<!--
  SPDX-License-Identifier: Apache-2.0
  SPDX-FileCopyrightText: 2020 The Gleam contributors
-->

# Changelog

## 1.19.0 - 2026-10-05

## 1.19.0-rc3 - 2026-10-01

### Bug fixes

- Fixed a bug where documentation pages configuration could be missing fields
  in `gleam.toml`.
  ([Louis Pilfold](https://github.com/lpil))

- Fixed a bug where documentation pages could be read from outside the package
  directory.
  ([Louis Pilfold](https://github.com/lpil))

## 1.19.0-rc2 - 2026-09-26

### Compiler

- The `--src-only` flag has been renamed to `--no-dev`.
  ([Louis Pilfold](https://github.com/lpil))

- The error message for failing to load data about already compiled modules has
  been improved.
  ([Louis Pilfold](https://github.com/lpil))

### Bug fixes

- Fixed a bug with `gleam compile-package` where `dev_dependencies` would not
  be included in the `.app` file regardless of whether `--no-dev` was provided.
  ([Rodrigo Álvarez](https://github.com/Papipo))

- Fixed a bug where `gleam compile-package` would not use Erlang abstract forms
  for compilation to BEAM.
  ([Louis Pilfold](https://github.com/lpil))

## 1.19.0-rc1 - 2026-09-22

### Compiler

- When compiling to Erlang, Gleam now compiles to the Erlang Abstract Format
  instead of textual Erlang.
  This format supports mapping every generated Erlang line back to its
  corresponding line in the original Gleam source file.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- When compiling to Erlang compile times have been reduced for a typical Gleam
  package.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- The compiler now emits a warning when trying to use the pattern of a spread
  and a variable when pattern matching inside a 'case' clause.
  This syntax may be removed in a future (major) release.

  Considering the following Gleam code:

  ```gleam
  pub fn main() {
    let letters = ["b", "c"]
    case letters {
      [] -> []
      [..x] -> x
    }
  }
  ```

  The compiler will emit the following warning:

  ```txt
  warning: Deprecated list pattern matching syntax
    ┌─ /main.gleam:6:9
    │
  6 │         [..x] -> x
    │         ^^^^^ Replace this with `x`.

      This syntax for pattern matching on lists is deprecated.
      This spread matches the entire list, so the variable x can be used as the
      pattern directly instead.
  ```

  ([Khalid Belkassmi E.H.](https://github.com/khalidbelk))

- The compiler now has a specific error message for when trying to use
  procedural operators that do not exist in Gleam, such as `+=` and `++`.
  ([0xda157](https://github.com/0xda157))

- The error message for calling a function in a guard expression or constant
  expression now points to the entire function call.
  ([mmustafasenoglu](https://github.com/mmustafasenoglu))

- Pattern matching on the JavaScript target now generates flatter code, with
  nested `if` statements collapsed into a single condition and fewer
  intermediate variables. This makes little difference to the size of a bundled
  application, but the resulting code has fewer branches for JavaScript engines
  to optimise. For example, this code:

  ```gleam
  pub fn go(x) {
    case x {
      Wibble(1, 2) -> 1
      _ -> 2
    }
  }
  ```

  Previously would generate this JavaScript:


  ```js
  export function go(x) {
    if (x instanceof Wibble) {
      let $ = x[0];
      if ($ === 1) {
        let $1 = x[1];
        if ($1 === 2) {
          return 1;
        } else {
          return 2;
        }
      } else {
        return 2;
      }
    } else {
      return 2;
    }
  }
  ```

  But now it generates this:

  ```js
  export function go(x) {
    if (x instanceof Wibble && x[0] === 1 && x[1] === 2) {
      return 1;
    } else {
      return 2;
    }
  }
  ```

  ([John Downey](https://github.com/jtdowney))

- When producing TypeScript annotations, the compiler now produces overloads to
  narrow the type of variants of a custom type. For example, given the following
  type:

  ```gleam
  pub type Box(value) {
    Full(value)
    Empty
  }
  ```

