fmt: A new formatting library

[ bsd3, library, text ] [ Propose Tags ]

A new formatting library that tries to be simple to understand while still being powerful and providing more convenience features than other libraries (like functions for pretty-printing maps and lists, or a function for printing arbitrary datatypes using generics).

A comparison with other libraries:

  • printf (from Text.Printf) takes a formatting string and uses some type tricks to accept the rest of the arguments polyvariadically. It's very concise, but there are some drawbacks – it can't produce Text (you'd have to T.pack it every time) and it doesn't warn you at compile-time if you pass wrong arguments or not enough of them.

  • text-format takes a formatting string with curly braces denoting places where arguments would be substituted (the arguments themselves are provided via a tuple). If you want to apply formatting to some of the arguments, you have to use one of the provided formatters. Like printf, it can fail at runtime, but at least the formatters are first-class (and you can add new ones).

  • formatting takes a formatting template consisting of pieces of strings interleaved with formatters; this ensures that arguments always match their placeholders. formatting provides lots of formatters and generally seems to be the most popular formatting library here. Unfortunately, at least in my experience writing new formatters can be awkward and people sometimes have troubles understanding how formatting works.

  • fmt (i.e. this library) provides formatters that are ordinary functions, and a bunch of operators for concatenating formatted strings; those operators also do automatic conversion. There are some convenience formatters which aren't present in formatting (like ones for formatting maps, lists, converting to base64, etc). Some find the operator syntax annoying, while others like it.


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Versions [RSS],,,,,,,,, 0.6, 0.6.1,,,,
Change log
Dependencies base (>=4.6 && <5), base64-bytestring, bytestring, call-stack, containers, formatting (>=7.0.0), microlens (>=0.3), semigroups (>=0.18 && <0.19), text, time, time-locale-compat [details]
License BSD-3-Clause
Author Artyom Kazak <>, Dmitry Kovanikov <>
Maintainer Chris Dornan <>
Category Text
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Source repo head: git clone git://
Uploaded by ChrisDornan at 2021-09-06T08:07:16Z
Distributions Fedora:, LTSHaskell:, NixOS:, Stackage:
Reverse Dependencies 31 direct, 14 indirect [details]
Downloads 18362 total (104 in the last 30 days)
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Status Docs available [build log]
Last success reported on 2021-09-06 [all 1 reports]