Hi everyone,
As part of the recent GHC 8.2.1 release and the associated boot-library
publishing to Hackage, I picked up again experimenting with a
reinstallable lib:ghc package primarily for the benefit of uploading
haddocks for lib:ghc (w/ `--hyperlinked-source`) to Hackage:
http://hackage.haskell.org/package/ghc
However, this also provides somewhat of a tech-preview of a
reinstallable lib:ghc package, which would help mitigate a big issue we
had in the past with lib:ghc and its dependencies being frozen to
specific versions, and which in combination with cabal's nix-style
pkg store that finally provide us with the ability to have multiple
instances of the same library versions co-existing in the same package
db, is no longer a requirement. So we could have lib:ghc more easily pick
dependencies up (but still within reason!) like e.g. lib:text without
forcing everyone to the single lib:text version bundled with GHC. And
we'd also gain the ability to publish patch-level releases to lib:ghc
only inbetween proper minor GHC releases (e.g. to support new versions
of library dependencies).
Consider this artificial example demo package which requires a different
version of binary than the one bundled with GHC 8.2.1:
--8<---cut here---start->8---
-- ghc-demo.cabal
name:ghc-demo
version: 0
build-type: Simple
cabal-version: >=2.0
executable ghc-demo
main-is: Main.hs
build-depends: base ^>= 4.10, ghc ^>= 8.2.1, binary == 0.8.5.0
default-language:Haskell2010
--8<---cut here---end--->8---
Trying to build this will have the cabal solver complain that the
installed instance of ghc requires a different (& installed) version of
binary, i.e. binary-0.8.5.1.
Moreover, the source version of lib:ghc on Hackage has the manual
`buildable` cabal flag which defaults to false, and currently prevents
lib:ghc to be reinstalled automatically (by enabling an unsatisfiable
constraint):
--8<---cut here---start->8---
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: base-4.10.0.0/installed-4.1... (dependency of ghc-demo-0)
next goal: ghc (dependency of ghc-demo-0)
rejecting: ghc-8.2.1/installed-8.2... (conflict: binary==0.8.5.0, ghc =>
binary==0.8.5.1/installed-0.8...)
trying: ghc-8.2.1
rejecting: ghc-8.2.1:-buildable (conflict: base==4.10.0.0/installed-4.1..., ghc
-buildable => base<0)
rejecting: ghc-8.2.1:+buildable (manual flag can only be changed explicitly)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: ghc-demo, binary, base, ghc,
ghc-8.2.1:buildable
--8<---cut here---end--->8---
However, if we unlock this by toggling the flag (NB: this only works on
linux/x86_64; it's explained later why), e.g.
--8<---cut here---start->8---
-- cabal.project file
packages: .
package ghc
flags: +buildable
--8<---cut here---end--->8---
The cabal solver would come up with an install-plan:
--8<---cut here---start->8---
Resolving dependencies...
Build profile: -w ghc-8.2.1 -O1
In order, the following will be built (use -v for more details):
- binary-0.8.5.0
{binary-0.8.5.0-7f686cf5c40c3c685540306709c4a2a30d740679f5d31ad196283d242a4d70ac}
(lib) (requires download & build)
- ghc-boot-8.2.1
{ghc-boot-8.2.1-7930c3d78690c5fa476415d38abbb7717b05cf65e80cc79c12e782c9886507cb}
(lib) (requires build)
- ghci-8.2.1
{ghci-8.2.1-6292924619a045fd18e9fde0cfa350632672ad7439cb84a5c56ff0e4781346b1}
(lib) (requires build)
- ghc-8.2.1
{ghc-8.2.1-bc10931b0f169622deb74d84cf10e315730dfd8f2edcf4cad9073d142cf5da0c}
(lib) +buildable (requires build)
- ghc-demo-0 {ghc-demo-0-inplace-ghc-demo} (exe:ghc-demo) (first run)
Configuring
binary-0.8.5.0-7f686cf5c40c3c685540306709c4a2a30d740679f5d31ad196283d242a4d70ac
(lib)
Building
binary-0.8.5.0-7f686cf5c40c3c685540306709c4a2a30d740679f5d31ad196283d242a4d70ac
(lib)
Installing
binary-0.8.5.0-7f686cf5c40c3c685540306709c4a2a30d740679f5d31ad196283d242a4d70ac
(lib)
Finished
binary-0.8.5.0-7f686cf5c40c3c685540306709c4a2a30d740679f5d31ad196283d242a4d70ac
(lib)
Configuring
ghc-boot-8.2.1-7930c3d78690c5fa476415d38abbb7717b05cf65e80cc79c12e782c9886507cb
(lib)
Building
ghc-boot-8.2.1-7930c3d78690c5fa476415d38abbb7717b05cf65e80cc79c12e782c9886507cb
(lib)
Installing
ghc-boot-8.2.1-7930c3d78690c5fa476415d38abbb7717b05cf65e80cc79c12e782c9886507cb
(lib)
Finished
ghc-boot-8.2.1-7930c3d78690c5fa476415d38abbb7717b05cf65e80cc79c12e782c9886507cb
(lib)
Configuring
ghci-8.2.1-6292924619a045fd18e9fde0cfa350632672ad7439cb84a5c56ff0e4781346b1
(lib)
Building
ghci-8.2.1-6292924619a045fd18e9fde0cfa350632672ad7439cb84a5c56ff0e4781346b1
(lib)
Installing
ghci-8.2.1-6292924619a0
