nanorepl

Level (micro/nano/mini) -- implementation style:

Default: nano-3 -- single file, one dependency, produces real results.

Reference points: micro-1 = microGPT gist (~200 LOC, pure Python autograd + GPT). nano-3 = nanoGPT (~600 LOC, PyTorch model + training). mini-5 = nanochat (~8000 LOC, full pipeline).

Invocation

nanorepl                           # Start new project
nanorepl <url>                     # Analyze a specific project
nanorepl <concept> <level>-<scale> # Quick start with parameters

Workflow

Step 1: Target Acquisition

Use AskUserQuestion to gather parameters:

Q1 -- Target: "What do you want to build a minimal version of?"

• URL -- GitHub repo, documentation page, or paper
• Local path -- existing project on disk
• Concept -- describe what it does (e.g., "a B-tree key-value store with WAL")

Q2 -- Scale and Level: "What complexity?"

• Show the scale/level matrix above
• Default: nano-3
• User can specify as {level}-{scale} (e.g., micro-2, mini-4)
• Or separately: "scale 3, level nano"

Q3 -- Language: "What language?" (default: Python)

Q4 -- Output location: "Where to create the project?" (default: ./nanorepl-{name}/)

If the user provided arguments with nanorepl, skip already-answered questions.

Step 2: Source Analysis

Analyze the target to identify the irreducible core. Approach depends on input type:

If the target is very large (e.g., an entire framework like PyTorch or Linux), ask the user to identify a specific subsystem or capability to reimplement. Do not attempt to analyze an entire large codebase.

Produce an anatomy -- classify every component of the original:

Read references/stripping-heuristics.md from this skill's directory for the full decision framework.

Step 3: Decomposition

Generate a layer plan. Number of layers = scale value (scale 3 = layers L0, L1, L2).

Rules:

• Layer 0 is always the irreducible core -- the thing that makes this project what it IS, not what makes it fast or convenient
• One concept per layer -- each layer adds exactly ONE conceptual component (one data structure, one mechanism, one I/O mode, one optimization). At low scale (1-2), the final layer may bundle 2-3 closely related concepts if they cannot be meaningfully separated (e.g., "HTTP handling" may include method filtering + path params). At scale 3+, strictly one concept per layer.
• Keep the I/O paradigm stable across layers -- if L0 uses a REPL, L1 should extend that REPL (not replace it with TCP). If an I/O paradigm shift is needed, it should BE the concept that layer introduces, and the core algorithm code should carry over unchanged so the diff stays clean.
• If a layer's diff from the previous would exceed ~200 LOC, split into sub-layers (2a, 2b)
• Every layer must be runnable and verifiable independently

Layer template from microGPT blog (example for a language model at micro-5):

• L0: bigram counting (simplest possible model)
• L1: MLP with manual gradients (add neural network)
• L2: autograd engine (add automatic differentiation)
• L3: self-attention mechanism (add the transformer's key idea)
• L4: full transformer block (add feedforward, norm, residuals)
• L5: Adam optimizer + LR decay (add real training)

For each layer, specify:

• What it adds (3-5 bullets)
• New capability unlocked (what it can do that the previous could not)
• Verification command and expected output
• Approximate LOC

Step 4: Plan Presentation

Present the decomposition as a clean overview:

nanorepl-{name} [{level}-{scale}]: {one-line description}

L0 (~{N} LOC): {core algorithm name}
  adds: {what}
  verify: {command}

L1 (~{N} LOC): + {concept added}
  adds: {what}
  verify: {command}
  diff: {key additions from L0}

...

Stripped from original:
- {thing}: {why it was removed}
- {thing}: {why it was removed}

Wait for the user to confirm or adjust before building.

Step 5: Progressive Building

Build one layer at a time, interactively.

For each layer:

1. Generate the layer file (layer{N}.py or appropriate extension)

   • Fully self-contained -- copy-paste runnable, no imports from other layers
   • Structured so diff layer{N-1}.py layer{N}.py shows clean, meaningful changes
   • Zero comments except rare "why" notes. Do not add section labels (# forward pass), step markers (# step 1), variable descriptions (# learning rate), or formula annotations (# dL/dw). If the code needs a comment to be understood, rename the variable or restructure the code instead. The only acceptable comment explains a non-obvious design choice (e.g., # ReLU instead of sigmoid to avoid vanishing gradients).
   • Short but clear variable names
   • No docstrings, no type annotations (unless language requires them)
   • At micro level: zero external deps, implement everything from scratch
   • At nano level: allow one key dependency
   • At mini level: allow essential deps, may use multiple files per layer

2. Present with context

   • Brief explanation of what's new relative to the previous layer
   • For L1+, highlight the key diff (what changed and why)

3. Show verification

   • Specific command to run
   • Expected output
   • What to check for correctness

4. Wait for user input. Supported commands:

   Command	Action
   next	Proceed to next layer
   explain {thing}	Walk through specific code, function, or concept
   diff	Show diff from previous layer
   modify {request}	Adjust current layer
   done	Stop here, skip remaining layers
   restart {N}	Regenerate from layer N

Step 6: Summary

After all layers are built (or user says "done"), generate README.md in the project directory:

# nanorepl-{name}

A minimal reimplementation of {project} in {total LOC} lines of {language}.

> "{one-line description of what the irreducible core does}"

## Layers

| File | LOC | Concept Added |
|------|-----|---------------|
| layer0.py | {N} | {core algorithm} |
| layer1.py | {N} | + {concept} |
| ... | ... | ... |

## What Was Stripped

| Original Feature | Why Removed |
|-----------------|-------------|
| {feature} | {rationale from stripping heuristics} |

## Run

{command to run the final layer}

## Compare

- Original: {link to original project}
- This reimplementation: {total LOC} lines vs {original LOC estimate}

Guiding Principles

These are non-negotiable -- they apply to every nanorepl project:

1. The algorithm is NOT simplified. Strip infrastructure, not the core logic. "Teeth over education" -- the hard parts stay hard.
2. Every layer runs. No layer is purely structural. Each produces verifiable output.
3. Code is the documentation. Zero comments by default. No section labels (# forward pass), no variable descriptions (# weights), no formula annotations. If the code isn't clear, rename variables or restructure -- don't add a comment.
4. Make implicit things explicit. If something is conceptually present but hidden by production code, expose it. Clarity > performance.
5. One concept per layer. If a layer adds two concepts, split it. At scale 1-2, the final layer may bundle closely related concepts that can't be meaningfully separated.
6. Standalone and diffable. Each layer file is self-contained. diff between adjacent layers reveals exactly what one concept looks like in code.
7. No magic. "It's a big math function." Every mechanism is visible and traceable.
8. Everything else is just efficiency. The gap between the nanorepl and production is engineering, not algorithmic understanding.

Fixed Behaviors