issues with configurations

[Einar Karttunen]

Hello

This is a formulation after more discussion on #haskell.

This is for build-depends with the configurable things.
Not sure whether this is the best way.

- Einar Karttunen


bexp = package-with-version
     | '(' bexp ')'
     | bexp '|' bexp
     | bexp ',' bexp
     | 'Flag(' string ')'
     | 'T'

Semantics:

package-with-version
   * satisfied iff a suitable version of P installed
   * depends on p

'(' e ')'
   * satisfied iff e satisfied
   * dependencies from e

a '|' b
   * satisfied iff a or b is satisfied
   * if a is satisfied then a else b

a ',' b
   * satisfied iff a and b are satisfied
   * dependencies are the combination of a and b
   * union of packages and intersection of versions

'Flag(' string ')'
   * satisfied iff string in environment
   * no dependencies

'T'
   * always satisfied
   * no dependencies


Code follows:

data BExp = BP String Version
          | BOr  BExp BExp
          | BAnd BExp BExp
          | BFlag String
          | BTrue


eval (BP n v)   = return [(n,v)]
eval (BOr a b)  = eval a `mplus` eval b
eval (BAnd a b) = do x <- eval a
                     y <- eval b
                     return [(name, intersectVersions $ map snd xs) | xs@((name,_):_) <- groupByFst (x++y) ]
liftM2 (comb) (eval a) (eval b)
eval (BFlag s)  = getFlag s >> return []
eval (BTrue)    = return []

groupByFst = groupBy eqFst where eqFst (a,_) (b,_) = a == b




- Einar Karttunen