Configurations proposal

[Einar Karttunen]

On 25.10 01:36, Duncan Coutts wrote:
> I would like to re-propose the last scheme that I cam up with. I'll try
> to make it a concrete proposal as well as giving some motivating
> examples to give the intuition of the meaning.

Nice to get this going, this is the most important missing feature in
Cabal from my perspective.

> So I propose two kinds of stanza: flags and configurations.
> 
> flag ::= "flag:" name
>          "default:" fexp
> 
> fexp ::= "available"( dependency )
>        | "os"( STRING )
>        | "arch"( STRING )
>        | fexp || fexp
>        | fexp && fexp
>        | !fexp
>        | True | False
> 
> conf ::= "configuration:" cexp
>          fields
> 
> cexp ::= "flag"( STRING )
>        | "using"( dependency )
>        | "os"( STRING )
>        | "arch"( STRING )
>        | cexp || cexp
>        | cexp && cexp
>        | !cexp
>        | True | False

Add '(' cexp ')' and '(' fexp ')' to the syntax.

> As for the meaning, it's not clear to me if it should be that each test
> is independent of if all constraints must be satisfied on a single
> package simultaneously. That is if we have a expression like:
>  available ( P > 1) && available ( P < 1)
> does there have to be a single package P with versions satisfying both
> constraints (ie impossible) or if the tests are independent and so may
> be satisfied by there being both P-0.9 and P-1.1 available.
> 
> I'm not sure it really matters, so we should go for the simple
> independent test meaning. Opinions?

We can already do "available( P > 1 & P < 1)" (the version syntax
supports such things). I think simple independent test makes things
easier to understand. But there were some counter examples in the
old discussion I think.

> Here's a nasty example:
> 
> configuration: using(P = 1.1)
> build-depends: P = 1.0
> 
> configuration: using(P = 1.0)
> build-depends: P = 1.1

This can be solved quite easily:

1) there is no prior build-depend on P => we are not using P so ignore these.
2) we depend on P version V.
3A) V=1.1:
   the first configuration matches, and we signal an error as we
   require two different versions of P.
3B) V=2:
   neither configuration matches and we continue happily ignoring them.

version: