Haskell Shell Scripting with GHCI and Turtle

haskell-shell.md

Shell scripting with Haskell

Problem

Shell scripting is kinda broken:

• Shell scripting languages differ or are interpreted with subtle differences
• Higher level language concepts are typically missing

Can Haskell be a good replacement for Bash—or similar languages—as a shell scripting language?

Using GHCI for System Programing

Setup

1. Install Haskell Stack

   brew install haskell-stack

2. Install turtle using Stack

   stack install turtle

3. Define functions to setup/init/extend Haskell shell and create an alias hsh

   hsh_setup() {
     local hsh_home="${HOME}/.hsh"
   
     mkdir -p ${hsh_home}
   
     printf "[INFO] Creating hsh config"
     printf ':set -XOverloadedStrings\n:set prompt "λ> "\n:set +m\nimport Turtle\nimport Prelude hiding (FilePath)' > ${hsh_home}/.ghci
     printf "\n[INFO] hsh setup done"
   }
   
   hsh_extend() {
     local hsh_home="${HOME}/.hsh" \
   	  command=${1}
   
     printf "\n[INFO] Creating function for command %-25s" "${command}"
     printf "\n%1\$s = \\\cl -> shell (\"%1\$s \" <> cl) empty\n%1\$s' = \\\cl -> shell (\"%1\$s \" <> cl)" ${command} >> ${hsh_home}/.ghci
   }
   
   hsh_init() (
     local hsh_home="${HOME}/.hsh"
   
     hsh_setup 
   
     while getopts "e:" option; do
     case ${option} in
     e) hsh_extend ${OPTARG}
     esac 
     done
   
     cd ${hsh_home}
     stack ghci
   )
   
   alias hsh=hsh_init

4. Run Haskell Shell and have fun 🙌

   $ hsh -e git -e latexmk -e typora
   [INFO] Creating hsh config
   [INFO] hsh setup done
   [INFO] Creating function for command git
   [INFO] Creating function for command latexmk
   [INFO] Creating function for command typora
   Note: No local targets specified, so a plain ghci will be started with no package hiding or package options.
   
   You are using snapshot: lts-17.2
   
   If you want to use package hiding and options, then you can try one of the following:
   
   * If you want to start a different project configuration
   than /Users/main/.stack/global-project/stack.yaml, then you can use stack init to create a new stack.yaml for the
   packages in the current directory.
   
   * If you want to use the project configuration
   at /Users/main/.stack/global-project/stack.yaml, then you can add to its 'packages' field.
   
   Configuring GHCi with the following packages:
   GHCi, version 8.10.3: https://www.haskell.org/ghc/  :? for help
   Loaded GHCi configuration from /Users/main/.hsh/.ghci
   Loaded GHCi configuration from /private/var/folders/yz/b655kl71057db1lhwxhc0rt00000gs/T/haskell-stack-ghci/2a3bbd58/ghci-script

In the ghci REPL we can now use the Turtle library to do system programming similar to what we awould do in Bash. 🚀

The Good

There are a couple cool aspects of Haskell/GHCI that are being used to provide a proper shell feeling:

1. monads: most Turtle functions like dir and echo are monadic

   λ> :t pwd
   pwd :: MonadIO io => io FilePath

   and GHCI executes code within an IO monad; that means we can write in do-notation style and read/write from/to input/output

   λ> dir <- pwd

2. overloaded strings:

   E.g. echo has type echo :: MonadIO io => Line -> io () (i.e. expects Line not String) which can however use the overloaded string literal syntax

   λ> echo "Hi!"

3. printing of return values: types like FilePath are printable and GHCI prints every value that is entered so we can immediately see results of commands

   λ> dir <- pwd
   λ> dir
   FilePath "/Users/friedrichk/.hsh"

The Turtle tutorial on Hackage has more examples. Be sure to check it out.

The Bad

• No Tab-Completion for commands/paths
• No Syntax-Highlighting

Bonus Round

Launching GHCI Directly in Your Terminal Emulator

In a terminal emulator like iTerm, we can start hsh in every new window/tab in order to get a complete Haskell based Terminal feeling.

Future Work

ptghci adds useful features like syntax highlighting and tab completion and might be a good extension.

---

SOURCES

• https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/ghci-dot-files.html
• https://bobkonf.de/2017/slides/thoma.pdf