Gleam Knowledge Patch

Covers Gleam 1.7–1.15 (2025-01-05 through 2026-03-16). Claude Opus 4.6 knows Gleam through v1.6.x including core language (types, pattern matching, pipes, use expressions, Result/Option, labelled arguments, generics, opaque types, externals/FFI), BEAM and JavaScript targets, and gleam build tool basics.

Index

Topic	Reference	Key features
Debug & testing	references/debug-and-testing.md	echo keyword, echo labels, assert for tests, let assert messages
Bit arrays	references/bit-arrays.md	JS parity (unaligned, 16-bit float, unit, UTF-16/32), float shorthand, size expressions
JavaScript FFI	references/javascript-ffi.md	$-prefixed external API, BitArray FFI, @external on types, CommonJS
Type system & patterns	references/type-system.md	Record updates across type params, const record updates, string <> in guards
Build tool & project	references/build-tool.md	Git deps, gleam dev, dev/ dir, gleam deps outdated, formatter controls

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Quick Reference — New Keywords & Syntax

Feature	Version	Syntax
echo (debug print)	1.9	echo expr or expr |> echo
echo with label	1.12	echo expr as "label"
assert (test assertion)	1.11	assert expr == expected
assert with message	1.11	assert expr as "message"
let assert message	1.7	let assert Ok(x) = expr as "msg"
Variant deprecation	1.7	@deprecated("msg") on variant
@external on types	1.14	@external(erlang, "mod", "type") on pub type
Record update type change	1.7	Named(..data, value: new_typed_value)
Record update in const	1.14	const dev = Config(..base, port: 4000)
String <> in guards	1.15	case x { s if a <> b == s -> ... }
Bit array float literal	1.10	<<1.5>> (no :float needed)
Bit array size expressions	1.12	<<data:bytes-size(size / 8 - 1)>>
Trailing comma → multiline	1.12	["a", "b",] forces multiline format
Git dependencies	1.9	pkg = { git = "url", ref = "..." }
gleam dev	1.11	Runs dev/ directory code
gleam deps outdated	1.14	Check for newer Hex versions

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Essential Patterns

Debug printing with echo (1.9+)

// Standalone — prints value + file:line to stderr
echo [1, 2, 3]

// In pipelines — passes value through
[1, 2, 3]
|> list.map(fn(x) { x + 1 })
|> echo
|> list.map(fn(x) { x * 2 })

// With label (1.12+)
echo config.port as "server port"

Test assertions with assert (1.11+)

pub fn hello_test() {
  assert telecom.ring() == "Hello, Joe!"
}

// Custom message:
pub fn system_test() {
  assert telecom.is_up(key, strict, 2025)
    as "My internet must always be up!"
}

JavaScript FFI — External API (1.13+)

// src/person.gleam
pub type Person {
  Teacher(name: String, subject: String)
  Student(name: String)
}

// src/my_ffi.mjs
import { Person$Teacher, Person$Student, Person$isTeacher,
         Person$Teacher$subject, Person$name } from "./person.mjs";

let teacher = Person$Teacher("Joe", "CS");
Person$isTeacher(teacher);         // true
Person$Teacher$subject(teacher);   // "CS"
Person$name(teacher);              // "Joe" (shared field)

Project structure with dev/ (1.11+)

my_app/
├── src/    # Production code (gleam run)
├── test/   # Tests (gleam test)
└── dev/    # Development scripts (gleam dev)

The $PACKAGENAME_dev module's main function runs via gleam dev.

Git dependencies in gleam.toml (1.9+)

[dependencies]
gleam_stdlib = { git = "https://github.com/gleam-lang/stdlib.git", ref = "957b83b" }

Supports git/HTTP URLs with a tag, branch, or commit ref.

@external on type declarations (1.14+)

@external(erlang, "erlang", "map")
@external(javascript, "../dict.d.mts", "Dict")
pub type Dict(key, value)

Produces precise Erlang type specs and TypeScript declarations instead of any.