PyF: Quasiquotations for a python like interpolated string formater

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Quasiquotations for a python like interpolated string formater.

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Dependencies base (>=4.9 && <5.0), containers (>=0.5 && <0.6), formatting (>=6.2 && <6.4), haskell-src-meta, megaparsec (>=6.0 && <6.6), template-haskell (>=2.11 && <2.14), text (>=0.11 && <1.3) [details]
License BSD-3-Clause
Author Guillaume Bouchard
Category Text
Source repo head: git clone
Uploaded by guibou at 2018-08-10T20:46:23Z
Distributions LTSHaskell:, NixOS:, Stackage:
Reverse Dependencies 4 direct, 7 indirect [details]
Downloads 13635 total (255 in the last 30 days)
Rating 2.0 (votes: 1) [estimated by Bayesian average]
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Status Docs available [build log]
Last success reported on 2018-08-10 [all 1 reports]

Readme for PyF-

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PyF is a Haskell library for string interpolation and formatting.

PyF exposes a quasiquoter f for the Formatting library. The quasiquotation introduces string interpolation and formatting with a mini language inspired from printf and Python.

Quick Start

The following Formatting example:

>>> import Formatting

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

>>> format ("Person's name is " % text % ", age is " % hex) name age
"Person's name is Dave, age is 36"

can be written as:

>>> import Formatting
>>> import PyF

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

>>> format [f|Person's name is {name}, age is {age:x}|]
"Person's name is Dave, age is 36"

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.

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"
>>> format [f|{name:<11}|]
"Guillaume  "
>>> format [f|{name:>11}|]
"  Guillaume"
>>> format [f|{name:|^13}|]

Padding inside = the sign:

>>> [fString|{-pi:=10.3}|]
"-    3.142"

Float rounding

>>> format [f|{pi:.2}|]

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

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


Using , or _.

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

Sign handling

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

>>> [fString|{pi:+.3}|]
>>> [fString|{pi: .3}|]
" 3.142"


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

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


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

>>> format [f|2pi = {2* pi:.2}|]
>>> format [f|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 ;)

>>> format [f|{pi:~>5.2}|]

Other quasiquoters

PyF main entry point is f but for convenience some other quasiquoters are provided:

  • f(StrictText|LazyText|String|Builder|IO) directly call the underlying Formatting runner and produce the specified type.
  • f' use type inference to deduce the type.

PyF reexport most of Formatting runners, such as format, sformat, formatToString, ...

For example:

>>> [f'|hello {pi.2}|] :: String
"hello 3.14"
>>> :type [fString|hello|]


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
>>> [f|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:
>>> [f|{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:
>>> [f|{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:
>>*> [fString|{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. This can fail in two flavors:
>>> [fString|{"hello":=10s}|]

<interactive>:88:1: error:
    • Exception when trying to run compile-time code:
        String Cannot be aligned with the inside `=` mode
CallStack (from HasCallStack):
  error, called at src/PyF/Internal/QQ.hs:143:18 in PyF-
      Code: quoteExp fString "{\"hello\":=10s}"
    • In the quasi-quotation: [fString|{"hello":=10s}|]


*PyF PyF.Internal.QQ> [fString|{"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:
>>> [fString|{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 = fStringWithDelimiters ('@','!')

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, such as {varname:.{precision}}, we should too. However should we accept String parameter (such as <), with a possible runtime error, or should we use the ADT such as AlignRight?
  • Python literal integers accepts binary/octal/hexa/decimal literals, PyF only accept decimal ones, hdece in to 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 [f|{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


  • Improve the error reporting with more Parsec annotation
  • Improve the parser for sub-expression (handle the : and } cases if possible).
  • Allow extension to others type / custom formatters (for date for example)
  • Improve code quality. This code is really ugly, but there is a really strong test suite so, well.
  • Work on performance, do we really care? For now, everything is internally done with String.
  • Directly expose the formatter to be used as a template haskell splice

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.