  Will now produce these two additional overloads:

  ```ts
  export function Box$isFull<I>(value: Box$<I>): value is Full<I>;
  export function Box$isEmpty<I>(value: Box$<I>): value is Empty;
  ```

  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- The compiler is now fault tolerant when it runs into unsupported binary
  operators used in constants. Instead than stopping to analyse the whole module
  the compiler now produces an error message explaining the operator is not
  supported. For example:

  ```gleam
  pub const wibble = 1.1 *. 11.0
  ```

  Results in the following error:

  ```
  error: Unsupported operator in constant expression
    ┌─ /Users/giacomocavalieri/Desktop/prova/src/prova.gleam:1:24
    │
  1 │ pub const wibble = 1.1 *. 11.0
    │                        ^^

  This operator is currently not supported in constants.
  ```

  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- When generating Erlang type definitions, types called `Record` will be renamed
  to `record_` to not overwrite Erlang's built-in `record` type introduced in
  OTP29.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- Creation of literal lists has been optimised on the JavaScript target.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- The compiler now provides a specific error message directing the user to the
  correct syntax when trying to update a record after providing fields.
  ([0xda157](https://github.com/0xda157))

- The compiler now provides a more specific error message when encountering a
  merge conflict indicator
  ([0xda157](https://github.com/0xda157))

- The compiler now provides hint about `|` in pattern matching
  ([n0kk23](https://github.com/n0kk23))

- The compiler now provides better error message when trying to import or use
  private value or type from modules within current package. For example:

  ```gleam
  import wibble
  //     ^^^^^^ `wibble` is a module in same package

  fn go() -> wibble.Wobble {
    todo
  }
  ```

  Results in following errors:

  ```
  error: Use of private module type
    ┌─ /workspaces/wobble/src/wobble.gleam:4:12
    │
  4 │ fn go() -> wibble.Wobble {
    │            ^^^^^^^^^^^^^

  `wibble.Wobble` is a private type.
  ```

  ([Andrey Kozhev](https://github.com/ankddev))

- When an invalid type alias is created the error messages no longer cascade
  through further usages.
  ([James Dolan](https://github.com/jamesdolan16))

- Matching on bit array patterns which contain empty string segments is now
  deprecated.
  ([Surya Rose](https://github.com/GearsDatapacks))

- The compiler now recognises more unreachable clauses in a `case` where an
  earlier clause matches an overlapping string.

  ```gleam
  pub fn go(input: String, x: Int) {
    case input, x {
      "ab", 1 -> 7
      "a" <> _, _ -> 2
      "ab", _ -> 3
      _, _ -> 4
    }
  }
  ```

  The compiler will emit the following warning:

  ```txt
  warning: Unreachable pattern
    ┌─ /main.gleam:5:5
    │
  5 │     "ab", _ -> 3
    │     ^^^^^^^

  This pattern cannot be reached as a previous pattern matches the same
  values.

  Hint: It can be safely removed.
  ```

  Anything reaching the third clause starts with `"a"`, so the second clause has
  already matched it. Code that previously compiled cleanly may now emit this
  warning.
  ([John Downey](https://github.com/jtdowney))

- The compiler's lexer now uses byte indexing with an ASCII fast path instead
  of iterating over characters, and builds tokens from source slices to reduce
  allocations.
  ([John Downey](https://github.com/jtdowney))

- The performance of `echo` for floats has been improved on the Erlang target.
  ([Andrey Kozhev](https://github.com/ankddev))

### Build tool

- The build tool now stores its build cache in a more compact binary format,
  making the cache files noticeably smaller.
  ([Andrey Kozhev](https://github.com/ankddev))

- The `ext` protocol is now explicitly disabled when when fetching git
  dependencies. This protocol is disabled by default since 2015, so this
  has no impact unless the user is using a very old version of git or has
  enabled this protocol in their configuration.
  ([Amr Kadry](https://github.com/Amrkadry) and
  [Louis Pilfold](https://github.com/lpil))

