Clojure Development Skill

Tool Preference

When clojure-mcp tools are available (e.g., clojure_eval, clojure_edit), always use them instead of shell commands like ./bin/mage -repl. The MCP tools provide:

• Direct REPL integration without shell escaping issues
• Better error messages and feedback
• Structural Clojure editing that prevents syntax errors

Only fall back to ./bin/mage commands when clojure-mcp is not available.

Autonomous Development Workflow

• Do not attempt to read or edit files outside the project folder
• Add failing tests first, then fix them
• Work autonomously in small, testable increments
• Run targeted tests, and lint continuously during development
• Prioritize understanding existing patterns before implementing
• Don't commit changes, leave it for the user to review and make commits

Metabase Clojure Style Guide

This guide covers Clojure and ClojureScript coding conventions for Metabase. See also: CLOJURE_STYLE_GUIDE.adoc for the Community Clojure Style Guide.

Naming Conventions

General Naming:

• Acceptable abbreviations: acc, i, pred, coll, n, s, k, f
• Use kebab-case for all variables, functions, and constants

Function Naming:

• Pure functions should be nouns describing the value they return (e.g., age not calculate-age or get-age)
• Functions with side effects must end with !
• Don't repeat namespace alias in function names

Destructuring:

• Map destructuring should use kebab-case local bindings even if the map uses snake_case keys

Documentation Standards

Docstrings:

• Every public var in src or enterprise/backend/src must have docstring
• Format using Markdown conventions
• Reference other vars with [[other-var]] not backticks

Comments:

• TODO format: ;; TODO (Name M/D/YY) -- description

Code Organization

Visibility:

• Make everything ^:private unless it is used elsewhere
• Try to organize namespaces to avoid declare (put public functions near the end)

Size and Structure:

• Break up functions > 20 lines
• Lines ≤ 120 characters
• No blank lines within definition forms (except pairwise let/cond)

Style Conventions

Keywords and Metadata:

• Prefer namespaced keywords for internal use: :query-type/normal not :normal
• Tag variables with :arglists metadata if they're functions but wouldn't otherwise have it

Tests

Organization:

• Break large tests into separate deftest forms for logically separate test cases
• Test names should end in -test or -test-<number>

Performance:

• Mark pure function tests ^:parallel

Modules

OSS Modules:

• Follow metabase.<module>.* pattern
• Source in src/metabase/<module>/

Enterprise Modules:

• Follow metabase-enterprise.<module>.* pattern
• Source in enterprise/backend/src/metabase_enterprise/<module>/

Module Structure:

• REST API endpoints go in <module>.api or <module>.api.* namespaces
• Put module public API in <module>.core using Potemkin imports
• Put Toucan models in <module>.models.*
• Put settings in <module>.settings
• Put schemas in <module>.schema

Module Linters:

• Do not cheat module linters with :clj-kondo/ignore [:metabase/modules]

REST API Endpoints

Required Elements:

• All new endpoints must have response schemas (:- <schema> after route string)
• All endpoints need Malli schemas for parameters (detailed and complete)
• All new REST API endpoints MUST HAVE TESTS

Naming Conventions:

• Query parameters use kebab-case
• Request bodies use snake_case
• Routes use singular nouns (e.g., /api/dashboard/:id)

Behavior:

• GET endpoints should not have side effects (except analytics)
• defendpoint forms should be small wrappers around Toucan model code

MBQL (Metabase Query Language)

Restrictions:

• No raw MBQL introspection outside of lib, lib-be, or query-processor modules
• Use Lib and MBQL 5 in new source code; avoid legacy MBQL

Database and Models

Naming:

• Model names and table names should be singular nouns
• Application database uses snake_case identifiers

Best Practices:

• Use t2/select-one-fn instead of fetching entire rows for one column
• Put correct behavior in Toucan methods, not separate helper functions

Drivers

Documentation:

• New driver multimethods must be mentioned in docs/developers-guide/driver-changelog.md

Implementation:

• Driver implementations should pass driver argument to other driver multimethods
• Don't hardcode driver names in implementations
• Minimize logic inside read-column-thunk in JDBC-based drivers

Miscellaneous

Examples:

• Example data should be bird-themed if possible

Linter Suppressions:

• Use proper format for kondo suppressions
• No #_:clj-kondo/ignore (keyword form)

Configurable Options:

• Don't define configurable options that can only be set with environment variables
• Use :internal defsetting instead

Linting and Formatting

• Lint PR: ./bin/mage kondo-updated master (or whatever target branch)
  • Call the command one time at the beginning, record the results, then work through the problems one at a time.
  • If the solution is obvious, then please apply the fix. Otherwise skip it.
  • If you fix all the issues (and verify by rerunning the kondo-updated command):
    • commit the change with a succinct and descriptive commit message
• Lint File: ./bin/mage kondo <file or files>
  • Use the linter as a way to know that you are adhering to conventions in place in the codebase
• Lint Changes: ./bin/mage kondo-updated HEAD
• Format: ./bin/mage cljfmt-files [path]

Testing

• Run a test: ./bin/mage run-tests namespace/test-name
• Run all tests in a namespace: ./bin/mage run-tests namespace
• Run all tests for a module: ./bin/mage run-tests test/metabase/notification Because the module lives in that directory.

