hwk: Simple cli text processing with Haskell functions

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A commandline tool for text processing with Haskell functions, which can used in addition to awk, grep, sed, etc. It applies the function supplied on the commandline using hint to lines of standard input and outputs the results.

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Versions [RSS] 0.2.0, 0.3, 0.4, 0.5, 0.6
Change log ChangeLog.md
Dependencies base (<5), directory (>=, extra, filepath, hint, simple-cmd-args (>=0.1.2) [details]
License MIT
Copyright 2016-2017 Lukas Martinelli, 2020 Jens Petersen
Author Lukas Martinelli
Maintainer Jens Petersen <juhpetersen@gmail.com>
Category Development
Home page https://github.com/juhp/hwk
Source repo head: git clone https://github.com/juhp/hwk.git
Uploaded by JensPetersen at 2020-10-12T20:06:46Z
Distributions Fedora:0.6, LTSHaskell:0.6, NixOS:0.6
Executables hwk
Downloads 568 total (9 in the last 30 days)
Rating (no votes yet) [estimated by Bayesian average]
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Status Docs not available [build log]
Last success reported on 2020-10-12 [all 2 reports]

Readme for hwk-0.4

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hwk MIT licensed


hwk (pronounced "hawk") is a simple Haskell-based text processing commandline tool, somewhat similar to tools like awk, grep, sed. hwk applies composed pure Haskell functions to a list of strings from stdin, enabling text processing without having to remember an obscure DSL or cli options. This tool can also help to encourage people to think functionally.

hwk was originally written by Lukas Martinelli in 2016-2017: see the original README file.

hwk is pretty similar to Hawk, so you may also want to try that for a different more sophisticated monadic implementation. Some of main differences are:

  • hwk uses String for input for type simplicity, whereas hawk uses ByteString
  • hawk has special options for controlling input and output delimiters, whereas in hwk everything is roughly just [String] -> [String] (more details below)
  • by default hwk applies a function to the list of all the lines of stdin: hwk -l corresponds to hawk -m and hawk -a to hwk.


Some simple use-cases are in the examples directory.

Change and append a string to each line:

$ seq 0 2 | hwk --line '(++ ".txt") . show . (+1) . int'

or without line-mode: hwk 'map ((++ ".txt") . show . (+1) . int)'.

Sum all negative numbers:

$ seq -100 100 | hwk 'sum . filter (< 0) . ints'

The ints function transforms a list of strings into a list of ints

Factorials in your shell scripts!:

seq 10 12 | hwk --line 'let {fact 0 = 1; fact n = n * fact (n - 1)} in fact . int'

Extract data from a file:

$ cat /etc/passwd | hwk -l 'reverse . filter (/= "x") . take 3 . splitOn ":"' | head -3
0 root
1 bin
2 daemon

(uses splitOn from the extra library; -l is the short form of --line).

The argument passed to hwk must be a valid Haskell function: a function that takes a list of strings and returns a new list or a single value.

Check whether the input contains a certain string:

$ cat /etc/passwd | hwk --all 'bool "no" "yes" . isInfixOf "1000"'


hwk uses a Haskell configuration file ~/.config/hwk/Hwk.hs which provides the context for the hint evaluation of the supplied function. Hint (ghci) also checks the current directory when loading so one can also override the configuration on a directory basis.

The default Hwk module configuration imports Prelude, Data.List, Data.Char, and System.FilePath into the hint interpreter.

The first time hwk is run it sets up ~/.config/hwk/Hwk.hs.

You can add other modules to import or define your own functions in ~/.config/hwk/Hwk.hs.

After a hwk version update you may wish or have to update up your Hwk.hs file to take account of new changes: a copy of the latest default Hwk.hs is also put in ~/.config/hwk/ with the version suffix.


Either use the install.sh script, or install by cabal-install or stack as described below:

Install script from source tree or git

Use stack unpack hwk or git clone https://github.com/juhp/hwk.

Then go to the source directory and run the install.sh script, which

  • first runs stack install
  • then moves the binary installed by stack install to ~/.local/lib/hwk, and sets up a wrapper script ~/.local/bin/hwk which runs it.

You may wish to change the resolver in stack.yaml first: it is also used to determine the resolver used by the created hwk wrapper script.


If you are on a Linux distro with a system installed ghc and Haskell libaries, you can install with cabal install to make use of them.


Installing by stack is better if you do not have a system ghc and/or global system Haskell libraries installed.

Alternatively to install by hand: run stack install, and then run it with stack exec hwk ... using the same resolver.

How does hwk work?

  • hwk use the hint library to evaluate haskell functions on standard input.
  • By default it splits the input to a list of lines and applies the function to them
  • Use -a or --all to apply a function to all the input, or -l/--line to map the function on each line separately.
  • You can only typecheck the function or an expr with -t/--typecheck or evaluate an expr with -e/--eval.

Supported return types

The following return values are supported:

  • String
  • [String]
  • [[String]]
  • Int
  • [Int]
  • [[Int]]


Open an issue or pull request at https://github.com/juhp/hwk to report problems or make suggestions and contributions. Usage examples are also welcome.

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