Dead Code Analysis in ReScript

This guide provides a detailed walkthrough on how to leverage ReScript’s powerful dead code analysis tools to maintain a clean, efficient, and distraction-free codebase.

Dead code refers to code that's present in your codebase but is never executed. It can lead to:

• Increased compilation times

• Confusion during development

• Misleading assumptions about functionality

ReScript’s language design allows for accurate and efficient dead code analysis using the ReScript Code Analyzer, available via the official VSCode extension.

Prerequisites

• ReScript VSCode extension (v1.8.2 or higher)

Activation

1. Open the Command Palette: Cmd/Ctrl + P

2. Run: > ReScript: Start Code Analyzer

Deactivation

• Run: > ReScript: Stop Code Analyzer

• Or click “Stop Code Analyzer” in the status bar

Result

• The “Problems” pane populates with dead code warnings and suggestions.

Real-World Use Cases

1. Unused Record Fields

RESCRIPT
type useReturn = {
  items: array<item>,
  toggleItemChecked: string => unit, // ← Never used
  setCheckedOnItem: (string, bool) => unit,
  checkAll: unit => unit,
  uncheckAll: unit => unit,
}

Remove unused fields to simplify code.

2. Unused Variant Cases

RESCRIPT
type textType =
  | Text(string)
  | TextWithIcon({icon: React.element, text: string})
  | Render(React.element) // ← Never constructed

Removing unused variants allows simplifying rendering logic.

3. Unused Parts of State

RESCRIPT
type validationState = Idle | Invalid | Valid

type state = {
  oldPassword: string,
  newPassword: string,
  newPasswordRepeated: string,
  validationState: validationState, // ← Never read
}

Old validation logic might remain after refactors—clean it up.

4. Unnecessary Interface Exposure

RESCRIPT
// DrilldownTarget.resi
MetricParam.parse // ← Never used
MetricParam.serialize // ← Never used

Keep interfaces minimal by removing unused exports.

5. Unused Functions

RESCRIPT
let routerUrlToPath = ... // ← Never used
let routeUrlStartsWith = ... // ← Never used

Removing these often uncovers further unused logic.

6. Unused Components

Components never referenced in production should be removed, unless explicitly preserved.

Keeping Some Dead Code

Use @dead and @live

@dead

Suppresses warnings but notifies if code becomes alive again.

RESCRIPT
type user = {
  name: string,
  @dead age: int,
}

@live

Permanently marks code as alive (no future warnings).

RESCRIPT
@live
let getUserName = user => user.name

Configuration

Add to your rescript.json:

JSON
"reanalyze": {
  "analysis": ["dce"],
  "suppress": ["src/bindings", "src/stories", "src/routes"],
  "unsuppress": [],
  "transitive": false
}

Options:

• analysis: Enables dead code analysis ("dce")

• suppress: Silences reporting for paths (still analyzes)

• unsuppress: Re-enables reports within suppressed paths

• transitive: Controls reporting of indirectly dead code

Recommendation: Set transitive: false for incremental cleanup.

Summary

ReScript’s dead code analyzer helps you:

• Incrementally clean up your codebase

• Avoid confusion and complexity

• Improve long-term maintainability

Use it regularly for the best results.