Note: the ./bin/mage run-tests command accepts multiple args, so you can pass ./bin/mage run-tests namespace/test-name namespace/other-test namespace/third-test to run 3 tests, or ./bin/mage run-tests test/metabase/module1 test/metabase/module2 to run 2 modules.

Code Readability

• Check Code Readability: ./bin/mage -check-readable <file> [line-number]
  • Run after every change to Clojure code
  • Check specific line first, then entire file if readable

REPL Usage

Note: If you have clojure-mcp tools available (check for tools like clojure_eval), always prefer those over ./bin/mage -repl. The MCP tools provide better integration, richer feedback, and avoid shell escaping issues. Only use ./bin/mage -repl as a fallback when clojure-mcp is not available.

• Evaluating Clojure Code: ./bin/mage -repl '<code>'
  • See "Sending Code to the REPL" section for more details

Sending Code to the REPL

• Send code to the metabase process REPL using: ./bin/mage -repl '(+ 1 1)' where (+ 1 1) is your Clojure code.
  • See ./bin/mage -repl -h for more details.
  • If the Metabase backend is not running, you'll see an error message with instructions on how to start it.

Working with Files and Namespaces

1. Load a file and call functions with fully qualified names:

To call your.namespace/your-function on arg1 and arg2:

./bin/mage -repl --namespace your.namespace '(your-function arg1 arg2)'

DO NOT use "require", "load-file" etc in the code string argument.

Understanding the Response

The ./bin/mage -repl command returns three separate, independent outputs:

• value: The return value of the last expression (best for data structures)
• stdout: Any printed output from println etc. (best for messages)
• stderr: Any error messages (best for warnings and errors)

Example call:

./bin/mage -repl '(println "Hello, world!") '\''({0 1, 1 3, 2 0, 3 2} {0 2, 1 0, 2 3, 3 1})'

Example response:

ns: user
session: 32a35206-871c-4553-9bc9-f49491173d1c
value:  ({0 1, 1 3, 2 0, 3 2} {0 2, 1 0, 2 3, 3 1})
stdout:  Hello, world!
stderr:

For effective REPL usage:

• Return data structures as function return values
• Use println for human-readable messages
• Print errors to stderr

REPL-Driven Development Workflow

• Start with small, fundamental functions:
• Identify the core features or functionalities required for your task.
• Break each feature down into the smallest, most basic functions that can be developed and tested independently.
• Write and test in the REPL:
  • Write the code for each small function directly in the REPL (Read-Eval-Print Loop).
  • Test it thoroughly with a variety of inputs, including typical use cases and relevant edge cases, to ensure it behaves as expected.
• Integrate into source code:
  • Once a function works correctly in the REPL, move it from the REPL environment into your source code files (e.g., within appropriate namespaces).
• Gradually increase complexity:
  • Build upon tested, basic functions to create more complex functions or components.
  • Compose smaller functions together, testing each new composition in the REPL to verify correctness step by step.
• Ensure dependency testing:
  • Make sure every function is fully tested in the REPL before it is depended upon by other functions.
  • This ensures that each layer of your application is reliable before you build on it.
• Use the REPL fully:
  • Use the REPL as your primary tool to experiment with different approaches, iterate quickly, and get immediate feedback on your code.
• Follow functional programming principles:
  • Keep functions small, focused, and composable.
  • Use Clojure's functional programming features—like immutability, higher-order functions, and the standard library—to write concise, effective code.

How to Evaluate Code

Bottom-up Dev Loop

1. Write code into a file.
2. Evaluate the file's namespace and make sure it loads correctly with:

./bin/mage -repl --namespace metabase.app-db.connection

3. Call functions in the namespace with test inputs, and observe that the outputs are correct Feel free to copy these REPL session trials into actual test cases using deftest and is.
4. Once you know these functions are good, return to 1, and compose them into the task that you need to build.

Critical Rules for Editing

• Be careful with parentheses counts when editing Clojure code
• After EVERY change to Clojure code, verify readability with -check-readable
• End all files with a newline
• When editing tabular code, where the columns line up, try to keep them aligned
• Spaces on a line with nothing after it is not allowed