"The pattern match does not match the subject type"

**Problem**: You're pattern matching on a value but your patterns don't align with the actual type structure. **Error**: ``` error: Type mismatch ┌─ /src/parser.gleam:15:9 │ 15│ Ok(content) -> parse_content(content) │ ^^^^^^^^^^^ This constructor does not match the subject type. Expected type: List(String) Found type: Result(String, _) ``` **Fix**: Check what type you're actually pattern matching on: ```gleam // Wrong - treating a List like a Result case string_list { Ok(content) -> parse_content(content) // List doesn't have Ok/Error Error(_) -> [] } // Right - match on the actual List type case string_list { [] -> [] [first, ..rest] -> parse_content(first) ++ parse_list(rest) } ```

"Unknown variable" errors that seem obviously correct

**Problem**: You're referencing a variable that looks like it should be in scope but the compiler can't find it. **Error**: ``` error: Unknown variable ┌─ /src/handler.gleam:23:12 │ 23│ user_id + 1 │ ^^^^^^^ The name `user_id` is not in scope here. ``` This happens because you accidentally discarded the variable with `_` prefix. Took me forever to figure this out: ```gleam // Wrong - discarded variables aren't accessible let _user_id = get_user_id() user_id + 1 // Can't access _user_id // Right - don't discard if you need it let user_id = get_user_id() user_id + 1 ```

"Cyclical imports detected" in module dependencies

Module A imports module B, which imports module A. Compiler gets dizzy and throws up. **Error**: ``` error: Cyclical imports ┌─ /src/user.gleam:1:8 │ 1 │ import app/auth │ ^^^^^^^^ /src/user.gleam -> /src/auth.gleam -> /src/user.gleam ``` You need to extract shared types to a separate module: ```gleam // Create shared_types.gleam pub type User { User(id: Int, name: String) } // user.gleam imports shared_types import app/shared_types.{type User} // auth.gleam imports shared_types import app/shared_types.{type User} // Now auth.gleam doesn't need to import user.gleam ```

"This function must return a value" for functions that look complete

**Problem**: Some code paths in your function don't return a value, or you have unreachable code after a case expression. **Error**: ``` error: Missing return ┌─ /src/calculator.gleam:12:1 │ 12│ pub fn calculate(op: String, a: Int, b: Int) -> Int { │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This function must return a value. ``` **Fix**: Ensure all code paths return a value: ```gleam // Wrong - missing return for unknown operations pub fn calculate(op: String, a: Int, b: Int) -> Int { case op { "add" -> a + b "subtract" -> a - b // Missing case for other strings } } // Right - handle all cases pub fn calculate(op: String, a: Int, b: Int) -> Int { case op { "add" -> a + b "subtract" -> a - b _ -> 0 // Default case } } ```

"Type `#(a, b)` is not compatible with type `#(b, a)`" tuple order issues

Tuple elements are in the wrong order. Gleam tuples are positional, so `#(String, Int)` is completely different from `#(Int, String)`. This is why tuples suck for anything complex. **Error**: ``` error: Type mismatch ┌─ /src/coords.gleam:18:20 │ 18│ process_coordinates(#(y, x)) │ ^^^^^^^ Expected type `#(Int, Int)`, found type `#(Float, Float)`. ``` **Fix**: Match the exact tuple structure expected: ```gleam // Wrong - order matters in tuples let coordinates = #(3.14, 42) // #(Float, Int) process_coordinates(coordinates) // Expects #(Int, Float) // Right - put elements in correct order let coordinates = #(42, 3.14) // #(Int, Float) process_coordinates(coordinates) // Or convert types explicitly let coordinates = #(float.round(3.14), int.to_float(42)) ```

"The imported module does not have this value" for functions that definitely exist

**Problem**: The function exists but it's not exported (no `pub` keyword), or you're importing it incorrectly. **Error**: ``` error: Unknown import ┌─ /src/main.gleam:3:18 │ 3 │ import utils.{helper_function} │ ^^^^^^^^^^^^^^^ The module `utils` does not have this value. ``` **Fix**: Check if the function is actually exported: ```gleam // In utils.gleam - function is private fn helper_function() -> String { // No 'pub' keyword "help" } // Fix: make it public pub fn helper_function() -> String { "help" } // Or import the whole module and use qualified access import utils utils.helper_function() // This works even if not in explicit imports ```

