New: making Plugin's Processor API chainable

[mysticatea]

The first RFC 🎉

• https://github.com/eslint/rfcs/blob/plugin-processor-chaining/designs/2018-plugin-processors-chaining/README.md

Summary

This proposal makes the processors being possible to combinate by that those preprocess can return the pair of code and filename. If the returned filename requires another processor, ESLint applies the processor recursively.

interface Processor {
    readonly supportsAutofix?: boolean
    preprocess(code: string, filePath: string): string[] //→ ({ code: string, filePath: string } | string)[]
    postprocess(messages: Problem[][], filePath: string): Problem[]
}

interface Plugin {
    processors?: {
        [ext: string]: Processor
    }
    // ... omit others ...
}

Related Issues

• Better support for multiple processors eslint#11035

[mysticatea]

I'm sorry for not a hight-level description...

[mysticatea]

I updated this PR.

[mysticatea]

Considering the following settings:

// .eslintrc.js
module.exports = {
    overrides: [
        {
            files: "*.md",
            plugins: ["markdown"],
            settings: {
                markdown: {
                    targetBlocks: ["js", "ts", "html", "vue"]
                }
            }
        },
        {
            files: "*.ts",
            parser: "typescript-eslint-parser", 
        },
        {
            files: "*.html",
            plugins: ["html"],
        },
        {
            files: "*.vue",
            plugins: ["vue"],
            parser: "vue-eslint-parser", 
        },
    ]
}

I expect the extracted blocks from *.md to use other overriden settings. For example, ESLint uses typescript-eslint-parser for ts blocks. However, in the current plan, CLIEngine makes configuration then Linter handles processors. So, the processors cannot change the configuration. :(

Fortunately, we don't have to find .eslintrc files again if the filename that preprocessors returned cannot change the directory which the file belongs. Probably we can move the logic that resolves overrides setting, from CLIEngine to Linter. Though, it may be a breaking change.

[mysticatea]

Merged to #3.