Gleam Compilation Errors - Fix Type System Issues & Build Failures

The Most Common Compilation Failures

Gleam's type system is designed to catch bugs at compile time instead of letting them explode in production. Gleam's type system catches bugs at compile time, which means more fighting with the compiler upfront. Here are the patterns that'll drive you nuts.

Pattern Matching Exhaustiveness Errors

You'll see something like:

error: Inexhaustive patterns
  ┌─ /src/app.gleam:15:3
  │
15│   case user_result {
   │   ^^^^ This case expression does not have a pattern for all possible values.

Missing patterns:
- Error(_)

You forgot to handle one of the possible values in your pattern match. Gleam forces you to handle every fucking case, which prevents runtime crashes but gets annoying as hell when you "know" certain cases won't happen. The exhaustiveness checking is stricter than your high school math teacher. Read more about pattern matching best practices and Result type handling in the official docs.

What actually works:

case user_result {
  Ok(user) -> render_user_page(user)
  Error(reason) -> {
    // Handle the error case you were trying to ignore
    io.println(\"User lookup failed: \" <> string.inspect(reason))
    show_error_page()
  }
}

Coming from JavaScript, this is annoying as hell. You can't just ignore errors and let everything explode at runtime. Gleam makes error handling explicit, which means more typing now but fewer angry phone calls at 3am when production goes down. BEAM's fault tolerance philosophy is baked into the language whether you like it or not. Check out the Gleam error handling guide and custom types documentation for more patterns. Stack Overflow Gleam questions often cover similar exhaustiveness checking issues.

Import Resolution Failures

You'll see something like:

error: Unknown module
  ┌─ /src/app.gleam:1:8
  │
1 │ import gleam/http
  │        ^^^^^^^^^^ The module `gleam/http` could not be found.

No module with this name was found in:
- /src
- The included libraries

The package isn't installed, the module name is wrong, or you're using an old package name that got refactored. Check the Gleam package index for current naming.

Try this first:

## First, check if you actually added the package
gleam deps list

## If it's missing, add it
gleam add gleam_http

## If it's there but still failing, check the correct module path
gleam docs build  # This will show available modules

Wasted my Saturday on this. Turns out gleam/http got renamed to gleam_http in one of the v0.30 releases and I was using old Stack Overflow answers. The error message tells you nothing. As usual. Had to dig through the package docs, GitHub issues, and package migration guides to figure out what happened. Package naming conventions have evolved since early releases. The Gleam package search helps find current names, and Hex.pm lists all BEAM packages. Check migration guides for breaking changes between versions.

Type Mismatch Hell

You'll see something like:

error: Type mismatch
  ┌─ /src/api.gleam:23:10
  │
23│   user.id
   │   ^^^^^^^ Expected type `String`, found type `Int`.

Your mental model of the data doesn't match the actual type definition. This bites you constantly when working with external APIs or database records where you assume string IDs but get integers. Type inference can be both helpful and confusing as fuck. Read up on Gleam's type system and JSON decoding patterns for handling external data.

Stop assuming and convert it:

// Instead of assuming the type
let user_id_string = user.id  // This fails if user.id is Int

// Convert explicitly (what you should have done from the start)
let user_id_string = int.to_string(user.id)

// Or handle both cases if you're feeling paranoid
let user_id_string = case user.id {
  id if is_string(id) -> id
  id if is_int(id) -> int.to_string(id)
  _ -> \"unknown\"  // This should never happen unless something's really fucked
}

Function Arity Errors That Confuse Everyone

You'll see something like:

error: Incorrect arity
  ┌─ /src/handlers.gleam:45:3
  │
45│   handle_request(req)
   │   ^^^^^^^^^^^^^^^^^^^ Expected 3 arguments, got 1

This function accepts these additional labelled arguments:
- context
- config

The function signature changed and you're calling it with the old parameters. Usually happens after you update dependencies and suddenly half your code is broken. Spent 2 hours debugging this when Wisp changed their handler signature in some recent version. Works fine in development, explodes in CI because of course it does.

