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[BUG][Elm] Root operation causes StringIndexOutOfBoundsException

Created by: Elewyth

Bug Report Checklist

  • Have you provided a full/minimal spec to reproduce the issue?
  • Have you validated the input using an OpenAPI validator (example)?
  • What's the version of OpenAPI Generator used?
  • Have you search for related issues/PRs?
  • What's the actual output vs expected output?
  • [Optional] Bounty to sponsor the fix (example)
Description

Generating client code for Elm (0.19) from an openapi.yaml specification with a root operation / causes a StringIndexOutOfBoundsException:

Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Default'
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:666)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:922)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:396)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)
Caused by: java.lang.StringIndexOutOfBoundsException: String index out of range: 0
	at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:47)
	at java.base/java.lang.String.charAt(String.java:693)
	at org.openapitools.codegen.languages.ElmClientCodegen.lambda$postProcessOperationsWithModels$3(ElmClientCodegen.java:449)
	at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:195)
	at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
	at org.openapitools.codegen.languages.ElmClientCodegen.postProcessOperationsWithModels(ElmClientCodegen.java:449)
	at org.openapitools.codegen.DefaultGenerator.processOperations(DefaultGenerator.java:1175)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:552)
	... 3 more
openapi-generator version

@openapitools/openapi-generator-cli@0.0.14-4.0.2 (installed via NPM)

OpenAPI declaration file content or url
openapi: 3.0.0

info:
  title: Elm Root Bug
  version: 1.0.0

paths:
  /:
    get:
      summary: Root endpoint that causes generator bug
      responses:
        204:
          description: No response body
Command line used for generation
npx openapi-generator generate -i bug.yaml -g elm -o output
Steps to reproduce
  1. Save sample spec as bug.yaml
  2. npm install -g openapi-generator-cli
  3. npx openapi-generator generate -i bug.yaml -g elm -o output
Related issues/PRs

I could not find a related issue, as this seems to be a bug specific to the Elm generator implementation.

Suggest a fix

ElmClientCodegen.java:449 contains a lambda with str.charAt(0) which fails on operations with a path of /. This should probably have a fallback on something like "root" or even an empty string. The Haskell generator produces an empty string and an operation named "rootGet".