Add runEnv: like run but uses $PORT

[pbrisbin]

When making smaller Yesod sites not using the scaffold, by far the most common
way I run it is to look at $PORT then fall back. I thought maybe the helper
we've been using for this would be generally useful. It would certainly be nice
not to have to define it again in all our projects.

As usual, I'm not tied to the name.

If this is accepted, and when the version bump happens, please also update the
"Since" comment.

[gregwebs]

Would it make more sense to call this something like runPortEnv since it is only checking the port?

[pbrisbin]

Maybe. I don't find that any clearer or more intention-revealing, myself.

I went with runEnv because where run uses the the Port given, runEnv gets it from the environment -- neither mention Port in their name even though both involve one. In other words, I'd be more on board with runPortEnv if run were already named runPort.

[snoyberg]

I'm OK with this as-is. @gregwebs I'll hold off on merging based on your objections.

[gregwebs]

I agree with @pbrisbin's reply. However, I do object to the use of the partial read function. It is fine to throw an exception, but it should mention the input value that was read in from the env variable and runEnv.

[pbrisbin]

That's a good point. I should be able to make that change on Monday.
On Feb 7, 2015 6:38 PM, "Greg Weber" notifications@github.com wrote:

I agree with @pbrisbin https://github.com/pbrisbin's reply. However, I
do object to the use of the partial read function. It is fine to throw an
exception, but it should mention the input value that was read in from the
env variable and runEnv.

—
Reply to this email directly or view it on GitHub
#334 (comment).

[pbrisbin]

I went with fail as a first swing since it seemed like the simplest thing that would work. If you'd prefer throwing some kind of actual exception type, let me know.

Another option would be to use the fallback on an invalid value (not just a missing value) but that felt too surprising for an end-user -- I'd rather a mis-configuration result in a useful failure than a silent change in behavior.

[gregwebs]

looks great, thanks!