elm-package.json modified (to something invalid) on install failure

[jessitron]

What I did:

• in a project with elm-http 1.0.0 <= v < 2.0.0
• tried to elm-package install one with elm-http 2.0.0 <= v < 3.0.0
  This gave an error about "Unable to find a set of packages that will work with your constraints." which is fine
• tried to elm-make my project again
  This gave an error about the package I tried to install being installed incorrectly: "your elm-package.json file says you depend on package jessitron/elm-http-with-headers,
  but it looks like it is not properly installed. Try running 'elm-package install'."

The request:
If elm-package fails to install a library, it should please not leave elm-package.json in an invalid state.

[dansisan]

+1

I'll add that this bug is arguably more severe than even the example above suggests. Probably the more common step after failing to install a package is (rather than elm-make) trying to install another package. That will fail too -- but with an error message that will lead you to think the new package is also out of date and to continue searching. (In the elm-make example above, I would think the user would be more likely to find the problem, since the error is unrelated to the current task.) Anyway, this happened to me and I wasted a lot of time. It seemed most packages were incompatible, and it almost put me off elm altogether!