chore: add a language support guide

CONTRIBUTING.md

@@ -9,22 +9,62 @@ We welcome contributions in the form of pull requests and issues.
 
 Note that this codebase isn't yet extensively documented. If you get stuck, please ask for help [on Discord](https://docs.grit.io/discord).
 
+## Feature Flags
+
+We use [feature flags](https://doc.rust-lang.org/cargo/reference/features.html) to control which parts of the codebase are compiled.
+
+Note that some proprietary server-only integrations are hidden behind the "server" feature flag. This flag is disabled by default and code should compile without any additions.
+
+For major changes, we put new features should be put into the `grit_alpha` feature flag. Features that are ready for broad release should be put into the `grit_beta` feature flag. This is used for all public releases.
+
+Features that should be tested in CI should be put into the `grit_ci` feature flag. This is used for all CI tests.
+
 ## Language Grammars
 
 If GritQL is failing to match a code snippet, this can typically be fixed simply by adjusting the metavariable grammar for the target language.
 
 Metavariable grammars are found under [./resources/metavariable-grammars](./resources/metavariable-grammars). Typical fixes include:
 - Adding a new named field for a relevant node you want to manipulate.
 - Adding a `grit_metavariable` node as a choice in the corresponding spot where you want to substitute the metavariable.
+- Check [this guide](https://github.com/tree-sitter/tree-sitter/wiki/Tips-and-Tricks-for-a-grammar-author) to debug grammars generally.
 
 After making your changes, run the [./resources/edit_grammars.mjs](./resources/edit_grammars.mjs) script to regenerate the matching grammar.
 
-## Feature Flags
+### Snippet contexts
 
-We use [feature flags](https://doc.rust-lang.org/cargo/reference/features.html) to control which parts of the codebase are compiled.
+Snippet contexts help when a snippet is a valid AST subtree, but needs to be in a larger tree to parse. For example, matching on a table name like ` $schema.$table` in SQL is not valid SQL by itself, only when surrounded by something like `SELECT x from $schema.$table`  is the snippet valid.
 
-Note that some proprietary server-only integrations are hidden behind the "server" feature flag. This flag is disabled by default and code should compile without any additions.
+Snippet contexts are defined by implementing the `snippet_context_strings` method in the `Language` trait. This method returns a list of strings that are used to match the snippet in the larger tree. For example, the SQL implementation returns `["SELECT 1 from ", ";"]` to match a table name in a SQL query.
 
-For major changes, we put new features should be put into the `grit_alpha` feature flag. Features that are ready for broad release should be put into the `grit_beta` feature flag. This is used for all public releases.
+## Adding a New Target Language
 
-Features that should be tested in CI should be put into the `grit_ci` feature flag. This is used for all CI tests.
+Note: Grit involves *two* languages:
+
+- GritQL is [our query language](https://docs.grit.io/language/reference) for searching and transforming codebases.
+- The “target language” is the language we are transforming (ex. Python). This document describes the process of adding new target languages to Grit.
+
+Most of these steps involve iteration over a set of sample programs to get closer to correctness. The initial work for a language can typically be done in a day or two.
+
+Here are the steps for adding a new target language:
+
+1. Find a tree sitter grammar for the language and add it as a submodule under `resources/language-submodules`.
+2. Add a simple parse test in `crates/core/src/test.rs` to ensure that the grammar is working.
+3. Copy the grammar file into `resources/metavariable-grammars`. This alternative grammar is used for parsing `snippets` in GritQL.
+4. Patch the metavariable grammar to include  `$.grit_metavariable` anywhere we want to substitute a metavariable. This is usually at least `$identifier` and `$literal`.
+    - For a snippet to match, it also needs to be a field. Often you’ll want to to wrap `$thing` like: `field('thing', choice($.grit_metavariable, $thing))`
+5. Add a new language implementation in `crates/core/src/languages`. This involves implementing the `Language` trait and adding a new `Language` enum variant.
+6. Add `snippet_context_strings` [like this](https://github.com/getgrit/gritql/blob/main/crates/language/src/sql.rs#L52) to provide context for snippets to match in.
+7. Add test cases for the language in `crates/core/src/test.rs`. This is a good time to add a few dozen test cases to ensure that the language is parsed correctly, and that the metavariable grammar is working.
+
+### Internal steps
+
+These steps are done in our cloud environment and are not necessary for contributors to do.
+
+- grep for an existing language like `Sol` for solidity, and add it to all the `Language` enums you find.
+    - Add the language to `apps/web/src/views/project/details.tsx`, so repos with this language don’t get an “unsupported language” warning. (5 minutes)
+    - LSP target languages list: https://github.com/getgrit/rewriter/pull/7734/files#diff-f9d4f097b08d33241c5c8d15a2fbde0e37086c265ce0eba8decac20d5cd989c6R23
+    - VS Code client list: https://github.com/getgrit/rewriter/blob/f992490394a4807789504f1cea6a04b934ad3b24/apps/poolish/src/lsp-client.ts
+    - VS Code command palette triggers: https://github.com/getgrit/rewriter/pull/7734/files#diff-b38f1d6304993a250903310722206e6c89c58c52c2d1bd4b6fdd8f7218810570R103
+- There are also `exhaustive` runtime checks that error if a switch case doesn’t handle a language, like `makeSingleLineComment`. Search for `exhaustive(lang` and fill those out too.
+- Regenerate both DB/prisma types to add it to the DB schema and GraphQL types.
+- Add the language to `language-selector.tsx`. Pick an icon from [https://react-icons.github.io](https://react-icons.github.io/), usually from the Simple Icons category.