- The ordering of import statements in generated JavaScript code is now stable.
  Previously the same code compiled twice could produce different output, as
  the singleton constants imported for a type's fieldless variants were emitted
  in a different order on each run.
  ([John Downey](https://github.com/jtdowney))

- Make links to Tangled repositories use their new domain & URL format.
  ([Naomi Roberts](https://github.com/naomieow))

- `compile-package` now supports a `--src-only` flag to compile only the `/src`
  folder.
  ([Rodrigo Álvarez](https://github.com/Papipo))

- The build tool now shows a better error when trying to add a package as a
  dependency when it is already a development dependency (or vice-versa).
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- `--out` argument in `export package-information` and `export package-interface`
  commands is no longer required. Those commands now print to stdout by
  default.
  ([Rodrigo Álvarez](https://github.com/Papipo))

- The `export javascript-prelude` and `export typescript-prelude` commands gain
  the `--out` parameter.
  ([Louis Pilfold](https://github.com/lpil))

- `gleam compile-package` now generates a .app file in the ebin folder, listing
  all .beam modules (this includes Erlang and Elixir ones).
  ([Rodrigo Álvarez](https://github.com/Papipo))

- `gleam compile-package` gains a new `--otp-app-names` flag, letting the
  caller specify dependencies' OTP application names when they differ
  from their Gleam package names, as comma separated `package=otp_app` pairs.
  ([Rodrigo Álvarez](https://github.com/Papipo))

### Language server

- The "Generate dynamic decoder" code action is now only offered when the
  `gleam_stdlib` package is available.
  ([0xda157](https://github.com/0xda157))

- Renaming a prelude item to its original name no longer unnecessarily imports
  it.
  ([0xda157](https://github.com/0xda157))

- The "add missing patterns" code action now has higher precedence than the code
  action to discard an unused result.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- The "pattern match on argument" code action now has higher precedence over the
  "discard unused argument" code action.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- The language server will now surround value completions right after dot in
  imports with braces. For example, consider following code:

  ```gleam
  import wibble.|
  ```

  where cursor is denoted with `|`. Completing value `Wibble` will result in
  following code:

  ```gleam
  import wibble.{Wibble}
  ```

  ([Andrey Kozhev](https://github.com/ankddev))

- The language server now supports triggering "Convert int to different base"
  code action in constants, patterns and bit array "size" options.
  ([Andrey Kozhev](https://github.com/ankddev))

- The language server now supports go-to-definition, find-references and
  renaming for function argument labels.
  ([Alistair Smith](https://github.com/alii))

### Formatter

### Compiler Wasm API

- The WebAssembly build of the compiler now exposes a `format_source` function
  for formatting Gleam source code.
  ([John Downey](https://github.com/jtdowney))

### Bug fixes

- Fixed a bug where the generated Erlang `.app` file's module list could be
  missing Erlang or Elixir native modules that hadn't been recompiled since
  a previous build.
  ([Rodrigo Álvarez](https://github.com/Papipo))

- Fixed a bug where on the JavaScript target a case clause whose guard's top
  level operator was `||` could run for a subject its pattern did not match.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where `gleam export erlang-shipment` would add Gleam contributor
  copyright metadata to the generated POSIX entrypoint.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where the `echo` on the JavaScript target would incorrectly print
  numbers like `1.0e300`.
  ([Andrey Kozhev](https://github.com/ankddev))

- Fixed a bug where the `echo` on the JavaScript target would incorrectly print
  `NaN` and `Infinity`.
  ([Andrey Kozhev](https://github.com/ankddev))

- Fixed a bug where the language server would incorrectly rename values and
  types with a same-named import alias.
  ([Andrey Kozhev](https://github.com/ankddev))

- Fixed a bug where the language server would crash if the client disconnected
  unexpectedly.
  ([Senthilnathan](https://github.com/ssenthilnathan3))

- Fixed a bug where a bit array segment pattern of an empty string would
  generate invalid code on JavaScript.
  ([Louis Pilfold](https://github.com/lpil))

