[Haskell-cafe] Standard package file format

[yogsototh]

> I guess the overriding question I have here is: what is the PROBLEM being 
> solved?
>

Let me share my experience with Clojure and lein. They use a clojure 
hash-map for their configuration. So yes arbitrary code could be executed 
and I believe this is a _very good thing_.

Why? Because it makes it very easy to add sub-configuration that can be 
used by third party plugin. For example:

- a plugin that help the use of environment variables (lein-environ) which 
is really helpful for application development (not so much for library 
development)
- a plugin that use S3 for our private dependencies (not supported by 
default by lein)


For deployment: we were able to add request to our API server that provide 
not only the written version but also the git commit hash. So we could be 
certain of the version of the server. Too much time there were sys/admin 
deployment errors. And that could only be achieved because we were able to 
run arbitrary command in the project description file.

I certainly forget many other advantages of having a package description 
format which is simply a data structure in the hosted language. But this 
has by far my preference.

- cabal is ok, but very imperfect, I generally need to have a lot of 
copy/paste, I need to change it very often while writing application with 
many dependencies
- JSON/YAML/TOML are simply not powerful enough to match all semantics we 
might need to configure a project. For example we might want to have Set 
instead of List for some properties. Or I don't know maybe ternary tree 
structures.

The point is: we pay a price by adding a step between the semantic and the 
syntax.
While if our configuration format was in Haskell we could express the 
semantic more directly.
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