hwk

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

hwk tries to demonstrate how a modern Haskell based stream manipulation tool could look like. It is similar to tools like awk or sed. hwk allows compact function sequences that operate on a list of strings. Because Haskell is lazy and has a powerful arsenal of functions, there is no need to invent another DSL and hopefully it encourages more people to think functionally.

Example

Prepend a string to each line:

$ seq 1 3 | hwk 'map (++ ".txt")'
1.txt
2.txt
3.txt

Sum all negative numbers:

$ seq -100 100 | hwk 'sum . filter (< 0) . ints'
-5050

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

Extract data from a file:

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

(a module defining splitOn from the extra or split library needs to be added to the Hwk.hs config file).

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.

Configuration

It uses a configuration module Hwk which provides the context for the hint evaluation of the supplied function.

It searches for Hwk.hs in ~/.config/hwk, then the package's installed data directory.

The default configuration Hwk module just sets the Prelude, Data.List, and Data.Char modules to be imported by default into the hint interpreter.

If you want to use other modules or define your own functions, you can copy the installed Hwk.hs or source data/Hwk.hs file to ~/.config/hwk/ to configure hwk.

Install

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/bin/hwk-bin, and sets up a wrapper script ~/.local/bin/hwk which runs it.
• and also copies the Hwk.hs configuration module to ~/.config/hwk/Hwk.hs (backing up any existing file).

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

cabal

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.

If you install with a recent cabal the Hwk.hs config module probably lives somewhere like ~/.cabal/store/ghc-*/hwk-*/share/data/Hwk.hs, or you can copy it from the source data/Hwk.hs.

stack

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, To customize hwk after a stack install it is probably easier just to copy the data/Hwk.hs source file.

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: [String] -> ToList a
• Use -a or --all to apply a function to all the input: String -> Tolist a

Supported return types

By default the following instances of the ToList class are defined:

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

Contribute

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

Related/alternative projects

• https://github.com/gelisam/hawk
• https://code.google.com/p/pyp/
• https://en.wikipedia.org/wiki/AWK
• https://en.wikipedia.org/wiki/Sed
• https://github.com/bawolk/hsp