[GHC] #16316: `-package-env=` in OPTIONS_GHC not supported

[GHC]

#16316: `-package-env=` in OPTIONS_GHC not supported
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I just realised we're missing an ingredient in order to support persistent
 package-environment workflows for standalone scripts:

 Consider the following use-cases:

 ----

 1.

 Script requires package environemnts disabled

 {{{#!hs
 #!/usr/bin/env runghc
 {-# OPTIONS_GHC -package-env=- #-}

 main = putStrLn "Hello World"
 }}}

 ----

 2.

 User manually maintains a package environment for scripts or teaching
 exercises (this is a quite common way to teach Haskell which I subscribe
 to, by letting students interact with ghc and ghci directly instead, and
 only properly introducing `cabal` lateron when it's actually needed)

 {{{
 cabal v2-install --lib lens lens-aeson --package-env=lets-lens
 }}}

 and the scripts shall be using that custom named package environment:

 {{{#!hs
 #!/usr/bin/env runghc

 {-# OPTIONS_GHC -package-env=lets-lens #-}

 import Control.Lens

 main = do
    putStrLn "Hello Lens"
    -- ...lens using code...
 }}}


 ----

 Currently both of these use-cases don't work because

 {{{
 $ ghci-8.6.3 Main.hs
 GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help

 Main.hs:3:16: error:
     unknown flag in  {-# OPTIONS_GHC #-} pragma: -package-env=lets-lens
   |
 3 | {-# OPTIONS_GHC -package-env=lets-lens #-}
   |                ^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I do realise that it's tricky to support package-db related commands in
 `OPTIONS_GHC` but it'd nevertheless be quite important to support this in
 some way (possibly with some restrictions) in order to properly support
 the package-env based workflows (NB: the legacy user pkg-db based ones
 aren't available anymore in future cabal versions -- package-environments
 have completely replaced the legacy user-pkg db there).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16316>
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