Request: 'cabal freeze --dry-run --no-installed-constraint'

[mietek]

Cabal automatically sets global installed constraints for installed packages, even during a dry run. Hence, if a local constraint conflicts with an installed package, Cabal will fail to resolve dependencies.

$ cat >fnord.cabal <<EOF
name:           fnord
version:        0.0
build-type:     Simple
cabal-version:  >= 1.2

executable fnord
  build-depends:  base == 4.7.0.0
EOF

$ cabal --no-require-sandbox freeze --dry-run
Resolving dependencies...
cabal: Could not resolve dependencies:
trying: fnord-0.0 (user goal)
next goal: base (dependency of fnord-0.0)
rejecting: base-4.7.0.1/installed-1a5... (conflict: fnord => base==4.7.0.0)
rejecting: base-4.7.0.1, 4.7.0.0, 4.6.0.1, 4.6.0.0, 4.5.1.0, 4.5.0.0, 4.4.1.0,
4.4.0.0, 4.3.1.0, 4.3.0.0, 4.2.0.2, 4.2.0.1, 4.2.0.0, 4.1.0.0, 4.0.0.0,
3.0.3.2, 3.0.3.1 (global constraint requires installed instance)
Dependency tree exhaustively searched.

This makes it difficult to automatically determine the version of GHC needed to install a package, as it is no longer sufficient to just look at Cabal constraints. An option to prevent Cabal from setting global installed constraints would be appreciated.

[23Skidoo]

/cc @benarmston

[dcoutts]

What we want here is the solver as a library. There's no call for this in cabal-install as it is now. (Though one could imagine a dedicated command for producing the plan in some format)

[bgamari]

For the record, #2768 does exactly what @dcoutts suggests.

[mietek]

Closing as no longer relevant.