PyF

PyF is a Haskell library for string interpolation and formatting.

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

Quick Start

>>> :set -XQuasiQuotes
>>> 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: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

Padding

Left < / Right > / Around ^ padding:

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

Padding inside = the sign

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

Float rounding

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

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"

Grouping

Using , or _.

>>> [fmt|{10 ^ 9 - 1:,}|]
"999,999,999"
>>> [fmt|{2 ^ 32  -1:_b}|]
"1111_1111_1111_1111_1111_1111_1111_1111"

Sign handling

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

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

0

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

>>> [fmt|{10:010}|]
0000000010
>>> [fmt|{-10:010}|]
-000000010

Sub-expressions

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.

Combined

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

>>> [fmt|{pi:~>5.2}|]
"~~3.14"

Multi-line strings

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

[fmt|\
- a
- b\
|]

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

Arbitrary value for precision and padding

The precision and padding width fields can be any Haskell expression (including variables) instead of a fixed number:

>>> [fmt|{pi:.{1+2}}|]
3.142

>>> [fmt|{1986:^{2 * 10}d}|]
"        1986        "

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.

Caveats

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 messages, 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:

foo = [fmt|{age:.3d}|]

File.hs:77:19: error:
  |
1 | {age:.3d}
  |        ^
Type incompatible with precision (.3), use any of {'e', 'E', 'f', 'F', 'g', 'G', 'n', 's', '%'} or remove the precision field.

Note: error reporting uses the native GHC error infrastructure, so they will correctly appear in your editor (using HLS), for example:

• Error in variable name are also readable:

test/SpecUtils.hs:81:33: error:
    • Variable not found: chien
    • In the quasi-quotation: [fmt|A missing variable: {chien}|]
   |
81 | fiz = [fmt|A missing variable: {chien}|]
   |                                 ^^^^^

• 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, you’ll get an awful error in the middle of the generated Template Haskell splice.

>>> [fmt|{3 + pi + "hello":10}|]

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

Custom Delimiters

If { and } do 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 the Template Haskell stage restriction, you must define your custom quasi quoter in another 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.

Differences with the Python Syntax

The implementation was unit-tested against the reference Python implementation (Python 3.6.4) and should match its result. Since 2025, the standalone test suite is not cross checked with the standard python implementation. 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 supports sub variables in the formatting options in all places, such as {expression:.{precision}}. We only support it for precision and width. This is more complexe to setup for others fields.
• Python literal integers accept 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 supports 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.

Differences

• 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 build
cabal test

There are a few available shells for you.

• nix-shell is the default, current GHC version with language server available.
• nix-shell ./. -A pyf_xx.shell is another GHC version (change xx) without language server.
• nix-shell ./. -A pyf_xx.shell_hls is another GHC version (change xx) with language server.

We also provide a few utility functions:

• nix-build ./ -A hlint will check hlint.
• nix-shell ./ -A ormolu-fix will format the codebase.

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
"+~~~~~~~~~~1111,1111"

GHC compatibility

This library is tested in CI with ghc 8.6 to 9.12.

Conclusion

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

Hacking

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

• nix develop will open a shell with everything you need to work on PyF, including haskell-language-server. It may be a bit too much, so you can instead:
• nix develop .#pyf_XY opens a shell with a specific GHC version and without haskell-language-server. That’s mostly to test compatibility with different GHC version or open a shell without HLS if you are in a hurry. Replace pyf_XY by a GHC version, e.g. pyf_912.

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

There is a cachix available, used by CI, and already configured in flakes. You can manually run cachix use guibou if you want.

You can locally build and test everything using:

• nix flake check

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

Formatting

Please run:

• nix run fmt

Before submitting.

Treesitter support

Have a look in the ./tree-sitter-pyf/ directory for a parser of PyF which can be integrated in your treesitter compatible editor to get syntax highlighting for PyF.