The fix:

// Check the actual function definition
// Old way that's now broken:
handle_request(req)

// New way that works:
handle_request(req, context, config)

// Or if you don't have the required arguments:
handle_request(req, default_context(), default_config())

Dependency Resolution Nightmares

The Error You See (Pre-v1.12.0):

error: Dependency resolution failed

An error occurred while determining what dependency packages and
versions should be downloaded.
The error from the version resolver library was:

Unable to find compatible versions for the version constraints in your
gleam.toml. The conflicting packages are:

- app
- wisp
- mist
- gleam_otp
- gleam_json

Your dependencies want different versions of the same underlying package, and the resolver can't find a combination that makes everyone happy. I've seen this exact error break CI for 3 different teams. Spent hours trying to figure out which package was causing the conflict.

Good news: Gleam v1.12.0 (released August 5, 2025) finally fixed this with better error messages. Instead of the cryptic stuff above, you get:

error: Dependency resolution failed

There's no compatible version of `gleam_otp`:
- You require wisp >= 1.0.0 and < 2.0.0
- wisp requires mist >= 1.2.0 and < 5.0.0
- mist requires gleam_otp >= 0.9.0 and < 1.0.0
- You require lustre >= 5.2.1 and < 6.0.0
- lustre requires gleam_otp >= 1.0.0 and < 2.0.0

Now you can actually see the conflict: mist wants gleam_otp < 1.0.0 but lustre wants gleam_otp >= 1.0.0.

Update your dependencies to compatible versions:

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

Custom Type Definition Errors

You'll see something like:

error: Unknown type
  ┌─ /src/models.gleam:12:15
  │
12│ pub fn process_user(user: User) -> String {
   │                         ^^^^ The type `User` is not defined.

You forgot to import the type, it's defined in a different module, or you have a typo in the type name. Spent 30 minutes once thinking the compiler was broken until I realized I spelled User as Uesr. Delete node_modules and try again won't work here - this is just a typo.

Fix:

// If User is defined in another module:
import app/models.{type User}

// Or import the whole module:
import app/models
pub fn process_user(user: models.User) -> String

// If you're defining it in the same file, make sure the type definition comes first:
pub type User {
  User(id: Int, name: String, email: String)
}

pub fn process_user(user: User) -> String {
  user.name <> \" (\" <> user.email <> \")\"
}

Record Update Syntax Confusion

You'll see something like:

error: Syntax error
  ┌─ /src/user_logic.gleam:34:21
  │
34│   User(..user, name: new_name, email: new_email)
   │                ^^^^ I was expecting `}` here.

The record update syntax has specific rules about field order and you're breaking them.

Fix:

// Wrong - you can't mix spread with individual field updates like this
User(..user, name: new_name, email: new_email)

// Right - use spread at the end
User(name: new_name, email: new_email, ..user)

// Or update specific fields only
User(..user, name: new_name)

Why This Trips People Up: The spread syntax (..user) must come at the end, unlike JavaScript where object spread can go anywhere. This is a deliberate design choice in Gleam but feels wrong as hell if you're coming from JS. Took me forever to unlearn this muscle memory.

External Function FFI Errors

You'll see something like:

error: Unknown external target
  ┌─ /src/crypto.gleam:5:1
  │
5 │ @external(erlang, \"crypto\", \"hash\")
  │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ External implementations must target \"erlang\" or \"javascript\".

You're trying to call external Erlang or JavaScript functions but the syntax is wrong or the target module doesn't exist.

Fix:

// Make sure the external function exists in the target runtime
@external(erlang, \"crypto\", \"hash\")
@external(javascript, \"./crypto.mjs\", \"hash\")
fn hash(algorithm: String, data: String) -> String

// And create the JavaScript implementation file crypto.mjs:
export function hash(algorithm, data) {
  const crypto = require('crypto');
  return crypto.createHash(algorithm).update(data).digest('hex');
}

Watch out: External functions are where type safety goes to die. Spent an entire afternoon debugging why our "type-safe" Gleam code kept crashing with undefined errors that took down prod for 2 hours. Turns out the JavaScript FFI function was fine for normal inputs but completely broke when someone passed an empty string. Type system can't see inside external code, so you're flying blind. FFI is where type safety goes to die.

