Version 1.12 (2025-06-04)

Nickel 1.12 includes the long-awaited addition of field punning, in the form of include expressions, which makes it easy to re-use a variable as a record field without running into infinite recursion errors or clumsy renamings. If foo is a symbol in the environment (e.g. a variable defined earlier or a function parameter), one can now write { include foo } instead of having to write let foo_ = foo in { foo = foo_ } to put it in a field with the same name. See the syntax section of the manual for more details.

The Nickel CLI now accepts any supported configuration format as file input, making the nickel binary a possible merge engine for JSON, YAML or TOML configurations. for example, nickel export conf1.json conf2.toml --format YAML --output result.yaml will merge a JSON configuration and a TOML configuration into a YAML one using Nickel's merge semantics. Together with the just added --apply-contract argument, it's also possible to use Nickel as a non-invasive validator for existing configurations without needing to convert them to Nickel first. For example, nickel eval config.json --apply-contract schema.ncl will validate config.json against the Nickel contract schema.ncl.

Core language

• [Feat] Field punning (include expressions) by @yannham in #2241
• [Feat] Support annotations on included expressions by @yannham in #2252

LSP

• Add context to error opening trace file in nls by @L0r3m1p5um in #2244
• Update behavior of debugLog flag in VSCode extension by @L0r3m1p5um in #2257
• Filter out LSP diagnostics for labels with no message by @jneem in #2263

Tooling

• Improved error message when subtyping an inhomogeneous record as a dict by @jneem in #2234
• Only output up to 10 warnings by @jneem in #2239
• Negation error messages by @jneem in #2242
• [Feat] Add --apply-contract argument to the CLI by @yannham in #2266
• [Feat] Determine the format automatically for input files on the CLI by @yannham in #2267

Fixes

• Don't quote enum tag keywords by @jneem in #2232
• Remove the unused merge keyword by @jneem in #2243
• Bigger stack on windows by @jneem in #2260

New Contributors

• @L0r3m1p5um made their first contribution in #2244

Full Changelog: 1.11.0...1.12.0