Gleam Knowledge Patch Skill

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 in guards | | Build tool & project | references/build-tool.md | Git deps, gleam dev, dev/ dir, gleam deps outdated, formatter controls |

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 } | | gleam dev | 1.11 | Runs dev/ directory code | | gleam deps outdated | 1.14 | Check for newer Hex versions |

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.

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