This covers most compilation errors you'll hit. Gleam's errors are usually precise about what's wrong, you just need to understand the type system's rules. Once you get used to the compiler's nagging, you'll actually miss it in other languages.

When Standard Fixes Don't Work

Sometimes you hit compilation errors that resist every fix you've tried. These are the edge cases that make you seriously consider going back to TypeScript where at least the pain is predictable. Here's how to debug the weird stuff that doesn't fit into neat categories.

Using echo for Compile-Time Debugging

The `echo` keyword got major improvements in Gleam v1.12.0, including custom messages and better display formatting. This isn't just for runtime debugging - you can use it to understand what types the compiler thinks your values have.

pub fn debug_types() {
  let user_data = get_user_from_api()
  echo user_data as "user data type"

  let processed = user_data
    |> echo as "before processing"
    |> transform_user_data()
    |> echo as "after processing"

  processed
}

Why This Works: echo prints the value and its inferred type to the console during compilation. When you're getting type mismatch errors, this shows you exactly what types the compiler sees versus what you expect. It's like printf debugging but for types.

Pro Tip: The language server provides a code action to remove all echo statements from a module when you're done debugging. Don't forget to use it before committing - the compiler will warn you about echo in published packages. Trust me, you don't want that embarrassment.

Compiler Flag Debugging

Most developers don't know about Gleam's debugging compiler flags. These can reveal what's happening under the hood when standard error messages aren't clear enough.

## Show detailed type inference information
gleam build --verbose

## See the generated Erlang code (helps debug FFI issues)
gleam build --target erlang && cat build/dev/erlang/*/ebin/*.erl

## See the generated JavaScript (helps debug JS target issues)
gleam build --target javascript && cat build/dev/javascript/*/index.mjs

Real-world mess: Had a function that worked perfectly on Erlang but completely failed when targeting JavaScript. Compiler just said "compilation failed" which tells you nothing. Turns out I was using a bit array pattern that JavaScript doesn't support. Spent 4 hours reading generated JS code to figure that out. Cross-platform development has its quirks. Check the JavaScript target documentation and target compatibility guide for platform differences.

Dependency Version Detective Work

Even with the improved dependency resolution errors in v1.12.0, sometimes you need to dig deeper into version conflicts.

## Show all dependencies and their versions
gleam deps list

## Show dependency tree (requires building first)
gleam build && find build -name "*.toml" -exec echo {} \; -exec cat {} \;

## Check what versions are actually available
gleam search your_problem_package

Advanced Technique: Create a minimal test project to isolate dependency issues:

mkdir debug_deps && cd debug_deps
gleam new test_project
cd test_project
gleam add the_problematic_package
gleam build

If it builds in the test project but not your main project, you have a dependency conflict. If it fails in both, the package itself has issues.

Type System Boundary Testing

For weird type errors, isolate the problem with small test functions:

// Create minimal reproduction cases
pub fn test_type_a() {
  let value = your_problematic_function()
  echo value as "actual type"
  value
}

pub fn test_type_b() {
  let expected: YourExpectedType = // some default value
  echo expected as "expected type"
  expected
}

// Then try to assign one to the other
pub fn test_compatibility() {
  let actual = test_type_a()
  let expected: YourExpectedType = actual  // This will fail with precise error
}

This shows you exactly where the type mismatch is.

External Function Debugging

FFI calls to Erlang or JavaScript are where type safety goes to die. When external functions fail, the error messages are often unhelpful because the compiler can't see inside the external code.

Systematic FFI Debugging Process:

1. Test the external function exists:

@external(erlang, "crypto", "hash")
fn test_external() -> String

2. Test with simple inputs:

@external(erlang, "crypto", "hash")
fn hash_simple() -> String {
  // Call with known good inputs first
}

3. Add type assertions:

@external(erlang, "crypto", "hash")
fn hash_with_check(algorithm: String, data: String) -> String {
  let result = hash_internal(algorithm, data)
  case result {
    s if is_string(s) -> s
    _ -> panic as "External function returned unexpected type"
  }
}

Common FFI Gotcha: Erlang functions often return tuples for errors ({error, reason}) but your Gleam type signature expects a plain string. You need wrapper functions to convert between Erlang conventions and Gleam types. Read the FFI documentation and Erlang interop guide for more patterns. Check NIFs and Ports documentation for complex integrations.

