loadPackage fails for hard to determine reason

[mikestillman]

Sometimes, calling loadPackage can fail, saying it cannot be reloaded.

This error occurred in a set of packages I wrote a few years ago. I have boiled it down to its essence:

In A.m2, place the code

newPackage("A", Headline => "A")

In B.m2, place the code

needsPackage "A"
newPackage("B", Headline => "B")

Now after creating these files, try the following code.

elapsedTime installPackage "A"
elapsedTime installPackage "B"
restart
loadPackage "A"

This last line fails, since the package ''A" has been loaded when "B" is being scanned for its newPackage code (I think), and so has already been loaded. This is very hard to determine, as any installed package could do this and cause the package to simply be loaded already... Using needsPackage works fine.

Also, the time to install the package "A" seems to take 4-5 seconds on my machine, which seems quite large. (Same with "B").

[mahrud]

I've heard of this complaint from a lot of people, and the answer has always been

¯\_(ツ)_/¯ yeah loadPackage is weird, just add Reload => true or use needsPackage!

I'd be in favor of ditching loadsPackage (or making it a synonym of needsPackage), but the trouble is that it's hard to get needsPackage to reload a local copy of a package (e.g. a modified copy in the same directory) if a distributed version already exists on the system. This is related to the same issue I described in #3307 (comment).

[mahrud]

Here's a proposal:

Currently we have load and needs, and you can call either on the same file however many times, but the difference being that needs only calls load if the timestamp of the file newer than when it was last loaded. However, loadPackage and needsPackage don't have the same relationship. Perhaps we should deprecate Reload, have loadPackage always dismiss the package and reload it, and needsPackage should call loadPackage only if the location and/or timestamp (of the top package file, or any file therein also?) has changed.

(in particular, a local copy of a package in the current directory will have a different location, so needsPackage will reload it)

[mahrud]

This was the suggestion in #3225, by the way. Any feedback or whether or not you agree with change would be useful.