"Cannot find type constructor" for custom types

**Problem**: You're trying to use a custom type constructor that doesn't exist or isn't imported. **Error**: ``` error: Unknown constructor ┌─ /src/state.gleam:15:12 │ 15│ Loading -> show_spinner() │ ^^^^^^^ The constructor `Loading` is not defined. ``` **Fix**: Import the type and its constructors: ```gleam // Wrong - only importing the type import app/ui.{type State} case current_state { Loading -> show_spinner() // Loading constructor not available Ready(data) -> show_data(data) } // Right - import type and constructors import app/ui.{type State, Loading, Ready} case current_state { Loading -> show_spinner() Ready(data) -> show_data(data) } ```

"Binary pattern contains unknown segment type" in bit array operations

**Problem**: You're using unsupported options in bit array pattern matching or construction. **Error**: ``` error: Unknown segment option ┌─ /src/parser.gleam:8:25 │ 8 │ let assert > = packet │ ^^^^^^^^ This segment option is not supported. ``` **Fix**: Use supported bit array segment options: ```gleam // Wrong - "unsigned-integer-32" isn't valid syntax > // Right - use valid options > // Or with explicit type > ```

"This expression creates an infinite type" in recursive type definitions

**Problem**: You're creating a type that references itself in a way that creates infinite recursion. **Error**: ``` error: Infinite type ┌─ /src/tree.gleam:3:1 │ 3 │ pub type Tree = Tree(Tree) │ ^^^^^^^^^^^^^^^^^^^^^^^^^^ This type definition creates an infinite type. ``` **Fix**: Use proper recursive type structure: ```gleam // Wrong - every Tree directly contains another Tree pub type Tree = Tree(Tree) // Right - use Option or List for termination pub type Tree(a) { Leaf(value: a) Branch(value: a, left: Option(Tree(a)), right: Option(Tree(a))) } // Or for lists pub type LinkedList(a) { Empty Node(value: a, next: LinkedList(a)) } ```

"Package has no compatible versions" in dependency resolution

**Problem**: The version constraints in your `gleam.toml` are too restrictive or conflicting. Newer Node versions can be pickier about dependency resolution. **Example Error (pre-v1.12.0)**: ``` error: Dependency resolution failed Unable to find compatible versions for the version constraints in your gleam.toml. ``` **Fix**: Relax version constraints or update dependencies: ```toml # Wrong - too restrictive [dependencies] gleam_json = "= 1.0.0" # Exact version only wisp = ">= 1.0.0 and = 1.0.0 and = 1.0.0 and 1.0" wisp = "~> 1.0" ```

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Gleam Compilation Errors: AI-Optimized Technical Reference

Overview

Gleam's type system catches bugs at compile time, resulting in more upfront compiler conflicts but fewer runtime failures. Error messages require translation from compiler-speak to actionable fixes.

Critical Context & Failure Modes

Compilation Behavior

Type system strictness: More restrictive than JavaScript's dynamic typing but prevents runtime crashes
Error timing: Catches bugs at compile time vs runtime explosions in production
Performance impact: Memory exhaustion possible with hundreds of modules and complex type hierarchies
Platform differences: Erlang vs JavaScript targets have different capabilities (bit arrays not fully supported on JavaScript)

Breaking Changes & Version Issues

Package renaming: Common between versions (e.g., gleam/http → gleam_http in v0.30 releases)
Function signature changes: Handler functions in frameworks like Wisp change arity between versions
Dependency resolution: Major improvements in v1.12.0 (August 2025) with detailed conflict trees vs cryptic errors in earlier versions

Most Common Compilation Failures

1. Pattern Matching Exhaustiveness Errors

Error Pattern: Inexhaustive patterns - Missing patterns: Error(_)

