module MyCustomQQ where

import Language.Haskell.TH.Quote

import PyF

myCustomFormatter :: QuasiQuoter
myCustomFormatter = fmtWithDelimiters ('@','!')

Later, in another module:

import MyCustomQQ

-- ...

[myCustomFormatter|pi = @pi:2.f!|]

Escaping still works by doubling the delimiters, @@!!@@!! will be formatted as @!@!.

Difference with the Python Syntax

The implementation is unit-tested against the reference python implementation (python 3.6.4) and should match its result. However some formatters are not supported or some (minor) differences can be observed.

Not supported

  • Number n formatter is not supported. In python this formatter can format a number and use current locale information for decimal part and thousand separator. There is no plan to support that because of the impure interface needed to read the locale.
  • Python support sub variables in the formatting options in every places, such as {expression:.{precision}}. We only support it for precision. This is more complexe to setup for others fields.
  • Python literal integers accepts binary/octal/hexa/decimal literals, PyF only accept decimal ones, I don’t have a plan to support that, if you really need to format a float with a number of digit provided as a binary constant, open an issue.
  • Python support adding custom formatters for new types, such as date. This may be really cool, for example [fmt|{today:%Y-%M-%D}. I don’t know how to support that now.


  • General formatters g and G behaves a bit differently. Precision influence the number of significant digits instead of the number of the magnitude at which the representation changes between fixed and exponential.
  • Grouping options allows grouping with an _ for floating point, python only allows ,.
  • Custom delimiters

Build / test

Should work with stack build; stack test, and with cabal and (optionally) nix:

nix-shell # Optional, if you use nix
cabal new-build
cabal new-test

Library note

PyF.Formatters exposes two functions to format numbers. They are type-safe (as much as possible) and comes with a combination of formatting options not seen in other formatting libraries:

>>> formatIntegral Binary Plus (Just (20, AlignInside, '~')) (Just (4, ',')) 255


Don’t hesitate to make any suggestion, I’ll be more than happy to work on it.


Revision history for PyF – 2019-12-29

  • Any type with Show instance can be formatted using :s formatter. For example, [fmt|hello {(True, 10):s}|]. This breaks compatibility because previous version of PyF was generating an error when try to format to string anything which was not a string, now it accepts roughly anything (with a Show instance). – 2019-11-08

  • Bump megaparsec bounds – 2019-10-13

  • Compatibility with GHC 8.8 – 2019-09-03

  • Precision can now be any arbitrary haskell expression, such as [fmt|hello pi = {pi:.{1 + 3}}|]. – 2019-08-27

  • (minor bugfix in tests): Use python3 instead of “python” to help build on environment with both python2 and python3 – 2019-08-27

  • Stack support – 2019-08-06

  • f (and fWithDelimiters) were renamed fmt (fmtWithDelimiters). f was causing too much shadowing in any codebase.
  • PyF now exposes the typeclass PyFToString and PyFClassify which can be extended to support any type as input for the formatters.
  • PyF now uses Data.String.IsString t as its output type if OverloadedString is enabled. It means that it behaves as a real haskell string literal.
  • A caveat of the previous change is that PyF does not have instances for IO anymore.

bugfixes and general improvements

  • An important amount of bugfixs
  • Error reporting for generic formatting (i.e. formatting without a specified type) is now more robust
  • Template haskell splices are simpler. This leads to more efficient / small generated code and in the event of this code appears in a GHC error message, it is more readable.
  • PyF now longer emit unnecessary default typing. – 2019-02-28

  • Tests: fix non reproducible tests – 2019-02-27

  • Fixed: PyF now uses the same haskell extensions as the one used by the current haskell file to parse sub expressions. – 2019-02-11

  • Fixed: PyF was wrongly ignoring everything located after a non-doubled closing delimiter.
  • New Feature: line break can be escaped with , thus allowing string to start on a new line ignoring the initial backspace – 2019-02-04

  • Bump dependencies to megaparsec 7
  • Error message are now tested
  • Name in template haskell splices are stable. This increases readability of error messages
  • Better error message for badly formated expression

Formatting removal

  • All monomorphic quasiquoters (f, fString, fText, fIO, fLazyText) are removed
  • Polymophic quasiquoter f' is renamed f and is the only entry point. Monomorphic users are encouraged to use the polymorphic quasiquoter with type annotation.
  • Formatting dependency is removed.
  • Previously named f quasiquoters which was exporting to Formatting.Format is removed. User of this behavior should use instead. – 2018-08-03

  • Custom delimiters, you can use whatever delimiters you want in place of { and }. – 2018-08-02

  • Fix the espace parsing of {{ and }} as { and } – 2018-04-16

  • Support for negative zero
  • Support for 0 modifier
  • Exponential formatter now behaves as python
  • Support for alternate floatting point represenation
  • Lot of documentation
  • Test are auto verified with the python reference implementation – 2018-04-13

  • Support for grouping option
  • Support for inner allignment
  • Correct display of NaN and Infinity
  • Fix a few cosmetic with python implementation
  • Introduce PyF.Formatters, type safe generic number formatter solution
  • Remove dependency to scientific – 2018-04-01

  • Support for haskell subexpression – 2018-01-07

  • Add support for the sign field. – 2018-01-03

  • First version. Released on an unsuspecting world.