[elm-discuss] Module intermittently missing from compiler output

[Kevin Yank]

For awhile now, we’ve been seeing intermittent issues with our build, where
a particular module in our application will occasionally be missing from
the JavaScript output produced by the Elm compiler. We’ve seen it happening
both in our CI environment and when building assets for production. We have
yet to see it happen on a development machine. As our codebase has grown,
this issue seems to be occurring more and more often, so I thought it might
be worth checking if anyone else in the community had experienced anything
like it.

As I said, the problem manifests itself by a build producing a JavaScript
output file where a particular module is simply missing. We then see an
error like this one at runtime:

ReferenceError: Can't find variable: _user$project$Dialog$init
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:172568:83
ReferenceError: Can't find variable: _user$project$Dialog$init
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:172568:83
at
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:66078
ReferenceError: Can't find variable: _user$project$Dialog$init
ReferenceError: Can't find variable: _user$project$Dialog$init
at
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:172568
at
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:160309 in
A2




When we repeat the build, we get a JavaScript bundle that works just fine.
Comparing the contents reveals a chunk of JavaScript corresponding to one
of our modules is simply missing. Where the Elm compiler seems to normally
generate a single line of whitespace between two modules’ JavaScript
output, there is a conspicuous two-line gap at the point where the missing
code should be. Here’s what this looks like in a diff tool:


Obviously this is a very long way from anything like a minimal test case
that we could submit as a compiler bug, and unfortunately we can’t
reproduce it reliably enough to be able to narrow it down.

What I’m hoping is that someone else in the community is seeing something
like this too. Actually, if I’m being honest, I’m hoping one of our friends
at NoRedInk has experienced this issue and one of them is going to pop in
and tell me “Oh yeah, that’s been fixed for the eventual release of 0.19.”

Of possible relevance: we’re building our JavaScript bundles using Webpack
and the elm-webpack-loader
<https://www.npmjs.com/package/elm-webpack-loader>. We have it configured
to run only a single instance of the Elm compiler at once, though, so I’m
not sure how it could be responsible for this issue.

Fingers crossed we’re not alone in this!

--
Kevin Yank
Lead Developer

twitter: @sentience
skype: kevinyank


Culture analytics for your company
www.cultureamp.com

[Noah Hall]

Never seen this before. Pop on to elm-dev on slack and I can help out. Will
be afk til Sunday though

Post by Kevin Yank
For awhile now, we’ve been seeing intermittent issues with our build,
where a particular module in our application will occasionally be missing
from the JavaScript output produced by the Elm compiler. We’ve seen it
happening both in our CI environment and when building assets for
production. We have yet to see it happen on a development machine. As our
codebase has grown, this issue seems to be occurring more and more often,
so I thought it might be worth checking if anyone else in the community had
experienced anything like it.
As I said, the problem manifests itself by a build producing a JavaScript
output file where a particular module is simply missing. We then see an
ReferenceError: Can't find variable: _user$project$Dialog$init
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:172568:83
ReferenceError: Can't find variable: _user$project$Dialog$init
http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.bundle.js:172568:83
at http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.
bundle.js:66078
ReferenceError: Can't find variable: _user$project$Dialog$init
ReferenceError: Can't find variable: _user$project$Dialog$init
at http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.
bundle.js:172568
at http://localhost:3001/assets/exit-34cc2ad64db7cb8f54fd.
bundle.js:160309 in A2


When we repeat the build, we get a JavaScript bundle that works just fine.
Comparing the contents reveals a chunk of JavaScript corresponding to one
of our modules is simply missing. Where the Elm compiler seems to normally
generate a single line of whitespace between two modules’ JavaScript
output, there is a conspicuous two-line gap at the point where the missing
Obviously this is a very long way from anything like a minimal test case
that we could submit as a compiler bug, and unfortunately we can’t
reproduce it reliably enough to be able to narrow it down.
What I’m hoping is that someone else in the community is seeing something
like this too. Actually, if I’m being honest, I’m hoping one of our friends
at NoRedInk has experienced this issue and one of them is going to pop in
and tell me “Oh yeah, that’s been fixed for the eventual release of 0.19.”
Of possible relevance: we’re building our JavaScript bundles using Webpack
and the elm-webpack-loader
<https://www.npmjs.com/package/elm-webpack-loader>. We have it configured
to run only a single instance of the Elm compiler at once, though, so I’m
not sure how it could be responsible for this issue.
Fingers crossed we’re not alone in this!
--
Kevin Yank
Lead Developer
skype: kevinyank
Culture analytics for your company
www.cultureamp.com
--
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