Root Cause: Gleam forces handling of all possible values in pattern matches

Critical Impact: Prevents runtime crashes but increases development time

Fix Strategy:

case user_result {
  Ok(user) -> render_user_page(user)
  Error(reason) -> {
    io.println("User lookup failed: " <> string.inspect(reason))
    show_error_page()
  }
}

Time Investment: 5-10 minutes per case
Difficulty: Low once pattern is understood
Prevention: Use systematic case expressions, let compiler guide implementation

2. Import Resolution Failures

Error Pattern: The module 'gleam/http' could not be found

Root Causes:

Package not installed
Module name changed between versions
Incorrect module path

Hidden Cost: Can waste entire afternoons debugging renamed packages

Fix Process:

gleam deps list  # Check installed packages
gleam add gleam_http  # Add missing package
gleam docs build  # Show available modules

Time Investment: Few minutes to several hours if using outdated documentation
Critical Resource: Gleam Package Index for current naming

3. Type Mismatch Errors

Error Pattern: Expected type 'String', found type 'Int'

Root Cause: Mental model mismatch with actual type definitions

High-Risk Scenarios: External APIs, database records with assumed string IDs but integer returns

Fix Strategy:

// Explicit conversion instead of assumption
let user_id_string = int.to_string(user.id)

// Handle multiple possible types
let user_id_string = case user.id {
  id if is_string(id) -> id
  id if is_int(id) -> int.to_string(id)
  _ -> "unknown"
}

Time Investment: 3-10 minutes
Prevention: Use explicit type annotations at module boundaries

4. Dependency Resolution Conflicts

Error Evolution:

Pre-v1.12.0: Cryptic "Unable to find compatible versions" with no detail
Post-v1.12.0: Detailed conflict trees showing exact version constraints

Example Conflict:

mist requires gleam_otp >= 0.9.0 and < 1.0.0
lustre requires gleam_otp >= 1.0.0 and < 2.0.0

Fix Strategy:

[dependencies]
wisp = ">= 1.1.0 and < 2.0.0"  # Use newer version compatible with gleam_otp 1.x
lustre = ">= 5.2.1 and < 6.0.0"

Time Investment: 10-30 minutes with v1.12.0, up to 3 hours with earlier versions

5. Function Arity Mismatches

Error Pattern: Expected 3 arguments, got 1

Root Cause: Framework API changes between versions

High-Risk Scenario: Dependency updates breaking existing handler functions

Production Impact: Works in development, fails in CI

Fix Strategy: Check function definition and add required parameters
Time Investment: Quick fix if you know the API, hours if documentation is outdated

Advanced Debugging Techniques

Compile-Time Type Debugging

Tool: echo keyword (enhanced in v1.12.0)

let user_data = get_user_from_api()
echo user_data as "user data type"

Purpose: Shows compiler's inferred types vs expected types
Critical: Remove before publishing (compiler warns in published packages)

Build System Nuclear Options

rm -rf build/
gleam deps download
gleam build

When to Use: Mysterious compilation failures with no clear error
Windows-Specific: PATH limit issues require shorter directory paths (use C:\dev\myapp)

FFI Debugging Process

High-Risk Area: External function calls bypass type safety

Systematic Approach:

Test external function exists
Test with simple inputs
Add type assertions for return values

Critical Gotcha: Erlang functions return tuples for errors {error, reason} but Gleam expects plain types

Binary Search Debugging

Last Resort: Comment out half the code, compile, repeat until problem isolated

Use Case: Large codebase with mysterious errors pointing to wrong lines
Time Investment: Entire afternoon but guaranteed to find the issue

Resource Requirements & Time Costs

Development Time Impact

Error Type	Time to Fix	Skill Level Required	Frequency
Pattern matching	5-10 minutes	Beginner	Very High
Import resolution	Minutes to hours	Beginner-Intermediate	High
Type mismatches	3-10 minutes	Beginner	High
Dependency conflicts	10-30 minutes (v1.12+)	Intermediate	Medium
FFI issues	5-60 minutes	Advanced	Low

Memory and Performance Thresholds

Large projects: Hundreds of modules can exhaust memory during type checking
UI performance: Breaks at 1000 spans, making debugging large distributed transactions impossible
CI impact: Memory issues can take down entire team builds

