stack support for yesod devel

June 29, 2015

GravatarMichael Snoyman

Since the release of the stack build tool, I think I've received four different requests and bug reports about stack support in Yesod. The sticking point is that yesod devel has not- until now- had support for using stack. The original plan was to wait for Njagi Mwaniki's Google Summer of Code project to finish the new ide-backend based yesod devel. However, demand for this feature was too high, so I'm happy to announce official stack support in yesod-bin-1.4.11.

This blog post should be consider a beta announcement for using Yesod with stack. Please test and report issues. Also, the workflow is not yet perfected. Once we get stack new written and add ide-backend support, things will be much smoother. For now, here's the workflow:

# Install both yesod-bin and cabal-install. Both are still necessary
$ stack install yesod-bin-1.4.11 cabal-install
# Initialize your project
$ stack exec yesod init
$ cd new-directory
# Create stack.yaml
$ stack init
# Build your project to get al dependencies
# Also rebuild yesod-bin with the current GHC, just to be safe
$ stack build yesod-bin-1.4.11 .
# Now run yesod devel
$ stack exec yesod devel

Like I said, this is a little kludgy right now, but will smooth out over time.

Technical details

If you're curious in how I added this support, the commit is really short. And there's not actually anything stack-specific in here, it's just working around a well known limitation in cabal which makes it incompatible with the GHC_PACKAGE_PATH environment variable. All we do in yesod devel is:

  • Detect the GHC_PACKAGE_PATH variable
  • Turn it into --package-db arugments to cabal configure
  • Remove GHC_PACKAGE_PATH from the environment of the cabal process

It would be nice if cabal got this functionality itself in the future, but I believe the current implementation was a design goal of the cabal team.

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