Functional Core Imperative Shell

by ed3dai

Functional Core Imperative Shell is a Quality skill for Claude Code, published by ed3dai in ed3d-plugins.

248 stars32 forkson ed3dai/ed3d-pluginsAdded 2026/09/02Repository updated 2026/06/16

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Install in seconds

Install Functional Core Imperative Shell

Copy Functional Core Imperative Shell into your Claude Code skills folder. Run the command in your terminal, or review the source on GitHub before installing.

terminal

npx degit https://github.com/ed3dai/ed3d-plugins/tree/main/plugins/ed3d-house-style/skills/howto-functional-vs-imperative ~/.claude/skills/howto-functional-vs-imperative

Requires Node.js. Downloads this skill only — not the rest of the repository — into your Claude Code skills folder.

Without Node.js

git clone https://github.com/ed3dai/ed3d-plugins.git

Clones the whole repository, then copy the skill’s own directory into your skills folder yourself.

In this catalog

Source file: plugins/ed3d-house-style/skills/howto-functional-vs-imperative/SKILL.md in ed3dai/ed3d-plugins
Installs to: ~/.claude/skills/howto-functional-vs-imperative
Collection: One of 25 skills cataloged from this repository
Category: Quality — 1418 skills

What Functional Core Imperative Shell does

Functional Core Imperative Shell separates pure business logic from side effects when writing, refactoring, or reviewing code. It classifies files before creation so runtime code is placed in a pure core or thin shell.

Functional Core Imperative Shell is cataloged under Quality on DirSkills. Functional Core Imperative Shell comes from a repository tagged claude, claude-code, claude-code-plugin, claude-code-plugin-marketplace and skills.

Documentation

README

Excerpt

Functional Core, Imperative Shell (FCIS)

Overview

Core principle: Separate pure business logic (Functional Core) from side effects (Imperative Shell). Pure functions go in one file, I/O operations in another.

Why this matters: Pure functions are trivial to test (no mocks needed). I/O code is isolated to thin shells. Bugs become structurally impossible when business logic has no side effects.

When to Use

Use FCIS when:

Writing any new code file
Refactoring existing code
Reviewing code for architectural decisions
Deciding where logic belongs

Trigger symptoms:

"Where should this function go?"
Creating a new file
Adding database calls to logic
Adding file I/O to calculations
Writing tests that need complex mocking

This is the opening of the README. Read the full README on GitHub.

Frequently asked about Functional Core Imperative Shell

What else does ed3dai publish alongside Functional Core Imperative Shell?
Functional Core Imperative Shell is one of 25 skills that DirSkills catalogs from ed3dai/ed3d-plugins, the repository it ships in. Its siblings there include Asking Clarifying Questions, Brainstorming and Coding Effectively. Each one is a separate skill with its own page in this directory, installs the same way Functional Core Imperative Shell does, and is maintained by ed3dai in that same repository. The rest of the collection is listed on the ed3dai/ed3d-plugins page.
How does Functional Core Imperative Shell compare to other Quality skills?
Functional Core Imperative Shell ranks #1296 by stars among the 1418 Quality skills in this catalog. The most-starred ones next to it are Benchmark, Benchmark Optimization Loop and API Design Patterns. DirSkills ranks by the star count of the repository each skill ships in, so that order reflects how popular those repositories are rather than any review of Functional Core Imperative Shell against them. Open each page to compare what they document and how they install.