Expertise Requirements

Type system understanding: Essential for efficient debugging
BEAM ecosystem knowledge: Required for FFI and external integrations
Version management: Critical for avoiding dependency hell

Critical Warnings & Operational Intelligence

What Official Documentation Doesn't Tell You

Package renaming frequency: Common between versions with no clear migration path
Windows PATH limits: Will break builds silently
Memory exhaustion: Large projects hit type checker limits
FFI safety loss: External functions bypass all type guarantees

Breaking Points & Failure Modes

1000+ UI spans: Debugging becomes impossible
Complex type hierarchies: Memory exhaustion during compilation
Cross-platform differences: Bit arrays fail on JavaScript target
Dependency age: Old Stack Overflow answers reference renamed packages

Community Wisdom

Discord #help channel: Real humans who've seen exact error patterns
GitHub issues: Search closed issues for specific error messages
Exercise tracks: Better learning than official docs for practical patterns

Migration Pain Points

v0.30 releases: Major package renames
Framework updates: Handler signature changes break existing code
Dependency resolution: Pre-v1.12.0 extremely difficult to debug

Configuration That Actually Works

Version Constraints

# Avoid exact versions - too restrictive
gleam_json = ">= 1.0.0 and < 2.0.0"  # Reasonable range
wisp = "~> 1.0"  # Let Gleam pick compatible versions

Development Workflow

# Regular maintenance
gleam clean  # Clear build artifacts
gleam deps list  # Verify dependencies
gleam format src/  # Validates gleam.toml syntax

IDE Setup

VS Code Gleam Extension: More detailed errors than command line
Language Server: Provides go-to-definition for debugging imports
Output panel: Check "Gleam Language Server" for traces not shown in CLI

Decision Criteria

When Gleam is Worth the Compilation Cost

Production stability > development speed
Team has patience for type system learning curve
Error prevention > rapid prototyping
Long-term maintenance > quick delivery

When to Consider Alternatives

Rapid prototyping requirements
Dynamic data from unreliable external APIs
Team unfamiliar with static typing
Legacy integration with many external systems

Resource Trade-offs

More time upfront debugging type errors
Fewer phone calls at 3am from production issues
Steeper learning curve but fewer hidden bugs
Explicit error handling vs runtime surprises

Essential Tools & Resources

Primary Debugging Resources

Gleam Language Tour: Interactive type system tutorial
Package Index: Current package names and versions
Discord #help: Community troubleshooting
GitHub Issues: Specific error solutions

Version-Specific Resources

v1.12.0 Release Notes: Dependency resolution improvements
Migration Guides: Breaking changes between versions
Changelog: Detailed version differences

Advanced References

BEAM Book: VM internals for platform issues
Erlang Docs: FFI and external function integration
Standard Library: API reference for import errors