- Fixed a bug where comments after the last item in a tuple, or after the last
  argument in a function call wouldn't be formatted properly.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- Fixed a bug where file permissions were not set in Hex tarballs.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- Fixed a bug where echo would print BitArrays like `<<1, 2, 3>>` as strings on
  the Erlang target.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- Fixed a bug where the compiler would not produce a type error when using a
  record constructor as the updated record in a record update expression in a
  constant.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- Fixed a bug where the compiler would crash when writing a constant record
  update depending on another invalid constant.
  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))

- Fixed a bug where the "Discard unused variable" language server code action
  would produce invalid code when discarding left side assignment of string
  prefix patterns.
  ([Andrey Kozhev](https://github.com/ankddev))

- Fixed a bug where the "Discard unused variable" language server code action
  would produce invalid code when discarding bindings in patterns with
  alternative patterns.
  ([Andrey Kozhev](https://github.com/ankddev))

- Fixed a bug where the language server would not suggest prefix pattern
  prefix alias.
  ([Andrey Kozhev](https://github.com/ankddev))

- Fixed a bug where on the JavaScript target a name shadowed inside a case
  clause that always matched would be used in place of the outer binding by the
  code that followed the case expression. This could produce a wrong value, or a
  runtime crash when the shadowed name was a module function or constant.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where on the JavaScript target the variables a clause guard
  needed would be declared in the enclosing scope of a case that always
  matched, so a later `let` binding one of those names produced a duplicate
  declaration and the whole module failed to parse.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where a package's optional dependency requirements could be
  ignored, or could pull a package into the dependency tree that nothing
  actually required.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where comparing `0.0` and `-0.0` resulted in a redundant
  comparison warning, which is incorrect on Erlang/OTP 27+.
  ([Jack Programs](https://github.com/jackprogramsjp))

- Fixed a bug where a project with multiple path dependencies would perform
  dependency resolution once per path dependency, so commands run after
  `gleam deps download` would needlessly resolve versions again.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where concatenating a string to a string in a block would
  produce an invalid bitstring on the Erlang target.
  ([Lillian Rose](https://github.com/lillianrubyrose) with
  [Mar Bloeiman](https://github.com/strawmelonjuice))

- Fixed a bug where the compiler would suggest wrapping a mismatched type in
  `Ok()` even when doing so would not fix the type unification error.
  ([Hari Mohan](https://github.com/seafoamteal))

- Fixed a bug where `gleam export hex-tarball` and `gleam publish` could fail on
  Windows with a "path is outside this Gleam project" error for files that were
  inside the project.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug in generating TypeScript annotations for function signatures
  that use types from aliased modules.
  ([Ian Chamberlain](https://github.com/ian-h-chamberlain))

- Fixed a bug where the generated `package-interface.json` would not include
  deprecation messages for constructors.
  ([Surya Rose](https://github.com/GearsDatapacks))

- Fixed a bug where on the JavaScript target a string prefix pattern could be
  skipped when an earlier clause matched an overlapping string, but then failed
  for another reason.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where on the JavaScript target a name given to a string prefix or
  bit array in a pattern would not be bound in every branch it was used in.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where the end position of the final token was incorrect when the
  source ended with a multi-byte character.
  ([John Downey](https://github.com/jtdowney))

- Fixed a bug where the build tool would exit with an error when the gleam.toml
  licence array contained a licence starting with `LicenseRef-`, which are used
  by hex.pm to support non-SPDX licences.
  ([Vivid](https://github.com/absolutely-vivid))

- Fixed a bug where packages published from Windows would record their file
  paths in `metadata.config` using `\` separators, which Erlang reads as
  escape characters, corrupting the paths or making the metadata unparseable.
  ([John Downey](https://github.com/jtdowney))

## v1.18.1 - 2026-08-01

- Fixed a bug where the Erlang code generator would generate wrong code when
  referencing constants that are aliases to other constants in bit array
  segments, string concatenation and clause guards.
  ([Andrey Kozhev](https://github.com/ankddev))