cabal-install common subcommands

cabal-install-commands.txt

The most common thing is to install a library (so your project can use it) or
an executable (like when you install `hoogle` so you can search documentation
locally). Behind the scenes, this involves a number of steps:

1. determine package version plan (a consistent Set (Library,Version))
2. download dependencies (i.e. their source)
3. install dependencies (i.e. do steps 1-6a for each dependency)
4. configure (think: what to build in which order, with which compiler flags etc.)
5. build
6a. register (libraries) in package repository
6b. copy the compiled (executables?) to destination directory

You might say: That sounds all reasonable, but why not wrap that in one command
and hide the details? There are two answers: Yes, this is exactly what
`cabal install` does: It includes all the above steps. The seconds answers is
that all steps take time, and during development, you do not need to execute
each step every time you make a change (e.g. when you change a few lines in
your project, you do not need to install new dependencies, or change the
configuration). That is why the user gets access to sub-commands that map
to certain steps above.

Some subcommands even allow different steps:

2c. export the install plan
5d. run directly / load in repl / run tests / run benchmark

The most important subcommands are:

1. `install --only-dependencies`, or `install --dep` for short, or `install --dep --enable-tests`
2. `configure`, optionally with flags such as `configure -ffoo` or `configure --enable-tests`
3. `build`
4. `run`, `test` and `repl`
5. `install` (which is necessary surprisingly rarely; mostly you need only install dependencies).

The table shows how some sub-commands map to the above steps. 

> x: always done
> m: maybe; depends on circumstances/flags
> d: this is required as precondition

                        dl    conf  reg   exp 
                     plan  inst  build copy  run
                     1  2  3  4  5  6a 6a 2c 5d
freeze               x                     x    (after freeze, the Set is constant, thus the `m`s in the 1 column)
fetch                m  x                   
install --only-deps  m  x  x                    (this involves 4 5 6a for deps)
configure                  d  x             
build                         d  x          
repl/run/bench/test        d  dm m            x (tests/benchmarks might have additional dependencies; sometimes there is auto-reconf, sometimes there is not)
copy                             d     x       
register                         d  x     
install              m  x  x  x  x  x  x    

As you can see, the mapping is rather straightforward; only `install` does
a lot more, or less, depending on flags.