[Hackage] #437: generalise the package db settings and flags

[Hackage]

#437: generalise the package db settings and flags
----------------------------+-----------------------------------------------
  Reporter:  duncan         |        Owner:         
      Type:  enhancement    |       Status:  new    
  Priority:  normal         |    Milestone:         
 Component:  Cabal library  |      Version:  1.2.3.0
  Severity:  normal         |     Keywords:         
Difficulty:  normal         |   Ghcversion:  6.8.3  
  Platform:                 |  
----------------------------+-----------------------------------------------
 Currently the user has just one knob to turn to select which package
 databases to use. This is not enough control.

 In general we can use a whole stack of package dbs to look for dependent
 packages and we can use at most one package db to register a package into.
 If we use a whole stack then it seems sensible that the destination
 package db be the top one.

 So we should generalise the setting from a single value to a list of
 package dbs. There are some constraints for example the global package db
 generally cannot be omitted and has to be first.

 Currently we have the flags `--global`, `--user` and `--package-db`. The
 global and user flags affect several settings. They select the default
 install prefix and which package db to use. The `--package-db` flag
 overrides the choice of package db. Of course the user and specific db
 choices are taken to be on top of the global db.

 It seems sensible to leave the user and global flags as kind of general
 policy flags that affect the defaults of other things in one convenient
 way.

 The `--global` flag in isolation would correspond to the db stack
 `[GlobalPackageDB]`. The `--user` flag would correspond to
 `[GlobalPackageDB, UserPackageDB]`, though perhaps only if the user does
 not specify any other specific dbs.

 So here's a proposal:

  * `--package-db=package.conf.local` appends that db to the stack
  * The special values `--package-db=$global` and `--package-db=$global`
 correspond to those package dbs.
  * The db stack is always taken to be prefixed with the global one.
  * If the db stack would be empty and the `--user` flag is used then the
 stack includes the user db. So it's absent if any `--package-db` flag is
 given and would have to be added explicitly using `--package-db=$user`.

 It may be useful to have a `--package-dbs` flag that sets several dbs in
 one go to use with `--package-dbs=$GHC_PACKAGE_PATH`. See #335. However
 note that  ghc considers `$GHC_PACKAGE_PATH` in the search path style
 where an earlier path overrides later where as we typically have later
 flags override earlier ones.

 Examples:

  * `--global`
    == `[GlobalPackageDB]`

  * `--user`
    == `[GlobalPackageDB, UserPackageDB]`

  * `--user --package-db=foo`
    == `[GlobalPackageDB, SpecificPackageDB "foo"]`

  * `--user --package-db=$user --package-db=foo`
    == `[GlobalPackageDB, UserPackageDB, SpecificPackageDB "foo"]`


 If people find this confusing that `--package-db` automatically overrides
 `--user` then we would instead need a way of undoing the effect of
 `--user` so that people can get the stack `[GlobalPackageDB,
 SpecificPackageDB "foo"]` while in other respects doing using an ordinary
 per-user install. I should note that `--user --package-db=foo` currently
 does mean `[GlobalPackageDB, SpecificPackageDB "foo"]` and not
 `[GlobalPackageDB, UserPackageDB, SpecificPackageDB "foo"]`

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/437>
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