Useful Links for Further Investigation

Gleam Compilation Troubleshooting Resources

Link	Description
Gleam Language Tour	Interactive tutorial that covers type system basics and common patterns. The best place to understand why certain errors occur.
Gleam Language Tour - Results	Interactive tutorial covering Result types and error patterns. Really helpful for understanding why pattern matching exhaustiveness matters.
Gleam Package Index	Search for packages and check their actual module names. Useful when import errors suggest the package exists but modules can't be found.
Gleam Changelog	Detailed release notes showing what changed between versions. Critical for understanding version-specific compilation behavior.
Gleam GitHub Issues	Search closed issues for specific error messages. The community often documents solutions to obscure compilation problems.
Gleam Discord #help Channel	Active community where experienced developers help with compilation errors. Real humans who understand Gleam's type system quirks.
Gleam on Stack Overflow	Tagged questions and answers about Gleam compilation issues. Good for finding solutions to specific error messages.
Exercism Gleam Track	122 coding exercises with mentor feedback. Excellent for understanding how pattern matching and type annotations should work in practice.
VS Code Gleam Extension	Official VS Code support with real-time error checking and type information on hover. Often provides more detailed error context than command-line compilation. Install from the [VS Code Marketplace](https://marketplace.visualstudio.com/search?term=gleam&target=VSCode).
Gleam Language Server	LSP implementation that works with multiple editors. Provides go-to-definition and error checking that can help debug complex import issues.
Gleam Vim Plugin	Vim/Neovim support with syntax highlighting and LSP integration. Good for developers who prefer terminal-based editing.
Fault Tolerant Gleam (May 2024)	Explains how the compiler handles errors and provides more helpful error messages. Useful for understanding why certain errors are reported the way they are.
No More Dependency Management Headaches (August 2025)	Details the v1.12.0 improvements to dependency resolution errors. Read this if you're dealing with dependency hell.
Convenient Code Actions (September 2024)	Covers language server features that help with debugging, including echo removal and error quick-fixes.
Gleam Documentation	Official documentation hub with guides, cheat sheets, and references. Helpful for double-checking pattern matching and type annotation syntax.
Learn X in Y Minutes - Gleam	Concise overview of Gleam syntax and type system. Good for understanding the mental model behind type errors.
Gleam for Elixir Users	Comparison guide that explains type system differences. Useful if you're coming from dynamic BEAM languages.
Gleam for PHP Users	Comparison guide explaining differences from dynamically typed languages. Helps understand why certain type errors occur.
Hex Package Manager	BEAM ecosystem package repository. Search here to check package names and version compatibility when debugging dependency issues.
Gleam Standard Library Docs	API reference for built-in modules. Check here when import errors suggest standard library modules don't exist.
PubGrub Version Resolution Algorithm	The dependency resolution algorithm used by Gleam. Understanding how it works helps debug complex version conflicts.
Erlang Documentation	Official Erlang docs for understanding FFI and external function integration. Necessary when debugging external function calls.
The BEAM Book	Deep dive into BEAM VM internals. Helpful for understanding platform-specific compilation behavior.
Erlang Efficiency Guide	Best practices for BEAM applications. Useful when compilation errors are related to performance or memory usage patterns.
Gleam Source Code on GitHub	The compiler source code. When error messages are unclear, reading the compiler code can reveal what specific conditions trigger certain errors.
Rust Language Reference	Since Gleam is written in Rust, understanding Rust error patterns can help when debugging build system issues or contributing fixes.
BEAM Blogosphere	Community blogs covering BEAM ecosystem topics. Often includes real-world troubleshooting examples and advanced debugging techniques.
Changelog Podcast #588 - Run Gleam run with Louis Pilfold	Creator interview discussing Gleam's design philosophy and type system decisions. Helps understand why certain compilation behaviors exist.
Gleam Debugging Early Days Blog Post	Developer experience setting up debugging workflows. Good for understanding advanced debugging setup beyond basic compilation.
Gleam v1.12.0 Release Notes	Major improvements to dependency resolution and error messages. Worth reading to understand current debugging capabilities.
Gleam Roadmap	Future development plans including planned improvements to error messages and debugging tools.
Migration Guides	Blog posts covering breaking changes and upgrade paths between major versions. Useful when compilation errors appear after version updates.

Gleam Compilation Errors: AI-Optimized Technical Reference

Overview

Critical Context & Failure Modes

Compilation Behavior

Breaking Changes & Version Issues

Most Common Compilation Failures

1. Pattern Matching Exhaustiveness Errors

2. Import Resolution Failures

3. Type Mismatch Errors

4. Dependency Resolution Conflicts

5. Function Arity Mismatches

Advanced Debugging Techniques

Compile-Time Type Debugging

Build System Nuclear Options

FFI Debugging Process

Binary Search Debugging

Resource Requirements & Time Costs

Development Time Impact

Memory and Performance Thresholds

Expertise Requirements

Critical Warnings & Operational Intelligence

What Official Documentation Doesn't Tell You

Breaking Points & Failure Modes

Community Wisdom

Migration Pain Points

Configuration That Actually Works

Version Constraints

Development Workflow

IDE Setup

Decision Criteria

When Gleam is Worth the Compilation Cost

When to Consider Alternatives

Resource Trade-offs

Essential Tools & Resources

Primary Debugging Resources

Version-Specific Resources

Advanced References

Useful Links for Further Investigation

Gleam Compilation Troubleshooting Resources

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