PyF: Quasiquotations for a python like interpolated string formatter

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Quasiquotations for a python like interpolated string formatter.

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Change log
Dependencies base (>=4.9 && <5.0), bytestring, ghc, ghc-boot, mtl, parsec, template-haskell, text, time [details]
License BSD-3-Clause
Author Guillaume Bouchard
Category Text
Source repo head: git clone
Uploaded by guibou at 2021-10-29T22:21:13Z


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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:

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:formattingOptions} where formattingOptions 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:

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:

>>> [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.
>>> [fmt|{toto}|]
<interactive>:78:4: error: Variable not in scope: toto
>>*> [fmt|{True:d}|]

<interactive>:80:10: error:
    • No instance for (Integral Bool)
        arising from a use of ‘PyF.Internal.QQ.formatAnyIntegral’
*PyF PyF.Internal.QQ> [fmt|{"hello":=10}|]

<interactive>:89:10: error:
    • String Cannot be aligned with the inside `=` mode
>>> [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 = mkFormatter "fmtWithDelimiters" (fmtConfig {
  delimiters = ('@','!')

Later, in another module:

import MyCustomQQ

-- ...

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

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

Have a look at PyF.mkFormatter for all the details about customization.

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


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

There are a few available shells for you.

We also provide a few utility functions:

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

GHC compatibility

This library is tested in CI with ghc 8.6 to 9.2.


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


Everything works with nix. But you can also try with manual cabal / stack if you wish.

Once in the shell, use cabal build, cabal test, cabal repl.

There is a cachix available, used by CI:

You can locally build and test everything using:

Don't hesitate to submit a PR not tested on all GHC versions.


The codebase is formatted with ormolu and this is checked in the CI. Please run:

Before submitting.