How can I depend on a lower version purs?

[chrissound]

spago build           
[error] Oh noes! It looks like the PureScript version installed on your system is not compatible with the package-set you're using.

installed `purs` version:    0.13.6
minimum package-set version: 0.13.8

Getting the very latest of purs installed sounds like a bit of work, if I can instead downgrade the dependency how do I do this? I assume I need to change this line:

let upstream =
      https://github.com/purescript/package-sets/releases/download/psc-0.13.8-20200708/packages.dhall sha256:df5b0f1ae92d4401404344f4fb2a7a3089612c9f30066dcddf9eaea4fe780e29

Though it's unclear what the sha256 refers to? How do I get this value for an older release like https://github.com/purescript/package-sets/releases/tag/psc-0.13.6-20200507 ?

[chrissound]

Hmmm I just spotted the actual packages.dhall file at the end but that doesn't seem to work as I expect.

wget https://github.com/purescript/package-sets/releases/download/psc-0.13.6-20200507/packages.dhall
sha256sum packages.dhall 
89d54bc428494100ebf4f3afcbe33747b7cee9cde3d76746d3e6a94fd26142f9  packages.dhall

However if I add those values to packages.dhall:

spago build
spago: 
↳ ./spago.dhall
  ↳ ./packages.dhall
    ↳ https://github.com/purescript/package-sets/releases/download/psc-0.13.6-20200507/packages.dhall sha256:89d54bc428494100ebf4f3afcbe33747b7cee9cde3d76746d3e6a94fd26142f9

Error: Import integrity check failed

Expected hash:

↳ 89d54bc428494100ebf4f3afcbe33747b7cee9cde3d76746d3e6a94fd26142f9

Actual hash:

↳ 9c1e8951e721b79de1de551f31ecb5a339e82bbd43300eb5ccfb1bf8cf7bbd62


122│       https://github.com/purescript/package-sets/releases/download/psc-0.13.6-20200507/packages.dhall
123│       sha256:89d54bc428494100ebf4f3afcbe33747b7cee9cde3d76746d3e6a94fd26142f9

/home/chris/NewProjects/PsCharts/packages.dhall:122:7

[chrissound]

Found a solution here: https://github.com/purescript/spago#override-a-package-in-the-package-set-with-a-remote-one

I omitted the hash and it worked. I'm leaving this open to clarify hash check though.

[f-f]

@chrissound I think we have two separate problems to look at here:

1. the title/main issue is a duplicate of Feature request: spago init chooses most recent package set compatible with available compiler #657 I think?
2. the story around hashes is not super clear - how do you think we could clarify it? What kind of explanation around this would you feel it's appropriate?

[chrissound]

1. Hard to say, that seems to be about 'picking' an alternative version. While my question was how do I actually do that?

2. Well, my question here was "how" do I determine the sha256sum? Is it from the tar file from github? Or something else? Cause thats what I tried and yet it generated an incorrect hash iirc.

:) I only looked at this briefly so I might be talking over my head here.

[f-f]

Right, great points:

1. there are many releases for package-sets, so one can just edit the package.dhall to point at the desired one
2. the hash is a Dhall hash, and Spago will generate one for you so you don't have to worry about it. It's there mostly to ensure that:
   1. the package set is cached
   2. the file it points to will not change over time

Makes sense?

[f-f]

I think this has been solved now that we have the --tag flag from #680?

See documentation here, your initial question would be solved by using

spago upgrade-set --tag psc-0.13.6

I wonder how we should document this so that it's discoverable?