[elm] Add support for nested Lists & Dicts

[eriktim]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Add support for nested Lists & Dicts, like List (List String).
And rename Union Types to Custom Types and Aliases to Records.

Fixes #1378. cc @wing328

[wing328]

Looks like the code/template is still referring to union.mustache, which has been renamed:

[main] INFO  o.o.codegen.AbstractGenerator - writing file /tmp/openapi-generator-test-fake-petstore/2.0/elm/src/Data/Dog.elm
[main] INFO  o.o.codegen.AbstractGenerator - writing file /tmp/openapi-generator-test-fake-petstore/2.0/elm/src/Data/EnumArrays.elm
[main] ERROR o.o.codegen.AbstractGenerator - elm/union.mustache (No such file or directory)
[main] ERROR o.o.codegen.AbstractGenerator - elm/union.mustache (No such file or directory)
Exception in thread "main" java.lang.RuntimeException: Could not generate model 'EnumClass'
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:474)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:865)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:349)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)
Caused by: java.lang.RuntimeException: can't load template elm/union.mustache
	at org.openapitools.codegen.AbstractGenerator.getTemplateReader(AbstractGenerator.java:81)
	at org.openapitools.codegen.DefaultGenerator$4.getTemplate(DefaultGenerator.java:889)
	at com.samskivert.mustache.Mustache$IncludedTemplateSegment.execute(Mustache.java:756)
	at com.samskivert.mustache.Template.executeSegs(Template.java:157)
	at com.samskivert.mustache.Mustache$IncludedTemplateSegment.execute(Mustache.java:774)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:870)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:881)
	at com.samskivert.mustache.Mustache$BlockSegment.executeSegs(Mustache.java:845)
	at com.samskivert.mustache.Mustache$SectionSegment.execute(Mustache.java:866)
	at com.samskivert.mustache.Template.executeSegs(Template.java:157)
	at com.samskivert.mustache.Template.execute(Template.java:134)
	at com.samskivert.mustache.Template.execute(Template.java:125)
	at org.openapitools.codegen.DefaultGenerator.processTemplateToFile(DefaultGenerator.java:895)
	at org.openapitools.codegen.DefaultGenerator.generateModel(DefaultGenerator.java:309)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:463)
	... 3 more

To reproduce the issue (as reported by Shippable CI), please run

java -jar ./modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g elm -o /tmp/openapi-generator-test-fake-petstore/2.0/elm

[eriktim]

Wow, how could I have missed that one. Committing a fix right now.

[wing328]

@trenneman thanks for the PR, which has been included in the v3.3.4 release: https://twitter.com/oas_generator/status/1068772409795207168