stack ghci usability

[alexanderkjeldaas]

stack ghci should not leave a user with no "next step" when trying to invoke ghci when the build is broken.

In this example, stack refuses to start ghci because it cannot satisfy -package new-template. That might not be the most important aspect of starting ghci for the user, and there seems to be no way out of this, at least not as documented in --help.

$ stack ghci
Configuring GHCi with the following packages: new-template
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
<command line>: cannot satisfy -package new-template
    (use -v for more information)
$ stack ghci --help
Usage: stack ghci [TARGET] [--ghc-options OPTION] [--with-ghc GHC] [--no-load]
  Run ghci in the context of project(s)

Available options:
  TARGET                   If none specified, use all packages defined in
                           current directory
  --ghc-options OPTION     Additional options passed to GHCi
  --with-ghc GHC           Use this command for the GHC to run
  --no-load                Don't load modules on start-up
$ stack ghci --no-load
Configuring GHCi with the following packages: new-template
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
<command line>: cannot satisfy -package new-template
    (use -v for more information)
$ ghci
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
Prelude>

[mgsloan]

We can't easily do anything about this, because when we invoke ghci, we pass over control of stdout and stderr. So, we have no idea if ghci initialized successfully. Moreover, when it fails, we have no way to know what the message is. Even if we did intercept stdout / stderr, we wouldn't know if the ghci process is failing immediately or due to some user interaction.

It also doesn't make much sense to allow ghci to attempt to load code if a -package is unsatisfiable. In this case, either the code won't load due to missing imports, or that package isn't necessary and should be removed from your cabal file.

How are you getting this error? Do you have a recent version of stack? stack ghci on the new template works fine. Maybe you want stack exec ghci, which runs ghci in the environment without setting any flags, controlling the package environment, or loading code.

I could imagine adding a --no-package-hiding, or some similar flag. This would omit the -package flags, which might help here. It would also be useful for the circumstance where you want to import modules that aren't dependencies of any of the targets.

[mgsloan]

I've gone ahead and added --no-package-hiding to stack ghci, since it's something I've wanted in the past, for other reasons.

Feel free to reopen if this explanation / flag addition doesn't resolve the issue.