[Nix-dev] Which is the best way to pass dependencies?

[Marc Weber]

passing deps:

dep graph ( C depends on B,D depending on A), thus A is found twice when traversing deps of C !

A - > B ->  C
  `-> D -> ` 


example what happens when building C:

(A) Using kind of register script (I think this is like the longer existing ghc implemetnation does it?)
     A1  using propagatedBuildIpnuts:
                # create local package db
                echo [] > localpkgdb.deps
                runSetupHooksOfDeps # all deps are registering themselves
                echo [] > localpkgdb.installto
                
                # build packages

                # expose this lib
                createSetupHook localpkgdb.installto # creates script registering package

(B) using env var to pass deps
     B1   using propagatedBuildIpnuts
                # create local package db
                runSetupHooksOfDeps # all deps are only adding their local dbs to GHC_PACKAGE_DB
                nix_ghc_tool --local-db-from-env localpkgdb.deps # reads GHC_PACKAGE_DB and joins all those listed files
                echo [] > localpkgdb.installto

                # build packages

                # expose this lib
                createSetupHook localpkgdb.installto # create script adding localdb to GHC_PACKAGE_DB

     B2   using buildinputs only
                # create local package db
                runSetupHooksOfDeps # all deps are only adding their local dbs to GHC_PACKAGE_DB
                nix_ghc_tool --local-db-from-env localpkgdb.installto # reads GHC_PACKAGE_DB and joins all those listed files
>>>                                                       ^^^^ we will install to this db thereby exposing the deps as well

                # build packages

                # expose this lib
                createSetupHook localpkgdb.installto # create script adding localdb to GHC_PACKAGE_DB

Here B2 makes some difference compared to B1 and A:

B2 is the only one exposing the build lib and its dependencies. This will be
more efficient if we get really big graphs somewhen in the future

I think B will be more efficient than A on cygwin because forking is expensive
(ghc-pkg is used to register packages)


Marc