Writing recipes over multiple source file types

Sometimes we need to examine multiple source files or multiple types of source files to make a decision in a visitor about whether we intend to make a change. For example, we could be looking for a particular condition to be present in a Maven POM file, and if the condition is met alter an application property in a YAML file.
Use the visit(List<SourceFile> before, ExecutionContext ctx) method on Recipe to iterate over and maintain decision state, and then ListUtils.map to execute the transformation.
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package org.openrewrite;
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​
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import org.openrewrite.internal.ListUtils;
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import org.openrewrite.maven.tree.Maven;
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import org.openrewrite.yaml.YamlVisitor;
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​
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import java.util.List;
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​
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public class MavenYamlRecipe extends Recipe {
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    @Override
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    public String getDisplayName() {
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        return "Something with Maven and something with YAML";
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    }
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​
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    @Override
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    protected List<SourceFile> visit(List<SourceFile> before, ExecutionContext ctx) {
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        boolean pomConditionMet = false;
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        for (SourceFile sourceFile : before) {
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            if(sourceFile instanceof Maven) {
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                Maven m = (Maven) sourceFile;
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                // pomConditionMet = ...
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            }
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        }
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​
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        return ListUtils.map(before, sourceFile -> (SourceFile) new YamlVisitor<Integer>() {
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            // whatever your YAML visitor does can now respond to pomConditionMet
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        }.visitNonNull(sourceFile, 0));
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    }
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}
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To write a unit test that tests multiple source file types, use their respective parsers:
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package org.openrewrite
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​
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import org.assertj.core.api.Assertions.assertThat
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import org.junit.jupiter.api.Test
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import org.openrewrite.maven.MavenParser
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import org.openrewrite.yaml.YamlParser
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​
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class MavenYamlTest {
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​
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    @Test
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    fun mavenYaml() {
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        val sources =
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            MavenParser.builder().build().parse(
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                """
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                    <project>
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                        <groupId>org.openrewrite</groupId>
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                        <artifactId>test</artifactId>
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                        <version>0.1.0</version>
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                    </project>
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                """
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            ).plus(
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                YamlParser().parse("""
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                  key: value
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                """)
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            )
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​
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        val results = MavenYaml().run(sources)
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        assertThat(results.map { it.after!!.printAll() }).containsExactly(
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            """
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                key: changedValue
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            """.trimIndent()
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        )
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    }
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}
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