仅在没有堆栈的情况下构建 LTS 依赖项。yaml

Question

I would like to implement a multistage Dockerfile with three stages:

1. pull fpco/stack-build:lts-17.12 and build all LTS dependencies
2. copy stack.yaml and package.yml and build all extra-deps
3. copy . and build the actual project

So far I managed to separate 2 and 3, but 1 and 2 are still executed together, which means that every single time either stack.yaml or package.yaml change, the entire LTS needs to be recompiled. This takes roughly 20 minutes every time, whereas in local dev (without Docker), when I modify stack.yaml or package.yaml , only the "new stuff" gets compiled and it's usually very quick.

Doing this "LTS-only" first compilation in a separate Docker stage would allow Docker to cache the results and avoid recompiling everything all the time.

When I try to stack build --dependencies-only with no YAML files, Stack complains that this folder is not a Stack project directory.

Is it possible to pre-compile only the LTS with no yaml file and, thereafter, compile the project based on stack.yaml ?

Answer 1

It turns out that my question, as it is phrased, doesn't make sense.

Stack doesn't "compile the entire LTS". It compiles only the modules that the project's dependencies need. Therefore, it can't do anything without a stack.yaml and a package.yaml (or project.cabal ).

A workaround is to use a long-term subfolder with a stack.yaml and a package.yaml files listing the dependencies that will change very rarely (say aeson , text , bytestring ...).

project-dir/
    long-term/
        stack.yaml
        package.yaml
    stack.yaml
    package.yaml

Then in the first stage of the Dockerfile:

FROM fpco/stack-build:lts-17.12 as dependencies

# ...

COPY ./long-term/stack.yaml ./long-term/package.yaml ./
RUN stack build --test --no-run-tests --dependencies-only

COPY ./stack.yaml ./package.yaml ./stack.yaml.lock ./
RUN stack build --test --no-run-tests --dependencies-only

As long as these two long-term files aren't touched, Docker will happily reuse the cached results.

And now, making a one-line change into the "actual" stack.yaml or package.yaml doesn't need to recompile the whole world.

--

Loose inspiration taken from: https://gist.github.com/and-pete/e8fd63d51121c61d25461bcc2b3fe743