Pattern Matching Debugging

Complex pattern matches can fail in subtle ways. Break them down systematically:

// Instead of one complex pattern match
case complex_data_structure {
  ComplexPattern(field1, SubPattern(nested_field, AnotherPattern(deep_field))) -> {
    // Complex logic
  }
}

// Debug step by step
case complex_data_structure {
  ComplexPattern(field1, sub_data) -> {
    echo field1 as "field1"
    echo sub_data as "sub_data structure"
    case sub_data {
      SubPattern(nested_field, another_data) -> {
        echo nested_field as "nested_field"
        echo another_data as "another_data structure"
        // Continue breaking down...
      }
    }
  }
}

Build System Debugging

Sometimes the issue isn't your code but the build system itself. Here are the nuclear options:

## Clear everything and rebuild
rm -rf build/
gleam deps download
gleam build

## Check for file system issues (especially on Windows)
find . -name "*.gleam" -exec echo "Checking: {}" \; -exec head -1 {} \;

## Verify your gleam.toml isn't corrupted
gleam format src/  # This will fail if gleam.toml has syntax errors

Windows-Specific Issues: Windows PATH limit will break things. If you're getting ENOENT: no such file or directory errors on Windows, try moving your project to a directory with a shorter path like C:\dev\myapp. Learned this the hard way after 2 hours of debugging. Check the Windows installation guide and cross-platform considerations for platform-specific issues.

Language Server Debugging

VS Code's Gleam extension can sometimes provide more detailed error information than the command line compiler:

1. Open the Output panel → "Gleam Language Server"
2. Look for detailed error traces that don't show up in gleam build
3. Restart the language server if errors seem stale (Command Palette → "Reload Window")

The language server maintains its own compilation state and sometimes gets out of sync with your actual code.

Memory and Performance Issues During Compilation

Large projects can hit memory limits during compilation, especially with complex type inference:

## Monitor memory usage during build
while true; do ps aux | grep gleam; sleep 1; done &
gleam build

## If you hit memory limits, try building incrementally
gleam clean
gleam build src/simple_module.gleam  # Build one module at a time

Enterprise Gotcha: Projects with hundreds of modules and complex type hierarchies can exhaust memory during type checking. I've seen this take down CI builds for an entire team. The only real solution is to break large modules into smaller ones or throw more RAM at the problem.

Version Compatibility Matrix Testing

When debugging cross-version compatibility issues, create a test matrix:

## Test with different Erlang versions
docker run -it erlang:24 bash -c "gleam build"
docker run -it erlang:25 bash -c "gleam build"
docker run -it erlang:26 bash -c "gleam build"

## Test with different Gleam versions (if you suspect version issues)
asdf install gleam 1.11.0
asdf local gleam 1.11.0
gleam build
asdf local gleam 1.12.0
gleam build

This systematic approach reveals whether your issue is Gleam version specific or Erlang version specific.

The Nuclear Option: Binary Search Debugging

When all else fails and you have a large codebase with mysterious compilation failures, use binary search:

1. Comment out half your code
2. Try to compile
3. If it compiles, the bug is in the commented code
4. If it doesn't, the bug is in the remaining code
5. Repeat until you isolate the problematic lines

This is tedious but works when you're completely lost. Used this once to find an issue where the error pointed to line 50 but the actual problem was a missing import 200 lines down. Took my entire afternoon.

Documentation and Community Resources

When you're stuck, these resources often have answers that aren't in the official docs:

• Discord #help channel: Real humans who've seen your exact error before
• GitHub issues: Search closed issues for similar error messages
• Exercism Gleam track: Look at solutions for similar patterns
• Gleam Weekly newsletter archives: Often covers common pitfalls
• Gleam Community Forum: Community discussions and troubleshooting tips
• Awesome Gleam: Curated list of resources and examples

The type system provides precise information - you just need to know how to extract it. These techniques help bridge the gap between what the compiler knows and what you understand.