Today I am going to introduce handy tools which help you build your Haskell project continuously so that you can see the list of errors and warnings quickly as you program.

Stack

stack build command has --file-watch option. When turned on, stack watches for changes in local files and automatically rebuild.

stack build --file-watch

Use --fast option if you want fast build which turns off optimizations (-O0). Also use --pedantic flag if you want to fix all warnings(-Wall and -Werror).

stack build --file-watch --fast --pedantic

ghcid

Neil Mitchell’s ghcid provides a similar functionality in a different way. It runs GHCi as a daemon and runs :reload whenever your source code changes.

ghcid executes stack ghci by default if you have stack.yaml file and .stack-work directory.

ghcid

If you would like to give a custom command, use --command option.

ghcid "--command=ghci Main.hs"

ghcid is much faster than stack build because it uses GHCi.

Steel Overseer

If you want to run arbitrary commands when arbitrary files change, use Steel Overseer instead. You can specify the pattern and commands in .sosrc file using YAML syntax. The following example has two rules.

• Watch *.hs files under System directory and run stack build.
• Watch *.hs files under test directory and run stack test.

- pattern: src/(.*)\.hs
  commands:
  - stack build
- pattern: test/(.*)\.hs
  commands:
  - stack test

sos command watches the specified files and runs the corresponding commands.

sos

Wrap-up

These small tools greatly increase your productivity. Please choose one and enjoy instant feedback!