This cabal project is an example of how Backpack module signatures are carried implicitly by intermediate "indefinite" packages which depend on the package which provides the signature, but without filling it.
-
coreis the package with the signature. -
intermediatedepends oncorebut it doesn't fill its signature. -
finaldepends onintermediateand fills its outstanding signature with an implementation.
Example compilation on Windows:
PS C:\Users\someuser\hs\backpack-intermediate> cabal clean
PS C:\Users\someuser\hs\backpack-intermediate> cabal build all
Resolving dependencies...
Build profile: -w ghc-9.0.1 -O1
In order, the following will be built (use -v for more details):
- core-0.1.0.0 (lib) (first run)
- final-0.1.0.0 (lib:impl-lib) (first run)
- intermediate-0.1.0.0 (lib) (first run)
- core-0.1.0.0 (lib with Core.SomeSig=final-0.1.0.0-inplace-impl-lib:Core.SomeImpl) (first run)
- intermediate-0.1.0.0 (lib with Core.SomeSig=final-0.1.0.0-inplace-impl-lib:Core.SomeImpl) (first run)
- final-0.1.0.0 (exe:final) (first run)
Configuring library for core-0.1.0.0..
Configuring library 'impl-lib' for final-0.1.0.0..
Preprocessing library 'impl-lib' for final-0.1.0.0..
Building library 'impl-lib' for final-0.1.0.0..
Preprocessing library for core-0.1.0.0..
Building library instantiated with Core.SomeSig = <Core.SomeSig>
for core-0.1.0.0..
[1 of 1] Compiling Core.SomeImpl ( lib\Core\SomeImpl.hs, C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\final-0.1.0.0\l\impl-lib\build\impl-lib\Core\SomeImpl.o )
[1 of 2] Compiling Core.SomeSig[sig] ( lib\Core\SomeSig.hsig, nothing )
[2 of 2] Compiling Core ( lib\Core.hs, nothing )
Configuring library for intermediate-0.1.0.0..
Configuring library instantiated with
Core.SomeSig = final-0.1.0.0-inplace-impl-lib:Core.SomeImpl
for core-0.1.0.0..
Preprocessing library for intermediate-0.1.0.0..
Building library instantiated with Core.SomeSig = <Core.SomeSig>
for intermediate-0.1.0.0..
[1 of 2] Compiling Core.SomeSig[sig] ( C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\intermediate-0.1.0.0\build\autogen\Core\SomeSig.hsig, nothing )
[2 of 2] Compiling Intermediate ( lib\Intermediate.hs, nothing )
Preprocessing library for core-0.1.0.0..
Building library instantiated with
Core.SomeSig = final-0.1.0.0-inplace-impl-lib:Core.SomeImpl
for core-0.1.0.0..
[1 of 2] Compiling Core.SomeSig[sig] ( lib\Core\SomeSig.hsig, C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\core-0.1.0.0\core-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\build\core-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\Core\SomeSig.o )
[2 of 2] Compiling Core ( lib\Core.hs, C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\core-0.1.0.0\core-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\build\core-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\Core.o )
Configuring library instantiated with
Core.SomeSig = final-0.1.0.0-inplace-impl-lib:Core.SomeImpl
for intermediate-0.1.0.0..
Preprocessing library for intermediate-0.1.0.0..
Building library instantiated with
Core.SomeSig = final-0.1.0.0-inplace-impl-lib:Core.SomeImpl
for intermediate-0.1.0.0..
[1 of 2] Compiling Core.SomeSig[sig] ( C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\intermediate-0.1.0.0\intermediate-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\build\intermediate-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\autogen\Core\SomeSig.hsig, C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\intermediate-0.1.0.0\intermediate-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\build\intermediate-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\Core\SomeSig.o )
[2 of 2] Compiling Intermediate ( lib\Intermediate.hs, C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\intermediate-0.1.0.0\intermediate-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\build\intermediate-0.1.0.0-inplace+Eus3r8iu6ksDpNBLnxENGw\Intermediate.o )
Configuring executable 'final' for final-0.1.0.0..
Preprocessing executable 'final' for final-0.1.0.0..
Building executable 'final' for final-0.1.0.0..
[1 of 1] Compiling Main ( Main.hs, C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\final-0.1.0.0\x\final\build\final\final-tmp\Main.o )
Linking C:\Users\someuser\hs\backpack-intermediate\dist-newstyle\build\x86_64-windows\ghc-9.0.1\final-0.1.0.0\x\final\build\final\final.exe ...
Example execution:
PS C:\Users\someuser\hs\backpack-intermediate> cabal run final
Up to date
****** 5 plus bar
React
Typescript
Django
Laravel
Facebook
Microsoft
Google
Alibaba
D3
Tencent