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    Top-level `freeze` command freezes dependency versions · c6138e81
    barmston authored
    Add new top-level `freeze` command, which resolves the dependencies to exact
    versions and writes a `constraints` section to `cabal.config`. This causes
    future builds to use the same fully constrained dependencies.
    
    The command takes a number of options related to resolving dependencies,
    namely, `--solver`, `--max-backjumps`, `reorder-goals` and
    `--shadow-installed-packages`. These are used to create an `InstallPlan` in
    much the same way that `install` does so. The `InstallPlan` is converted to a
    list and all `PlanPackage`s are inspected to determine their exact version.
    These versions are then either written to `cabal.config` or to standard output
    depending on the presence of `--dry-run`.
    
    Limitations in resolving dependencies
    -------------------------------------
    
    In order to keep the initial implementation of this new command simpler, a
    number of options are not yet supported.  There should be no great difficulty
    in supporting the options `--flags`, `--enable-{tests,benchmarks}`,
    `--constraint` and `--preference`.  However, the options concerned with
    compilers may prove more difficult.
    
    Different versions of GHC ship with different library versions, the
    constraints that are written currently include all dependencies, including
    `base`. This prevents the constraints, as written, from being used with
    alternate versions of GHC.
    
    There are three solutions to this problem: 1) have the user edit the
    constraints section, 2) exclude certain packages from the list of constraints,
    3) develop a mechanism for per-arch-os-compiler constraining. As neither (2)
    nor (3) have been developed we default to (1).
    
    The lack of a good story for per-compiler constraints has lead to the options
    `--with-compiler`, `--ghc`, `--uhc` et al, not being supported.
    
    Further limitations:
    --------------------
    
     - The `cabal.config` file is completely overwritten. Just the `constraints`
       section should be overwritten.
    c6138e81
Setup.hs 61.4 KB