Build your Haskell project continuously

- January 28, 2017
Kwang's Haskell Blog - Build your Haskell project continuously

Build your Haskell project continuously

Posted on January 28, 2017 by Kwang Yul Seo

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 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


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.


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

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



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

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GHC typecheck build (no codegen)

- January 27, 2014
Kwang's Haskell Blog - GHC typecheck build (no codegen)

GHC typecheck build (no codegen)

Posted on January 27, 2014 by Kwang Yul Seo
Tags: Haskell, GHC, build

When programming in Haskell, I rebuild my program as frequently as possible to check if there is any type error. This is a good practice because Haskell finds most of bugs just by type checking.

However, as a program gets bigger, the longer it takes to build the program. This is problematic for a large project. Fortunately, GHC provides -fno-code option. When this option is given, GHC omits code generation and only performs type checking.

In combination with -fforce-recomp, one can force typecheck build with the following command:

cabal build --ghc-options="-fforce-recomp -fno-code"
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